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Online Content Code: ASY2009 SEPTEMBER 2020
Enter this code at: www.astronomy.com/code
VOL. 48, NO. 9
to gain access to web-exclusive content

NASA, ESA/HUBBLE AND THE HUBBLE HERITAGE TEAM

CONTENTS 18 ON THE COVER

FEATURES 38 54 Ethereal glowing structures in the
Eagle Nebula let us reflect on the
18 COVER STORY Star Dome and Deep-sky observing possibilities of life elsewhere in
Paths of the Planets from Costa Rica the cosmos.
How we’ll find life
in the universe RICHARD TALCOTT; Some of the greatest COLUMNS
ILLUSTRATIONS BY ROEN KELLY splendors of the cosmos lie
Is there life out there? in the Southern Hemisphere. Strange Universe 14
Researchers are pursuing 44
three paths to find out. DAVID J. EICHER BOB BERMAN
The great asterism hunt
ROBERT NAEYE 60 For Your Consideration 16
You can find lots of unofficial
28 star patterns using binoculars, We test Chroma filters JEFF HESTER
a telescope, or just your eyes.
What does Titan With perfectly applied Secret Sky 62
smell like? MICHAEL E. BAKICH and highly tuned coatings,
Chroma’s series of parfocal STEPHEN JAMES O’MEARA
A bouquet of musky 50 filters offer top-notch quality
sweetness, bitter almonds, for a reasonable price. Observing Basics 64
gasoline, and decomposing A shadow crosses
fish would likely fill the air South America TONY HALLAS GLENN CHAPLE
on Saturn’s largest satellite.
With a bit of planning, you can 68 Binocular Universe 66
MORGAN L. CABLE witness this year’s total solar
eclipse. MICHAEL E. BAKICH Ask Astro PHIL HARRINGTON
36
Messages from the farside. 7
Sky This Month
QUANTUM GRAVITY
Catch the outer planets.
Everything you need
MARTIN RATCLIFFE to know about the
AND ALISTER LING universe this month:
SpaceX brings astronauts
ONLINE Dave’s News My Science Picture of to space, MEERKAT
FAVORITES Universe The latest Shop the Day observations solve a
The inside updates from Gorgeous mystery, and more.
Go to www.Astronomy.com scoop from the science Perfect gifts for photos from
the editor. and the hobby. your favorite our readers. IN EVERY ISSUE
for info on the biggest news and science geeks.
From the Editor 5
observing events, stunning photos, Astro Letters 6
Advertiser Index 65
informative videos, and more. New Products 67
Reader Gallery 70
Breakthrough 74

Astronomy (ISSN 0091-6358, USPS 531-350)
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4 ASTRONOMY • SEPTEMBER 2020

FROM THE EDITOR

What is life? Editor David J. Eicher
Design Director LuAnn Williams Belter
Sunlight glints off It’s one of the oldest philosophical
a hydrocarbon lake questions, and one that only in EDITORIAL
near Titan’s north pole recent times have we come to accept- Production Editor Elisa R. Neckar
in this near-infrared able scientific terms with. Living organisms Senior Associate Editor Alison Klesman
image, snapped by display seven distinct characteristics: homeo- Associate Editor Jake Parks
the Cassini spacecraft stasis (regulation of their internal environ- Copy Editor McLean Bennett
in 2012. NASA/JPL-CALTECH/ ments), organization into one or more cells, Editorial Assistant Hailey McLaughlin
metabolism, growth, adaptation, response to
UNIVERSITY OF ARIZONA/ stimuli, and reproduction. Those properties ART
have led to 8.7 million known living species Contributing Design Director Elizabeth M. Weber
UNIVERSITY OF IDAHO on our planet. And scientists believe that may Illustrator Roen Kelly
represent only 20 percent of the total. That also doesn’t include Production Specialist Jodi Jeranek
things like viruses: Our nemesis of recent days, the coronavirus, is
not alive. It is merely an infectious agent that hijacks cells once CONTRIBUTING EDITORS
within them to make copies of itself. Michael E. Bakich, Bob Berman, Adam Block,
Bob Naeye’s story, “How we’ll find life in the universe,” on Glenn F. Chaple Jr., Martin George, Tony Hallas,
page 18, describes where we are in the earliest stages of the search Phil Harrington, Korey Haynes, Jeff Hester, Alister Ling,
for life elsewhere in the cosmos. The visible universe contains a Stephen James O’Meara, Martin Ratcliffe, Raymond Shubinski,
minimum estimated 10,000 billion billion stars. Given our heritage Richard Talcott
of thinking Earth is the center of everything and finding out that’s
not quite right, there’s probably an immense amount of life out SCIENCE GROUP
there. Spectroscopy tells us that chemistry is uniform throughout Executive Editor Becky Lang
the universe, and we’ve detected complex organic compounds else-
where in the solar system. Life must be spread widely throughout EDITORIAL ADVISORY BOARD
the Milky Way and in other galaxies. Buzz Aldrin, Marcia Bartusiak, Jim Bell, Timothy Ferris,
And yet we know of just one planet that contains life — right here. Alex Filippenko, Adam Frank, John S. Gallagher lll,
The point of origination is unknown, although it’s clear that life Daniel W. E. Green, William K. Hartmann, Paul Hodge,
began relatively quickly after the violent period of bombardment Edward Kolb, Stephen P. Maran, Brian May, S. Alan Stern,
in the early solar system. The best betting odds put forth a deep James Trefil
oceanic black smoker as the place where organic molecules first
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ASTRO LETTERS

The Apollo 13 capsule A piece of Apollo history we sky lovers are fortunate if we’re able to enjoy a
after recovery. NASA transit. — Jim McLeod, Charlotte, NC
Richard Talcott’s interview of Jim Lovell
We welcome in the April issue brought back memories. Future cosmologist
your comments While the Apollo 13 capsule was in Paris,
at Astronomy Letters, the control panels and seats ended up at the I am 17 years old and received your March 2020 edition
P.O. Box 1612, Kentucky Science Center (formerly known as on my birthday. I want to be a cosmologist and prove
Waukesha, WI 53187; the Museum of Natural History and Science). the existence of primordial black holes and prove
or email to letters@ A decade after the mission, the Apollo 13 crew Hawking right. I’ve been doing research and studying
astronomy.com. came to Louisville to celebrate and the lines to black holes for the past eight months now. The article
Please include your meet the astronauts went out the door. “Is Planet Nine a tiny black hole?” by Jake Parks has
name, city, state, and About five years later, I worked at the museum allowed me to extend my study. Thank you for this
country. Letters may as the curator of science. One of my annual tasks was to amazing edition. I’m looking forward to receiving
be edited for space enter the capsule and gently clean the interior. I could more. — Aayush Roy, Mumbai, India
and clarity. appreciate the claustrophobic nature the astronauts had
worked in as I carefully sat in the same seats as Lovell, Finally informed
Haise, and Swigert. — Alan Goldstein, Louisville, KY
I want to write and commend Ron Voller and
The lucky ones Astronomy for the wonderfully informational story
“How the Ritchey-Chrétien telescope was born” in the
Leave it to Roen Kelly to remind me why Mercury March 2020 issue. I have wondered for a long time what
and Venus transit the Sun so rarely. When I com- makes a Ritchey-Chrétien telescope different than a
bine the visually appealing illustration on page 13 Cassegrain. Your story and the diagrams successfully
of May’s issue with the fact that the Sun’s angular educated me. — Michael VanVooren, Ellisville, MO
diameter is only ½°, it is easy to understand why

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6 ASTRONOMY • SEPTEMBER 2020

QG QUANTUM GRAVITYEVERYTHING YOU NEED TO KNOW ABOUT THE UNIVERSE THIS MONTH

NASA, ESA, THE HUBBLE HERITAGE TEAM (STSCI/AURA), A. NOTA (ESA/STSCI), AND THE WESTERLUND 2 SCIENCE TEAM. BOTTOM FROM LEFT: NASA/JUDE GUIDRY; NASA/BILL INGALLS; NASA/GODDARD/UNIVERSITY OF ARIZONA SNAPSHOT HOT READY ROCKETS FAREWELL, ANNIE GO FOR COLLECTION
BYTES NASA has contracted Annie Glenn, a strong NASA’s OSIRIS-REx
LOCATION California’s Aerojet advocate for people spacecraft will attempt
MATTERS Rocketdyne to produce with speech disorders to collect its first
18 Space Launch and widow of astronaut sample from asteroid
A peek inside a System RS-25 rocket John Glenn, died May 19 Bennu on October 20.
packed cluster engines in preparation due to complications The action was delayed
shows it’s harder for upcoming Artemis from COVID-19. She was two months due to the
to birth planets Moon missions. 100 years old. ongoing pandemic.
in the center.

Planets are commonplace
throughout our galaxy, but some
neighborhoods are tougher than
others when it comes to planet
formation. Astronomers looking
at the properties of the disks of
gas and dust from which planets
ultimately form around nascent
stars have found that environ-
ment plays a role in how well
— and how long — those disks
can stick around. In a region
like the star cluster Westerlund
2, imaged here by the Hubble
Space Telescope for its 25th
anniversary, stars in the crowded
inner portion of the cluster are
less likely to have planet-forming
disks than those on the sparser
outskirts. That’s because in the
cluster’s center, massive, hot stars
blast out intense radiation and
fast-moving winds that evaporate
or blow away their neighbors’
circumstellar disks. Any dust
that does remain has been altered
by the plentiful, high-energy
starlight, making it less likely
to clump together into planets.
Outside the center, however,
planet formation appears to
be proceeding as expected,
highlighting that location is
an important factor when it
comes to forming stars and their
orbiting worlds. —ALISON KLESMAN

WWW.ASTRONOMY.COM 7

QUANTUM GRAVITY

HISTORIC FIRST

SPACEX’S CREW DRAGON DELIVERS
NASA ASTRONAUTS TO ISS

Elon Musk’s rocket company is now the first private enterprise
to put a human into orbit. But, more importantly, the mission
restored America’s ability to launch astronauts from its own soil.

Private spaceflight company astronaut Doug Hurley, who com- A DOCK TO REMEMBER. The SpaceX Crew
SpaceX made history on manded May’s Crew Dragon Demo-2
May 30, when its Crew Dragon capsule mission. He was accompanied by fellow Dragon spacecraft approaches the space station
safely reached orbit with two NASA astronaut Robert Behnken. for docking in this artist’s concept. On May 31,
astronauts aboard. The next day, Crew 2020, the real Crew Dragon, carrying two NASA
Dragon went on to successfully dock LAUNCH AMERICA astronauts, successfully carried out the depicted
with the International Space Station docking maneuver. NASA/SPACEX
(ISS). The historic mission marks the For nearly a decade, NASA has had
first time NASA astronauts have blasted to buy its astronauts seats on Russian space agency branded May’s historic
off from U.S. soil since the Space Soyuz spacecraft to reach the ISS, event “Launch America.”
Shuttle Program ended in 2011. which was built with some $200 billion
Appropriately, that final flight of in U.S. taxpayer dollars, according to “This is a unique opportunity
space shuttle Atlantis was piloted by some estimates. To celebrate the return to bring all of America together in
of crewed launches from U.S. soil, the one moment in time and say: ‘Look
at how bright the future is.’ That’s
what this launch is all about,” NASA
Administrator Jim Bridenstine said at a
press briefing before the launch.

As Crew Dragon blasted off, roughly
10 million people watched live — both
online and on TV; the event was carried
by all the major news networks. Back

8 ASTRONOMY • SEPTEMBER 2020

in 2019, over 100,000 people traveled PRACTICE RUN. Robert Behnken enters QUICK
to Cape Canaveral in Florida for Crew TAKES
Dragon’s uncrewed demonstration commands during a test run of the Crew
flight. But the crowds at the Kennedy Dragon ahead of the launch. SPACEX/ASHISH SHARMA CLOTTING IN SPACE
Space Center were significantly smaller
this time around, primarily due to the when they come back, the return trip A study of 38 female astronauts with
social distancing restrictions in place will serve as yet another pivotal test flights between 2000 and 2014 found
to combat the COVID-19 pandemic. for Crew Dragon. The capsule must
survive being superheated by Earth’s that taking oral contraceptive pills
Nonetheless, a palpable silence, atmosphere before it uses a collection while in space does not appear to
followed by raucous cheers, rang out of parachutes to further slow down
at Launchpad 39A as the astronauts and gently land in the Atlantic Ocean, increase the risk of potentially
climbed to the heavens aboard Crew just off Florida’s east coast. life-threating blood clots.
Dragon. This same launch site has been
used to vault space shuttle flights into If the mission ends successfully, it INTERSTELLAR CENTAURS
orbit, as well as send Apollo astronauts will complete SpaceX’s validation to
to the Moon. fly crewed missions to orbit — which Researchers modeled and rewound
would be yet another first for the the orbits of a group of Centaurs —
As Crew Dragon entered low Earth private company. It should also allow
orbit, the rocket boosters from its them to start flying tourists into orbit icy asteroids that usually orbit
Falcon 9 launch vehicle descended back in the not-too-distant future. between Jupiter and Neptune —
to Earth, landing safely on SpaceX’s and found that 19 were likely ripped
drone ship Of Course I Still Love You. The company currently has two from a nearby star system when our
This allowed them to be retrieved for available Crew Dragon capsules it can
use in future missions. Such reusability use to ferry NASA astronauts to the ISS, newborn Sun still lived in a
has proven to be the key feature that as well as a number of Cargo Dragon stellar nursery.
makes SpaceX’s technology cheaper capsules for sending supplies. But in
than anything that has come before addition to its NASA contracts, SpaceX MOTHER OF HUBBLE
it. And with “cheap” flights to space already has been in talks with space
now available, many think NASA is tourism companies about bringing NASA’s Wide Field Infrared Survey
in a prime position to reestablish its tourists to orbit. Each Crew Dragon Telescope, set to launch in the
dominant role in space. ship is designed to hold four NASA mid-2020s, is now named the
astronauts or seven tourists, so human- Nancy Grace Roman Space
“We’re bringing America back, ity’s presence in space should increase Telescope in honor of the space
as it relates to human spaceflight,” in the years ahead.
Bridenstine said at a joint press agency’s first chief astronomer, who
conference with President Donald Actor Tom Cruise could be among was pivotal in making the Hubble
Trump. “There was a day when there the first non-astronauts to take one Space Telescope a reality.
was grass growing out of the runways of those seats. SpaceX and NASA are
[at Kennedy Space Center]. But now, we reportedly in talks with the star about PRESSURE RELIEF
not only have the policy directive from filming an upcoming movie in space.
the administration, we also have the So stay tuned, because the next few Pluto’s atmospheric pressure
budgets to match that policy directive years of human spaceflight are sure increased from 1988 to 2016, but
to put America preeminent in space.” to have many more surprising twists. has since decreased by more than
20 percent. Although astronomers
DAWN OF SPACE TOURISM — ERIC BETZ expected some drop as the world
recedes from the Sun, they did not
Crew Dragon autonomously docked predict such a precipitous fall.
with the ISS on May 31 at 10:16 A.M.
EDT. And at the time of this writing, CHAOTIC ORIGINS
the crew is aboard the orbiting research
laboratory, carrying out experiments. Data from ESA’s Gaia spacecraft
It’s not yet clear exactly how long suggest the Milky Way’s small
Behnken and Hurley will remain on satellite galaxy Sagittarius has
board before returning to Earth, but repeatedly slammed into our
they’re expected to stay until sometime
in late summer or early fall. No matter galaxy, triggering major episodes of
star formation. One such episode
4.7 billion years ago roughly
corresponds with the Sun’s birth.

CLEAN SPACE

The commercial satellite industry
could be worth $3 trillion USD by
2040 if an “orbital-use fee” were
imposed on satellites, according to
a new study. The tax would reduce
the risk of collisions and resulting
space junk by discouraging the use
of new satellites until old ones are

deorbited. — JAKE PARKS

WWW.ASTRONOMY.COM 9

QUANTUM GRAVITY

TESS unlocks the secret
of Delta Scuti stars

Brightness
variation

PRODUCED BY DR. CHRIS BOSHUIZEN (TWITTER.COM/DRCHRISPYMUSIC), WITH ASSISTANCE FROM DR SIMON
MURPHY AND PROF. TIM BEDDING (TWITTER.COM/TIMBEDDING)

ESO/BOCCALETTI ET AL.
1% A PLANET
IS BORN
0%
Astronomers think they may
–1% 6 12 Hours 18 24 have captured the first direct
evidence of a planet being
Twinkling variable stars are vital tools for COMPLEX PATTERNS. Delta Scuti stars rotate so born in the image above,
astronomers. Each flicker is tied to the taken with the European
expansion and contraction of the star, rapidly that they are flattened at the poles and Southern Observatory’s Very
allowing us to probe its internal structure. bulging in their center. Such fast rotation has Large Telescope. On display
But one class of variables, Delta Scuti made spotting a regular pattern to their pulsations are bright spiral arms within
stars, has long been shrouded in mystery. difficult, but a recent study has finally done so. the dusty disk around the
That is, until a recent mission designed to young star AB Aurigae,
find exoplanets spotted something new. the flickering of variable stars, too. So by located some 500 light-years
combing through TESS data, researchers away. Such strong spirals,
Delta Scuti stars are young stars identified several Delta Scuti stars with researchers say, suggest that
between 1.5 and 2.5 times the Sun’s mass. apparently regular pulsations. The team a baby planet is stirring up
They offer a window into our own Sun’s then went back through data from the gas in the disk like a boat
past, revealing how stars and planets form Kepler Space Telescope — a similar planet creates a wake in water.
and evolve over time — if astronomers hunter that’s now defunct — and followed And although astronomers
can discern what they’re saying. Delta up with ground-based observations. In previously suspected this
Scuti stars rotate rapidly, which muddles total, they identified 60 Delta Scuti stars system was growing a planet,
their complex intrinsic variations, like the with regular, predictable variations. “It’s the new, clearer images reveal
cacophony that occurs when several like notes of a song finally falling into place a remarkable “twist” (inset,
instruments play different parts of a song to play a beautiful melody,” said study bright yellow). This knotlike
all at once. That makes it hard to pick out co-author Daniel Huber, of the Institute for feature, researchers say, is the
their pulsing variations amid other changes Astronomy at the University of Hawai‘i, in result of two inflows from the
astronomers might see. a press release. larger disk converging on the
fledgling planet’s location,
But in a paper published May 13 in the All 60 are particularly young Delta Scuti allowing material to accrete
journal Nature, researchers using NASA’s stars with fast pulsations, which the team onto the growing world. — J.P.
Transiting Exoplanet Survey Satellite (TESS) says are easiest to detect. As these stars age,
announced they’d identified, for the first their pulsations get slower, which means
time, regular pulsations in Delta Scuti stars. they’re more likely to get jumbled by the
TESS watches stars for the telltale dimming star’s rotation. The new find will not only help
that indicates a planet passing in front of astronomers better understand the structure
them. But that also means TESS catches and nature of Delta Scuti stars, but also hint
at a star’s age based on its variations. — A.K.

10 ASTRONOMY • SEPTEMBER 2020

BYE-BYE, SOLAR SYSTEM!

TOP-DOWN VIEW

Voyager 1

Pioneer 11 SatEarth Jupiter
New urn
Horizons
Uranus
Voyager 2
Pioneer 10
NRAO/AUI/NSF; SARAO; DES
ASTRONOMY: ROEN KELLYNeptune

Pluto

MeerKAT solves Voyager 1 OBLIQUE
a boomerang- SIDE VIEW
shaped mystery Pioneer 11
New Uranus
Astronomers often see twin jets erupting Horizons Neptune
in opposite directions from galaxies with
actively feeding supermassive black holes. Saturn Pluto
But in the galaxy PKS 2014-55 (shown
here) and other so-called X-galaxies like Pioneer 10
it, the jets from the central black hole are
oddly boomerang- or X-shaped. And new, Ecliptic plane
detailed observations with the MeerKAT
radio telescope in South Africa have Jupiter
finally uncovered why. PKS 2014-55’s
X-shaped jets of radio waves (blue), Voyager 2 FAST FACT
which extend 2.5 million light-years into
space, are being turned back onto the The Voyager spacecraft were able to
galaxy as they encounter the pressure visit all four outer planets thanks to a
of intergalactic gas around them. But as rare alignment of the giant worlds that
the material is pushed back toward the takes place just once every 175 years.
core of the galaxy, it’s again deflected
by higher pressure gas, curving it back LEAVING HOME. In the past 45 years, NASA has launched five ambitious
outward and creating the horizontal arms spacecraft that are destined to exit our stellar neighborhood. The
of the X. This result reveals the intriguing first two, Pioneer 10 and Pioneer 11, were launched in 1972 and 1973
battle between the black hole’s jets and respectively, and studied the asteroid belt, Jupiter, and more. The next
the gas through which they’re moving. pair, Voyager 1 and Voyager 2, blasted off a couple of weeks apart in
1977 and went on to explore all the giant planets. Nearly 30 years later,
— HAILEY ROSE MCLAUGHLIN in 2006, NASA launched New Horizons, which made its dramatic
flyby of Pluto in 2015 before venturing on to visit the Kuiper Belt
13.6 billion object Arrokoth on New Year’s Day 2019. — J.P.

The age, in years, of globular

cluster NGC 6652. This makes

it one of the oldest objects in

the Milky Way Galaxy.

WWW.ASTRONOMY.COM 11

QUANTUM GRAVITY

Astronomers spot a bowed bridge

X-RAY: NASA/CXC/SAO/V. PAREKH, ET AL. & ESA/XMM-NEWTON; RADIO: NCRA/GMRTNorthernCollision siteAbell 2384 is a busy place. It’s causing it to warp as a result.
ASTRONOMY: ROEN KELLYgalaxy clusterthe site of a cosmic collisionAstronomers traced the nearly
Radio jet that occurred several hundred 2-million-light-year-long jet via
X-ray bridge Supermassive million years ago, when two its radio emission (pink). The
black hole massive clusters of galaxies jet is brighter and extends
Southern Radio jet passed through each other. farther to the south than the
galaxy cluster The event pulled hot gas north, where astronomers
from within the clusters into a believe it’s being slowed
massive bridge that still links down by the material within
the two, stretching roughly the bridge as the jet smacks
3 million light-years long and into it. Studying the sites of
containing 6 trillion Suns’ such cosmic collisions allows
worth of gas. Because it’s so astronomers to learn more
hot, the gas glows brightly in about how galaxy clusters
X-rays (blue). But there’s still and the material within them
more to the story: A jet from interact, which includes not
the supermassive black hole only galaxies and hot gas,
in one of the clusters’ galaxies but also plentiful dark matter.
is blasting into the bridge,
— A.K.

JUPITER SATURN URANUS NEPTUNE

<0.1% 0.5% 2.3% 1.5%
10.2% Other (ammonia, Methane <0.1% Methane <0.1% Other
methane, ethane, 3.2% Other (ammonia, Other (ammonia,
Helium water) methane, ethane, 15.2% water) 19% (ammonia,
Helium water) ethane, water)
Helium Helium

89.8% 96.3% 82.5% 80%

Hydrogen Hydrogen Hydrogen Hydrogen

GIANT PLANET ATMOSPHERES FAST FACT

GAS UP. Our newborn Sun was surrounded by a disk of gas and dust left over after it ignited. This The upper atmosphere of
material later formed the planets in our solar system. The giant planets — Jupiter, Saturn, Uranus, and Neptune is one of the solar
Neptune — pulled in gas directly from the disk to form their atmospheres. Unlike the inner planets, which system’s coldest places,
lost their early atmospheres and formed secondary ones later on, the outer planets are still swaddled where temperatures hover
largely in hydrogen and helium, the two most common elements in the Sun and the nebula from which at –361 degrees Fahrenheit
it formed. (Due to uncertainties, some of these measurements add up to more than 100 percent.) —A.K.
(–218 degrees Celsius).

12 ASTRONOMY • SEPTEMBER 2020

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WWW.ASTRONOMY.COM 13

STRANGE UNIVERSE

Lunar storyline

The Moon is a history book just waiting to be read.

On the list of sub- Mare Nectaris, that Sea of Nectar, is one of the Moon’s

jects you can explore oldest visible features, at about 3.9 billion years. Its for-

with your telescope, mation marks the beginning of the Nectarian period.

history probably doesn’t Anything earlier comprises the pre-Nectarian. At the

appear at the top. other end of the chronological yardstick is Mare

At least, not usually. Most Imbrium, the biggest round blotch at the top, or north

deep-space objects look fro- part, of the Moon. It formed after most other maria,

zen, as if the cosmos is being making it younger, and dates from around 3.8 billion

run as a wax museum. If only years ago. That timestamp thus marks the end of the

someone could snap one pic- Nectarian and the beginning of the Imbrian period.

ture of the Orion Nebula every Picture each mare as an enormous meteorite impact

thousand years and thereby creating a vast depression that soon filled with hot, dark,

create a movie with sped-up iron-rich magma. And sometimes the bowl didn’t quite

motion. What grandeur would get filled: Mare Imbrium’s center is the lowest spot on

materialize! We’d then see the Moon. But then things quieted down and no blotches

swirling animated eddies and were made in the past 3 billion years. So, craters now

intense blue stars popping into carry our story forward. Lava covered up many of the

view like holiday lights. oldest ones. And, logically, all craters seen on top of the

Instead, most of our eye- dark mare basins are automatically newer.

piece vistas are as unchanging There are other clues to crater ages, too. The bottom

as the vacation slides my uncle (southern) part of the Moon is the highlands, where lava

Copernicus, a young sadistically made us watch on never flowed. It’s old terrain, and its craters have a weath-
lunar crater, as
sketched in pencil repeat. A static scene is not a storyline. But there are ered look, with worn-down rims and intrusions by
by the author at the
eyepiece of his 60mm exceptions. One of them offers a secret narrative avail- smaller, newer craters. Erosion came from tiny meteorites
refractor when he
was 15 years old. able to anyone who knows how to read it. It’s the Moon! hammering the surface for eons. But keep looking, and

BOB BERMAN Not long ago, lunar features looked dramatic and you’ll see craters, like Eratosthenes, which do have sharp

BY BOB BERMAN cool but told me no story beyond the obvious rims and an absence of superposed craterlets.
Bob’s newest book,
Earth-Shattering fact that our nearest neighbor has been badly A static If a fresh, sharp crater is not bright and white
(Little, Brown and mistreated. Then, 60 years after I made the around Full Moon, and doesn’t have white rays
Company, 2019),
explores the greatest pencil sketch shown on this page, I finally scene is not streaming away from it, it dates from 1 billion
cataclysms that have awakened to the storyline hovering in the a storyline. to 3 billion years ago — the Eratosthenian
shaken the universe. eyepiece. It’s an epic narrative that, thanks to period. (Try saying it with emphasis on the
lunar research such as that done by the But fourth, stretched-out theen syllable.)
Apollo program, lets us create an actual there are
Finally let’s turn to the newest chapter, the

chronicle of what the Moon has experienced exceptions. last billion years. These youngest craters have
since its birth. And because the violence that sharp rims, no superposed craterlets, bright

happened there was unfolding here at the interiors around Full Moon, and straight white

same time, it’s a history that helps make sense of our lines radiating from them. Copernicus is typical — a

own planet’s impacts and mass extinctions. meteorite created it just 0.8 billion years ago. So is Tycho,

Start with the Moon’s dark blotches, the only features whose lengthy white rays dominate the Full Moon. It’s

easily seen with the unaided eye. Each solidified lava only 100 million to 200 million years old. This most

flow, called a mare (MAR-ay), started with an enor- recent era — still in progress — is the Copernican period.

mous impact around 4 billion years ago. They’re vivid Pre-Nectarian, Nectarian, Imbrian, Eratosthenian,

through binoculars. If you observe from the Northern and Copernican. Now you know all five lunar geologi-

Hemisphere, look for a small, round, isolated mare near cal periods and how to recognize them when your

the Moon’s right limb. This is Mare Crisium, the Sea of telescope explores the Moon, chapter by nightly chapter,

Crises. To its left, two large blotches form a crude figure as a history book.

8, with the Sea of Tranquillity on the bottom and the

smoother-edged Sea of Serenity at the top. The small BROWSE THE “STRANGE UNIVERSE” ARCHIVE
mare beneath them is the Sea of Nectar. AT www.Astronomy.com/Berman

14 ASTRONOMY • SEPTEMBER 2020

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WWW.ASTRONOMY.COM 15

FOR YOUR CONSIDERATION When white blood cells start showing up to clean up
destroyed cells, they signal that there is a battle afoot by

releasing small proteins called cytokines. In response,

nearby blood vessels swell and release fluid into their

Learning the surroundings, causing inflammation.
Inflammation can help with infection, but in this case,

it backfires. All of that fluid dilutes surfactants even

hard way more, further robbing the lungs of their elasticity.
Increased pressure causes the tiny air sacks called alveoli
to collapse and also fill with fluid. Secondary infections

Next time, let’s listen to the scientists, OK? can make it worse.
Cytokines do more than trigger inflammation. High

levels of cytokines activate additional white blood cells,

A few months ago, a virus which release more cytokines, which activate more white

began circulating in blood cells, and so on. As the resulting cytokine storm

China’s Hubei Province. rages on, chemicals released by white blood cells to fight

The virus attacks lung cells that the virus destroy healthy tissue instead. The lungs become

produce surfactants, chemicals useless. Each breath takes more and more work, but brings

that reduce surface tension and in less and less oxygen. The patient is suffocating.

help make tissue pliable instead of If you get to this point, it’s unlikely that you are com-

rigid. From the beginning, epide- ing back. Deprived of oxygen and awash in the chemical

miologists understood that the maelstrom of the battle, organs throughout your body

virus had the potential to become a start shutting down.

global pandemic of unimaginable I deeply apologize to those readers who have seen this

scale. For some nations, the U.S. in agony themselves at close range. I almost didn’t write this

particular, there was ample warn- for fear of adding to your pain. But what happened needs

ing and plenty of time to act. to be laid bare, and there is no painless or pretty way to

The spikes that adorn But Wall Street doesn’t like that do that.
the outer surface of
the COVID-19-causing kind of news; talk of a pandemic might hurt the stock I suppose people can throw rhetoric, ideology and
coronavirus are clear
in this illustration. market. Sure, the bodies were piling up in China by the wishful, magical thinking at physical reality if that’s what
Under an electron
microscope, the thousands, but here, the powers that be and their cable they choose. But it’s as pointless and irresponsible as it is
spikes appear as a
halo, or “corona,” news lackeys spun COVID-19 as a “mild flu.” unforgivably stupid. If you try to fight
giving the virus its
name. CDC/ALISSA ECKERT, Because no one on the planet was If this physical reality with demagoguery and
immune, the virus responsible for COVID- propaganda, reality spits in your face and
MS; DAN HIGGINS, MAMS
19 found fertile ground in every new person pandemic has then stomps your hide into the ground
it encountered. As humans carried armadas a silver lining, every time.
of these little molecular machines around it is that we
the world on airliners and cruise ships, sci- might start to Such is the lesson of COVID-19.
entists spoke of “when” — not “if” — the show physical If you’ve followed my writing for
virus would arrive in the U.S. Astronomy, you know that I frequently talk
about the nature of scientific knowledge

But for powers that be, that didn’t com- reality the and the ways it is under attack. This — what
pute: “COVID-19 is a hoax!” respect it we are going through right now with
demands. COVID-19 — this is why that matters!
When cases of COVID-19 started show-
ing up on American shores, scientists and If this pandemic has a silver lining, it is

public health officials sent up every warning that we might start to show physical reality

flare they could. If we acted immediately, the respect it demands, and accept that real-

they said, we could still save untold thousands of lives. ity doesn’t give a rodent’s hindquarters about beliefs,

But those who might have prevented the carnage had ideology, convenience, or anything of the sort.

long ago decided that scientists were just a bunch of party And maybe, just maybe, we will start listening when

poopers who say things nobody wants to hear. A year scientists say there’s a problem.

before, they had fired the very scientists who might have At the moment, the reality that can’t be tweeted away

stopped COVID-19 in its tracks. Why should they start is spelled C-O-V-I-D. But there are other deadly realities,

listening now? Instead of taking action, they proclaimed, some of which are easier to ignore because they move

“One day, it’s like a miracle, it will disappear.” more slowly. One such is spelled C-L-I-M-A-T-E. If we

BY JEFF HESTER There is no miracle to be had. Viruses aren’t evil. don’t start believing scientists about that reality very
Jeff is a keynote Viruses aren’t even alive. They are just little packages of soon, the consequences will make COVID-19 look like
speaker, coach, RNA that tell a cell to stop what it’s doing and start mak- a warm spring day in the park.
and astrophysicist. ing more virus particles instead.
Follow his thoughts BROWSE THE “FOR YOUR CONSIDERATION”
at jeff-hester.com Interestingly, COVID-19’s real danger comes less from ARCHIVE AT www.Astronomy.com/Hester
the virus itself than from our own immune systems.

16 ASTRONOMY • SEPTEMBER 2020

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Is there life out there?
Researchers are pursuing
three paths to find out.

BY ROBERT NAEYE

lifeHouiwn tnwheeıv’ll feinrd se

18 ASTRONOMY • SEPTEMBER 2020

Tau Ceti f is a rocky
super-Earth orbiting a
Sun-like star. This artist’s
concept shows the view
from the potentially
habitable planet, including
two hypothetical moons
and a sky full of comets,
which have created a
massive dust belt around
the star. In our solar system,
impacts carried water to
the inner planets; the same
process may be happening
to Tau Ceti f. RON MILLER

WWW.ASTRONOMY.COM 19

MANY OF SCIENCE’S GREATEST direct observation, no longer suffering variety of forms — from acid-loving
the ignominy of being a science whose bacteria to kangaroos — that attempts to
mmysteries surround questions of how subject matter is not proven to exist. define it inevitably leave out entire class-
much life is in the universe and what es of critters. Plus, what’s out there in
forms it might take. And, given the pop- Although there’s no official competi- the greater universe might be even more
ularity of sci-fi movies depicting alien tion or roadmap, scientists across multiple extreme than anything we can imagine
creatures, interest is clearly not limited disciplines are pursuing three general from our limited geocentric perspective.
to researchers alone. pathways for detecting extraterrestrial
For millennia, great minds have life. First, they’re hunting for life in the In their recent book, Imagined Life,
contemplated the origin, nature, and prev- solar system using robotic or sample planetary scientist Michael Summers and
alence of extraterrestrial life. But despite return missions. Second, they’re searching physicist James Trefil identify three kinds
the impressive brainpower brought to for compelling evidence of life-bearing of life: life like us, life not like us, and life
bear, the frustrating reality is that we worlds by probing exoplanet atmospheres. really not like us. The first centers around
really don’t know who or what is out Third, they’re chasing the ultimate jack- all terrestrial biota: life based on organic
there. Scientists have good reason to think pot: evidence of intelligent life through (carbon-based) chemistry using liquid
billions of inhabited worlds are sprinkled purposefully seeking out alien signals or water as a solvent. The second involves
throughout our galaxy in a universe teem- receiving them by serendipity. chemistry based on elements other than
ing with life — perhaps even technologi- carbon, such as silicon. The third is the
cally advanced life. But maybe the genesis Life on Mars wild card: life-forms so far outside our
and long-term survival of life on Earth conceptual horizon that we might not
has been a once-in-a-galaxy fluke. Maybe A major challenge in any search for life is even recognize them as being alive.
we’re living on one of the precious few defining exactly what we’re looking for.
miracle worlds where life evolved to stag- Terrestrial life assumes such a dizzying Scientists are familiar with the first
gering levels of diversity and complexity. type of life, so they have some idea of what
Scientists can debate these questions they’re looking for. Better yet, solar system
until they’re blue in the face. And they exploration over the past five decades has
have. But the only way to find out the greatly increased the inventory of rela-
definitive answer is to observe and tively nearby candidate worlds that might
explore. And here’s the good news: Never harbor some form of familiar biology.
before have scientists had so many tools
at their disposal. Excitement is palpable Mars remains the most compelling
that in 10 to 20 years, astrobiology could target due to its proximity and over-
make the critical leap from theory to whelming evidence that liquid water once
covered much of its surface. Claims of
NASA’s Viking 1 lander used its sampling arm to scoop several troughs in the rust-colored martian regolith. martian life date back more than a cen-
The lander placed the material into three experiments designed to examine its composition and look for tury, to Percival Lowell’s popularization
signs of biological activity. NASA/ROEL VAN DER HOORN (WIKIMEDIA COMMONS) of its fabled canals. In 1976, the Viking
Labeled Release experiments returned
positive test results for metabolizing
microbes, a result that most (but not all)
scientists attribute to active soil chemis-
try. Two decades later, a NASA and

20 ASTRONOMY • SEPTEMBER 2020

Although it shines brightly in this
Hubble Space Telescope image,
Proxima Centauri (below) is a red dwarf
invisible to the naked eye. It is, however,
the Sun’s closest neighbor, and hosts an
exoplanet — shown in the artist’s concept
at left — within its habitable zone.

ESA/HUBBLE & NASA; ESO/M. KORNMESSER

Stanford University team led by David NASA/JPL/MALIN SPACE SCIENCE SYSTEMS
McKay reported evidence for ancient
microorganisms in Mars meteorite ALH NASA’s Mars Global Surveyor spotted this 8-mile-wide (13 km) fanlike feature, typical of river deltas on Earth,
84001, a claim that remains in dispute. inside an unnamed Mars crater. It is just one of many such features on the Red Planet, and one piece of an
extensive body of evidence hinting that liquid water flowed on the planet’s surface over a long period of
Since Viking, NASA’s Mars explora- time. A similar feature appears in Jezero Crater, the landing site of the Mars 2020 Perseverance rover.
tion strategy has been to “follow the
water.” A series of orbiters and rovers has MARS’ METHANE CYCLE
conclusively demonstrated that, billions
of years ago, Mars had conditions that Researchers used SPRING SUMMER AUTUMN WINTER 0.8 Methane (CH4) parts per billion
would enable the development of life as Curiosity to track faint
we know it. But did the spark of life ever wisps of methane in Mars year 32 0.6
grace the Red Planet, and could the Mars’ atmosphere Mars year 33 0.4
world still host life today? over three years, Mars year 34
while also tallying its
Intriguingly, NASA’s Mars rover possible sources and Surface ASTRONOMY: ROEN KELLY, AFTER NASA/JPL-CALTECH
Curiosity, along with various telescopes sinks — including release
and orbiters, have detected faint wisps microbial production
of methane (CH4) in the planet’s atmo- of the gas. Their Surface uptake Abiotic or Microbial 0.2
sphere at levels up to a few parts per results show that Fissures production production Surface uptake
billion. About 95 percent of Earth’s methane levels spike
atmospheric methane is biogenic, during the peak of Microseepage Faults
raising hopes that there could be summer in the Cracks
methane-producing martian microbes northern hemisphere,
(methanogens) eking out an existence then wane during
around underground aquifers. But the other parts of the year.
detections remain controversial. For
example, the European Space Agency’s designed to detect extant life, but its will drill down several inches to collect
(ESA) Trace Gas Orbiter has detected imaging systems could theoretically dozens of rock and regolith samples for
nary a whiff. And even if the methane is reveal macroscopic life-forms or fossils. later return to Earth. NASA and ESA are
real, scientists have identified numerous collaborating on a $7 billion sample
geological and chemical processes that “Most of the professional community return mission that could return martian
can produce the gas without any contri- thinks that’s extremely unlikely, but not material by 2031. NASA will provide the
butions from life. impossible,” says team member Jim Bell lander and ascent vehicle to collect the
of Arizona State University. Bell thinks samples — left by Perseverance — and
The next Mars rover, Perseverance, it’s more likely that Perseverance’s pow- launch them into Mars orbit. ESA will
is planned to arrive in Jezero Crater, an erful instruments could detect complex contribute the spacecraft that will snatch
ancient lakebed, on February 18, 2021. It’s molecules associated with biological pro- the samples and transport them to Earth.
not carrying any instruments specifically cesses. More important, Perseverance

WWW.ASTRONOMY.COM 21

About as wide as the state of Arizona, Saturn’s icy moon Enceladus is one of several locations in our solar In Bell’s view, Perseverance’s mission
system where life might currently exist. Plumes of water have been seen spouting from fractures in the won’t be complete until those samples are
moon’s surface, indicating a body of liquid water beneath. NASA/JPL/SPACE SCIENCE INSTITUTE brought to Earth. Studying these samples
in a lab, he says, “will allow us to do life-
Jupiter’s moon Europa is a prime place to search for life in the solar system. Slightly smaller than Earth’s detection experiments at much higher
Moon, it is covered in an icy shell that lies above what researchers think is a salty liquid ocean. NASA/JPL/DLR fidelity than we could do on Mars.”

22 ASTRONOMY • SEPTEMBER 2020 ESA is also sending a Mars rover,
named Rosalind Franklin after the great
British scientist. The rover was originally
scheduled to launch this July, but has been
delayed until 2022 because of problems
while testing its parachute. Rosalind
Franklin will drill about 6.5 feet (2 meters)
below the surface, which is ravaged by
solar ultraviolet radiation that quickly
breaks down organic molecules. The rover
carries cameras and spectrometers capable
of detecting chemical and mineral signa-
tures of past or current life.

Finding life on Mars would be a mon-
umental breakthrough. If that life turns
out to be similar to that on Earth, it
would imply that either it was transported
inside a meteoroid from one planet to
another or that both worlds were seeded
from a common outside source. It would
be amazing to learn that planets can
exchange life and that terrestrial life
might have originated on Mars. But if
martian life has a different chemical
makeup, is not based on cells, or utilizes
information-encoding molecules other
than DNA and RNA, it would strongly
suggest that life originated independently
on two different worlds and took two dif-
ferent paths. This would then imply that
life is widespread in the galaxy.

NASA Chief Scientist James Green says
that even if Mars’ life came together
exactly like it did here on Earth, its current
form would be the result of many unique
environmental factors. “When we bring it
[martian life] back, we’re going to see that
evolutionary difference. That’s what makes
the whole field very exciting,” he says.

Ice worlds of the
outer solar system

Humanity has long focused on Mars
as the most promising abode of life.
But interplanetary probes have greatly
expanded the list to include worlds
that might contain liquid-water oceans
beneath their icy facades. Calculations by
the late University of Arizona planetary
scientist Adam Showman suggest there
could be as many as 12 to 15 solar system
worlds that harbor subsurface oceans.

These include planets, moons, and even ENCELADUS’ SUBSURFACE OCEAN
Kuiper Belt objects such as Pluto.
Surface jets
“Most of those are speculative possi-
bilities at this stage. The ones with real Surface (35.1kmmi)les
evidence are Europa, Ganymede (tenta- Ice
tive), Callisto, Enceladus, and Titan,” said
Showman, who tragically passed away Ocean (4605.4kmm)iles
during the reporting of this article.
Rocky Hot spots
Like other scientists, Showman identi- interior on seafloor
fied Europa and Enceladus as the most
promising candidates. Both of these Hydrothermal circulation
moons are tidally heated through gravita-
tional interactions with their host planets UNDERNEATH EUROPA’S ICE
(Jupiter and Saturn, respectively) and
their nearby fellow moons. In both cases, Plume
models of these moons’ interior structures
indicate that their oceans are sandwiched Surface
between an icy layer above and a rocky
layer below. “This opens the possibility of Ice 6 miles
water-rock interactions that could intro-
duce biologically interesting chemicals (10 km)
into the ocean,” said Showman.
Liquid Ocean 62 miles
Life on such worlds could resemble ocean currents (100 km)
the myriad bacteria, tube worms, clams,
and other life-forms that thrive around Hydrothermal
hydrothermal vents on Earth’s ocean vents
floors. These creatures draw their suste-
nance from nutrients expelled by the Rocky
vents, not from sunlight. interior

The icy shells of Europa and Icy moons such as Europa and Enceladus likely hide liquid oceans beneath their outer shells. And if those
Enceladus are relatively thin. In 2005, oceans are in contact with hydrothermal vents on the seafloor below, such features could form warm,
NASA’s Cassini spacecraft returned spec- nutrient-rich pockets where life could thrive — as it does in similar regions on Earth. ASTRONOMY: ROEN KELLY
tacular images of jets venting salty-ice
particles into space from Enceladus’ evidence that life is possible. And the only satellite with a thick atmosphere,
south polar region. And in 2014, the results could, in turn, motivate the which is similar to Earth’s in terms of its
Hubble Space Telescope first spotted launch of a lander to sample surface surface pressure and nitrogen-dominated
evidence of plumes emanating from material or even drill or melt through composition. Better yet, Cassini radar
Europa’s south pole. In 2019, researchers the ice to explore the ocean directly. found dozens of lakes on Titan filled with
released the best evidence yet that liquid methane and ethane (C2H6). It is a
Europa’s plumes contain water. All these Finding life on a solar system ocean world oozing with organic compounds.
discoveries suggest that biogenic materi- world would be an astrobiologist’s dream
als could exist at or near the surface. come true. It would imply a universe At –290 degrees Fahrenheit (–179
brimming with life — for, as Summers degrees Celsius), Titan’s surface is prob-
NASA’s planned Europa Clipper says, “Ocean worlds are probably com- ably too cold for life as we know it. But
mission will fly by its namesake moon monplace in the galaxy.” Dragonfly’s suite of instruments will
44 times starting in the early 2030s. study the moon’s chemistry, perhaps
Scientists expect it to confirm the exis- NASA is also planning to fly a robotic finding clues to how life got started on
tence of the world’s ocean, measure the rotorcraft named Dragonfly on Saturn’s Earth. It might even get really lucky by
ice shell’s thickness, and help researchers large moon Titan in 2034. Titan is the
ascertain the moon’s biological potential.
Green says that the spacecraft will be
able to sample material in Europa’s
plumes for possible evidence of organics
and other indicators of life. This mate-
rial, he says, might be coming from
hydrothermal vents at the bottom of its
global subsurface ocean. The spacecraft
won’t be detecting actual life-forms, but
it could conceivably provide compelling

WWW.ASTRONOMY.COM 23

Instruments such as the
New Mexico Exoplanet
Spectroscopic Survey
Instrument (NESSI) on
Palomar’s Hale Telescope are
already probing exoplanet
atmospheres to help scientists
learn about their chemistry.
Although NESSI, pictured here
alongside Jet Propulsion
Laboratory astronomer Mark
Swain, is designed to focus on
gas giant planets, researchers
say that in the future it may
peer at Earth-like exoplanets
as well. NASA/JPL-CALTECH

detecting signs of life as we don’t know it. even atmospheric composition of many monoxide (CO), iron, magnesium,
“Titan is a compelling target because it of these worlds. sodium, potassium, and even the rare
really would be life 2.0 — a very different elements scandium and yttrium.
basis for biology,” says University of For now, the best information on
Arizona astronomer Chris Impey. exoplanet atmospheres comes from One particularly interesting planet is
transit spectroscopy. In short, when a K2-18 b, a mini-Neptune orbiting within
Exoplanet atmospheres planet crosses, or transits, the face of the habitable zone of its red dwarf host
its host star as seen from Earth’s line star. (The habitable zone is the region
Thanks to revolutionary advances in of sight, starlight passes through the around a star where planets with an
the discovery and characterization of planet’s upper atmosphere and interacts atmosphere have the right temperature
exoplanets over the past 20 years, it is with the chemicals there. By comparing to support liquid water on their surfaces.)
now conceivable that scientists could spectra taken during a transit to observa- Using Hubble, two research teams have
find the first strong evidence for extra- tions of the star alone when the planet is found significant amounts of water
terrestrial life on a planet outside the not transiting, astronomers can extract vapor in its hydrogen- and helium-rich
solar system. information about the planet’s atmo- atmosphere. The water probably con-
spheric chemistry. denses into clouds that might produce
As of late June, astronomers had dis- rain. It remains unknown whether
covered 4,274 planets outside our solar Make no mistake, these are exceed- K2-18 b has a rocky or watery surface
system. And thanks to space telescopes ingly difficult and time-consuming that would be conducive to life.
such as Hubble and Kepler, as well as observations. But several teams have
numerous ground-based instruments, used this technique to reveal atmospheric “It is really the first time that we have
we have surprisingly detailed informa- constituents on a few dozen exoplanets, probed the conditions on a habitable
tion about the size, mass, density, and finding chemicals such as water, carbon zone planet,” says Björn Benneke of the

PEERING INTO EXOPLANET ATMOSPHERES 1.2

Astronomers can learn a lot CO2 H2O CO2 CO
about a planet when it transits, 1.0
or passes across, the face of its
host star. In some cases, Brightness 0.8
researchers can use a Flux planet/Flux star (10-3)
technique called transmission 0.6
spectroscopy. As starlight
filters through the planet’s 0.4
upper atmosphere, molecules
such as carbon monoxide (CO), 0.2
(cHa2rObo) nabdsiooxrbidteh(eClOig2h),t.aAnfdtewr ater
removing the star’s contribution 0.0
from the signal we receive,
astronomers are left with Time –0.2 1.6 1.8 2.0 2.2 2.4
information about the chemical
makeup of the planet’s Wavelength (micron)
atmosphere. ASTRONOMY: ROEN KELLY

24 ASTRONOMY • SEPTEMBER 2020

Université de Montréal in Canada, leader THE HABITABLE ZONE
of one of the two teams that detected
water vapor. Hotter stars

Current telescopes and instruments Sun-like stars
lack the resolution and sensitivity to
reveal combinations of chemicals indica- Cooler stars
tive of life on temperate terrestrial plan-
ets, such as free oxygen (oxygen such as The habitable zone (shown in green) is the region around a star within which water can exist as a
O2 and O3 that is not bound to other liquid on the surface of an orbiting planet. This region is closer in for smaller, cooler stars and farther
elements), carbon dioxide, and methane. out for larger, hotter stars. NASA/KEPLER MISSION/DANA BERRY
Scientists are particularly interested in
finding evidence for “disequilibrium three of which orbit within the habitable tool astronomers use to block a star’s light
chemistry” — a blend of compounds that zone. NASA’s Transiting Exoplanet and probe the area around a star for
should not exist together in a planetary Survey Satellite mission is currently find- fainter objects. This will enable research-
atmosphere without the existence of life. ing additional targets for follow-up study, ers to study a planet without its feeble
For example, without photosynthetic as will ESA’s PLAnetary Transits and light getting lost in the star’s intense glare.
life to constantly replenish the supply, Oscillations of stars, or PLATO, mission
oxygen and methane would not coexist when it launches in 2026. And ELF could do much more than
in Earth’s atmosphere. But, as Stephanie spectroscopy. “We’ve demonstrated how
Olson of the University of Chicago notes, Astronomers are also planning new ELF can make images of the surfaces of
“Photosynthesis could look very different techniques for observing the much larger exoplanets within about 30 light-years of
on other planets,” so the manifestation number of planets that don’t transit their the Sun,” says Kuhn. He adds that by
of disequilibrium chemistry would also parent stars. Probing a nontransiting watching a planet as it rotates on its axis,
look different. planet enables a telescope to see deeper ELF could even “get an image of the sur-
into the atmosphere and perhaps all the face and clouds.” The team is currently
Although the current generation way down to the surface. One of the first building a prototype telescope in the
of telescopes is not quite up to the task, targets of such studies is likely to be the Canary Islands, miniELF, to test some
the next generation might be. Once it roughly Earth-mass planet orbiting of the technologies.
launches and deploys (expected in 2021), within the habitable zone of Proxima
NASA’s James Webb Space Telescope Centauri, the closest star to the Sun at But even if astronomers find biosigna-
will conduct infrared observations of just 4.2 light-years away. tures in exoplanet atmospheres, it remains
relatively nearby transiting exoplanets, unclear whether the broader community
especially super-Earths orbiting red One international team is developing will accept them as evidence of extrater-
dwarfs. But the telescope’s best bet will the ExoLife Finder (ELF), which will be a restrial life. “I don’t think any detection
be finding planets with primitive life 20- to 30-meter telescope (depending on outside of the solar system is going to give
resembling that on early Earth, as most funding) optimized for near-infrared 100 percent confidence that there is life
biosignatures present in exoplanetary observations of relatively nearby exoplan- on another planet. I think there will
atmospheres might not show up clearly ets. According to team member Jeff Kuhn always be some chance that it isn’t life,
due to the relatively modest spectral of the University of Hawai‘i, ELF will
resolution of its instruments. employ a combination of advanced tech-
nologies to control the incoming light
The scientific community has higher waves from the star so that the primary
hopes for upcoming ground-based obser- mirror behaves like a coronagraph — a
vatories such as the 24.5-meter Giant
Magellan Telescope, the Thirty Meter Titan is the only solar system satellite with
Telescope, and the European Southern a thick atmosphere and surface liquid
Observatory’s 39-meter Extremely Large lakes and seas, which appear as dark,
Telescope, all of which could see first irregularly shaped regions in this Cassini
light by the end of the decade. These image. The largest sea, at the far right of
behemoths will be equipped with the grouping shown here, is Kraken
high-resolution spectrographs and Mare. To its upper left is Ligeia Mare
other high-tech instruments to look for and to its immediate left is Punga
biosignatures at optical and near-infrared Mare, the moon’s second- and third-
wavelengths. They will be particularly largest seas, respectively. NASA/JPL-CALTECH/
well-suited to detect free oxygen, a clear
hallmark of life on our world. UNIVERSITY OF ARIZONA/UNIVERSITY OF IDAHO

Probably the juiciest target will be
TRAPPIST-1, a red dwarf 40 light-years
away. This diminutive star hosts seven
transiting terrestrial-sized planets, at least

THE DRAKE
EQUATION

In 1961, astronomer Frank N = R* x f nx x fl x fi x fx L
Drake developed an pe c
equation to estimate
the number of alien The number of The average The fraction The average The fraction The fraction The fraction of The average
civilizations in the civilizations in rate of of stars that number of of those of planets
Milky Way capable of the galaxy with host planets those planets where life planets with length of time
transmitting interstellar which we could galactic star that could planets where becomes
signals (which humanity communicate formation support life life actually intelligent intelligent life such a civilization
could receive through develops
SETI searches). Although that develops continues
there is no agreed-upon
value for this number, interstellar sending
Drake has suggested
it could be as large as communication messages
10,000. ASTRONOMY: ROEN KELLY

some low-probability way that signature (HabEx) mission would launch a 4-meter Seeking alien signals
could be produced by nonlife processes,” telescope around 2035 to conduct spec-
says astronomer Johanna Teske of the troscopy and imaging of terrestrial-size And then there’s the ultimate jackpot,
Carnegie Institution for Science. planets in habitable zones. NASA’s Large which could come from the Search for
UV/Optical/IR Surveyor (LUVOIR) Extraterrestrial Life, or SETI. Finding
But she also points out that if astrono- mission would launch an 8- or 15-meter direct evidence of an alien civilization
mers can find multiple signatures that telescope with a coronagraph in the late would irrevocably change the course of
are difficult to produce via nonbiological 2030s, for detailed characterization of history in ways nobody can predict. It
processes, or if they find biosignatures exoplanet atmospheres and surfaces. As would also show that intelligent life can
on multiple planets, that would be University of California, Santa Cruz, survive technological adolescence.
“incredibly convincing.” astronomer Joshua Krissansen-Totton
points out, LUVOIR could find life on Astronomers continue to scan the skies
NASA and ESA are considering space planets with alternative biospheres, with using large radio telescopes in the hopes
telescopes dedicated to exoplanet observa- “weird combinations of atmospheric of catching signals from advanced civili-
tions. ESA’s Atmospheric Remote-sensing gases, hazes, or surface pigments.” zations. In February, the Breakthrough
Infrared Exoplanet Large-survey mission, Listen project publicly released nearly
ARIEL, is scheduled for a 2028 launch “I think, ultimately, something like 2 petabytes of observational data, includ-
and will probe the atmospheres of 1,000 LUVOIR or HabEx is going to give us the ing scans of the galactic center and a brief
transiting planets with an oval-shaped, best chances of finding biosignatures survey of 20 nearby stars whose civiliza-
1.1-by-0.7-meter primary mirror. because of the number of planets these tions would see Earth transiting the Sun.
telescopes plan to survey,” says Teske.
Further down the road, NASA’s No definitive alien radio signals have
Habitable Exoplanet Observatory ever turned up — a situation that skeptics
call “The Great Silence.” But SETI astron-
The Allen Telescope Array in California’s Cascade Mountains is dedicated to the search for intelligent omers rightly counter that the radio
extraterrestrial life. Its 42 dishes scan the skies for radio signals of alien origin. SETH SHOSTAK/SETI INSTITUTE surveys conducted to date have barely
scratched the surface in terms of stars in
the sky and radio frequencies searched.
The same goes for current optical SETI
attempts to detect alien laser signals.

Traditional SETI using radio tele-
scopes will progress in the coming years
as more telescopes and new capabilities
are brought to bear. But creative astrono-
mers are looking to expand these
approaches. Breakthrough Listen princi-
pal investigator Andrew Siemion says, “As
SETI astronomers, we conduct our own
experiments. But we also try to do our
best to convince our colleagues to be on
the lookout for things they don’t expect
and not discount the possibility that their
experiments might uncover evidence of
an extraterrestrial intelligence.”

26 ASTRONOMY • SEPTEMBER 2020

EARTH’S BIOSIGNATURES

EARLY EARTH

Methane Water
vapor

Carbon Nitrogen
dioxide

PRESENT-DAY EARTH

Oxygen

Carbon Nitrogen
dioxide

Not all of SETI’s efforts are passive. In 1974, Frank Drake used the Arecibo Observatory in Puerto Rico to
deliberately transmit a simple pictorial message in the direction of the globular cluster M13, which lies
25,000 light-years away. NAIC ARECIBO OBSERVATORY, A FACILITY OF THE NSF

Water Ozone advanced civilizations could use to har- sign from, for instance, a free-floating
vapor vest the energy of their host stars. These solar system artifact,” says Wright.
structures would presumably show up in
Methane infrared observations. Detecting technologically capable
extraterrestrial life could help answer the
Life’s very presence on Earth affects the Wright also points out that we have profound question of whether life has a
composition of our planet’s atmosphere. On barely begun to explore our own solar post-biological future, meaning com-
present-day Earth, gases such as oxygen, system. Other than Earth, the Moon is puter or machine intelligence. “This is
ozone, nitrogen, methane, water, and carbon the only world whose entire surface has all conjecture, of course, but some sort
dioxide hint that our planet is teeming with life. been imaged at high resolution. And of biological-machine hybrid seems a
But planets with life in earlier stages of evolution right here on Earth, ground-penetrating possible outcome for humanity. Or one
might appear more like early Earth, whose radar has only just recently revealed could imagine self-replicating probes
atmosphere was composed of methane, carbon ancient cities whose remains were hiding being sent out to prepare the ground for
dioxide, nitrogen, and water vapor. ASTRONOMY: ROEN just a few inches underground. us biological beings. And if we think that
lies in our future, then it could already
KELLY, AFTER NASA/WIKIMEDIA COMMONS/JOSHUA KRISSANSEN-TOTTON In 2019, Adam Frank of the University have occurred elsewhere,” says physicist
of Rochester and Gavin Schmidt of Stephen Webb of the University of
If ELF is built and observes according NASA’s Goddard Institute for Space Portsmouth in England.
to its specifications, it could potentially Studies published a peer-reviewed paper
detect signs of advanced aliens through on the Silurian Hypothesis. This is the Any detection of extraterrestrial life
technosignatures, such as thermal idea that an industrial civilization might — primitive or advanced, near or far —
images of cities on the nighttime hemi- have inhabited Earth millions of years will give humanity new insights into our
spheres of nearby exoplanets. Future ago. They concluded that erosion and place in the universe. Future discoveries
telescopes could potentially find exo- plate tectonics would have almost com- will tell us how common life-bearing
planet atmospheric constituents such as pletely erased all signs of such life from planets are, what types of planets can
chlorofluorocarbons, which could only the geological record. But even though in support which kind of life, and perhaps
be produced by artificial means. their paper they “strongly doubt” such a even whether radically different life-forms
past civilization ever existed, they sug- exist at all. Finding life elsewhere is one of
Penn State University astronomer gested ways that geologists could look for the holy grails of science, destined to give
Jason Wright has touted the idea that signs that it did, such as “deeper explora- us a new cosmic perspective.
astronomers should be on the lookout tion of elemental and compositional
for alien megastructures, large-scale anomalies” in ancient sediments. Robert Naeye wrote numerous articles
astro-engineering projects such as Dyson related to astrobiology as an Astronomy
spheres — vast swarms of spacecraft that Who knows what lies below the sur- staff editor from 1995 to 2000. Please visit
faces of our neighboring worlds or could his website: www.RobertNaeye.com. He
You can dig deeper into the factors be orbiting the Sun, unbeknownst to us? dedicates this article to Adam Showman.
that allow planets to support life at “We might detect either a signal or other
astronomy.com/magazine/2020/
09/planetsandlife

WWW.ASTRONOMY.COM 27

TitanWhat does
SMELL LIKE?

A bouquet of musky sweetness,
bitter almonds, gasoline, and
decomposing fish would likely fill
the air on Saturn’s largest satellite.

BY MORGAN L. CABLE

28 ASTRONOMY • SEPTEMBER 2020

Titan, Saturn’s largest
moon, is a dynamic world
with a dense atmosphere,
hydrocarbon lakes, and
lots of different smells.

RON MILLER

WWW.ASTRONOMY.COM 29

Titan is Saturn’s biggest Titan’s dense atmosphere (dark blue) extends hundreds of
moon. It is so big, in fact, miles above the world’s surface, as seen in this false-color
that it rivals Jupiter’s moon mosiac captured by the Cassini spacecraft. NASA/JPL-CALTECH/SPACE
Ganymede for the title of
the solar system’s largest SCIENCE INSTITUTE
satellite. If you were to strip
Titan bare by removing its insulated gas mask for oxygen, and maybe a heater
atmosphere, Ganymede is to prevent supercooled air from freezing your lungs.
slightly larger. But that’s OK; The suit wouldn’t even need to be completely air-
Titan is still so fascinating tight as long as oxygen remained circulating. It
that we can let Ganymede have that win. would be like the masks on airplanes: “Even though
your bag may not inflate, oxygen is still flowing.”
Yes, Titan has an atmosphere. And not a small,
inconsequential one, either: Titan’s atmosphere is This leads to an interesting thought experiment:
four times denser than Earth’s. And while you If an airtight spacesuit is not a necessity on Titan,
wouldn’t need a spacesuit to protect you against its then it might be possible to get a whiff of your sur-
pressure, you would need one heck of a parka. The roundings through your oxygen mask. In that case,
surface temperature on Titan is a frigid –290 degrees what would Titan smell like?
Fahrenheit (–179 degrees Celsius). For reference, the
coldest temperature ever recorded on Earth was An inventory of Titan’s smells
when the mercury dropped to only about –129 F
(–89 C) at Vostok Station in Antarctica in 1983. Thanks to the instruments aboard NASA’s Cassini
spacecraft, as well as those on the Huygens probe
So, an astronaut walking on the surface of Titan built by the European Space Agency (ESA), we have
would have to keep their skin covered to avoid rapid a fairly good understanding of the composition of
frostbite, but they wouldn’t need a super bulky Titan’s atmosphere. Therefore, we can hazard an
spacesuit like they would on the Moon or in the educated guess as to what the world may smell like.
vacuum of space. The astronaut would have to bring
their own oxygen, though, as Titan’s atmosphere is The two main ingredients of Titan’s atmosphere
devoid of it. Instead, Titan’s skies, like Earth’s, are — nitrogen and methane (CH4) — are both odorless.
The distinctive smell you might associate with meth-
If an airtight spacesuit is not a ane is actually another gas (methanethiol), which is
necessity on Titan, then it might be added to consumer methane for safety. Methanethiol
is often described as smelling like rotten eggs, and
possible to get a whiff of your the human nose can detect it down to the parts-per-
surroundings through your oxygen mask. billion level. By adding this putrid-smelling chemical
to methane and natural gas, we humans can quickly
mostly nitrogen, plus about 5 percent methane and detect and address potentially dangerous leaks.
a few other trace gases.
So, is there anything else in Titan’s atmosphere
For a trip to Titan, one could envision donning that would have a scent? A quick inventory of the
attire similar to that worn by researchers working
in Antarctica during the frigid winters: clothing
with multiple insulating layers, perhaps embedded
with futuristic NASA-quality heating elements;
thick gloves; hats and balaclavas; and extreme cold
vapor barrier boots (fondly referred to by Antarctic
researchers as bunny boots or Mickey Mouse boots).
The only necessary modification would be an

30 ASTRONOMY • SEPTEMBER 2020

Titan, though dwarfed WHAT’S IN TITAN’S ATMOSPHERE?
by the giant planet
Saturn (top), is one of With a dense atmosphere, hydrocarbon seas, and active
the largest moons in the geology, Titan is a prime candidate for future human
solar system. At about exploration. But although we know the main ingredients
1,600 miles (2,600 km) of Titan’s lower atmosphere, the secret scent of its skies
wide, the moon is about depends on a mysterious blend of trace gases.
half the diameter of
Earth’s Moon. But unlike Nitrogen (N2): 94.2% by mole fraction
our satellite, Titan sports Methane (CH4): 5.6%
a very dense and hazy Hydrogen (H2): 0.1%
atmosphere (bottom). Trace gases: Less than 0.1% (including carbon monoxide
[CO], ethane [C2H6], ethylene [C2H4], acetylene [C2H2]],
BOTH IMAGES: NASA/JPL-CALTECH/ hydrogen cyanide [HCN], ammonia [NH3], propyne [C3H4],
cyanoacetylene [C3HN], benzene [C6H6], acetonitrile
SPACE SCIENCE INSTITUTE [CH3CN], methylamine [CH3NH2], and more)

WWW.ASTRONOMY.COM 31

REPLICATING TITAN’S SKIES

Titan aerosol

Laboratory aerosol

Low-frequency vibrations

In the lab, researchers attempted Vibrating aerosol molecules
to re-create the complex spectral
signature that Cassini detected in Titan’s hydrocarbon seas slosh against the alien shoreline in this
Titan’s atmosphere at far-infrared artist’s illustration. Such mixing could help instigate the formation
(low-frequency) wavelengths. of increasingly complex molecules on the moon. MICHAEL CARROLL
Although they couldn’t get a
perfect match, they got closest
when they included complex
molecules called polycyclic
aromatic nitrogen heterocycles
(PANHs), seen at right.

NASA/GSFC/JPL-CALTECH

Carbon Nitrogen Hydrogen is propyne, also known as methylacetylene, which is
used both for welding and as rocket fuel.
measured or predicted trace components of Titan’s
atmosphere includes molecular hydrogen (H2), Then there’s hydrogen cyanide, which is
carbon monoxide (CO), ethane (C2H6), ethylene described as having a bitter almond odor. As you
(C2H4), acetylene (C2H2), hydrogen cyanide (HCN), may know from spy flicks, this gas is also incredibly
ammonia (NH3), propyne (C3H4), cyanoacetylene poisonous. But the threshold for smelling hydrogen
(C3HN), benzene (C6H6), acetonitrile (CH3CN), and cyanide is less than 5 parts per million, while a
methylamine (CH3NH2). lethal dose is around 135 ppm. Although you
wouldn’t want to take a big whiff, you can smell it
Many of these have no odor, although some may and survive the experience. But please do not try.
seem to. Molecular hydrogen, carbon monoxide, and Roughly 25 percent of people cannot smell the gas,
ethane have no smell at all. But acetylene is a trick- due to a recessive genetic trait. So, you could easily
ster — it is odorless just like methane, but anyone breeze past smelling and into the afterlife.
who has used an acetylene torch might dispute this.
That’s because impurities in welding-grade acetylene TO EACH THEIR OWN SCENT
typically include phosphine, which can give it a
garliclike smell. But acetylene itself is not the culprit. Many people experience smells the same, but due to
cultural, environmental, and genetic factors, certain
Don’t fret, though — a few of Titan’s trace odors are unappealing to some and pleasant to others.
components do smell. Ethylene is the simplest Regardless, humans can’t perceive a scent unless its
alkene (two carbon atoms connected with a double concentration is above a certain odor threshold.
bond) and is a common plant hormone that influ- Below are the thresholds for common Titan scents.
ences fruit ripening and seed germination. In its
pure form, it is reported to have a sweet and musky Methylamine: 0.002 parts per million
scent. Similarly, acetonitrile, which is used in many Hydrogen cyanide: 0.6 to 5 ppm
chemical and organic synthesis reactions, has a Benzene: 4.7 ppm
sweet, etherlike odor. Rounding out the sweet smells Acetonitrile: 20 ppm
Ammonia: 47 ppm
Propyne: 100 ppm
Ethylene: 270 ppm

32 ASTRONOMY • SEPTEMBER 2020

When the Huygens probe
touched down on Titan, it
provided an unprecedented
look at a visually stunning
and surprisingly complex
moon. But if humans ever
reach the alien world, as
seen in this artist’s concept,
the first thing they would
notice probably wouldn’t be
the sights, but the smells.

MICHAEL CARROLL

HOW DOES SMELL WORK?

Smell is a wondrous sense. Not only does our sense of smell allow us to avoid
potentially rotten or poisonous food, but certain scents can often trigger power-
ful physiological responses. The latter attribute of our sense of smell stems from
the fact that our brain is wired to receive and process scents in a different way
than it does sights or sounds.

When enough of a smelly molecule binds with some of the tens of millions
of olfactory receptors in the back of the human nasal cavity — that is, when the
odor threshold is reached — a signal is sent into the brain via the olfactory tract.
Unlike olfactory signals, visual and auditory signals first get routed through a
central relay center before getting passed along to different parts of the brain.
But when we encounter a scent, however, the resulting olfactory signal
bypasses the brain’s central command, going directly through regions like the
amygdala (emotional processing), the thalamus (consciousness and alertness),
and the neocortex (cognition and motor control).

This is why smells can trigger immediate and very powerful responses, often
subconsciously. These response range from the unexpected recollection of a
long-forgotten memory to an unstoppable fight-or-flight reaction. — Jake Parks

RIGHT: After spending Cryothermal vents, (allowing them to easily turn into vapor). Only
seven years traveling to like those seen in this those will be able to float up through a gas mask
the Saturn system with artist’s concept, are one and into an astronaut’s nose.
Cassini, the Huygens way that smelly and
probe finally touched often noxious gases All of the odorous compounds we’ve listed so far
down on Titan on could be pumped into are frozen solid on Titan’s surface — meaning they
January 14, 2005, Titan’s atmosphere. won’t be very volatile — with the exception of one:
revealing a world rife ethylene. This hydrocarbon has a freezing point of
with hydrocarbon lakes RON MILLER –272 F (–169 C), which is just warmer than Titan’s
and signs of erosion. ESA/ surface temperature. And in some lab experiments,
There may be less-appetizing scents on Titan as scientists have seen it remain liquid down to –298 F
NASA/JPL/UNIVERSITY OF ARIZONA well. Ammonia has a very strong, characteristically (–183 C). The general thinking is that this is due to
pungent odor. It is commonly used in fertilizers and freezing point depression, where salts that have dis-
as the primary component of smelling salts, and solved in the liquid lower the mixture’s freezing
evokes the scent of particularly concentrated urine. point. (This is why snowplows salt the roads in
wintertime to prevent icing.) So, ethylene is the
Rounding out the list of pungent smells, we have most likely molecule to be volatile on Titan, which
methylamine — the precursor Walter White used would give the world a sweet and musky aroma.
to brew methamphetamine in the TV series
Breaking Bad. Although the smell was not a focus But that’s not the end of the story of Titan’s
in the show, it should have been, as it carries smells. We also need to factor in the sensitivity of
undertones of putrid fish. This chemical is a the human nose. Human odor thresholds are a real
byproduct of decomposing animal tissue and is
used by some organisms to generate energy via thing, and you can find them for
methanogenesis. all sorts of compounds. So even if
a substance is frozen solid, at least
Benzene, a compound found in a few molecules will escape into
crude oil and known to cause cancer, the vapor phase. And if the nose is
is also present in Titan’s atmosphere. sensitive enough to detect them,
Despite being carcinogenic, benzene the brain will register it as a smell.
has a sweet, aromatic, gasoline-like
odor that’s sometimes compared to In terms of odor thresholds,
the smell of solvents. three molecules stand out: hydro-
gen cyanide and benzene have
A whiff of Titan about the same odor threshold of
up to about 5 ppm. Meanwhile,
At this point, for Titan’s potential humans can detect methylamine
smells, we have: sweet, musky, bit- down to a scant 0.0021 ppm. By
ter, pungent, and rotten, with a few contrast, to smell ethylene would
other subtle notes thrown in. But require a concentration of around
remember, Titan’s surface is very a whopping 270 ppm. This brings
cold. This means that only small our gasoline, almond, and dead
molecules with weak intermolecular fish fragrances back into the mix.
interactions will have a low enough
boiling point to remain volatile

34 ASTRONOMY • SEPTEMBER 2020

Currently planned for
launch in April 2026,
Dragonfly is a cutting-
edge NASA mission
that will send a flying
rotorcraft to Titan.
Thanks to the moon’s
thick atmosphere and
weak gravity, Dragonfly
will be able to efficiently
explore the alien world.

RON MILLER

And finally, we must also consider that a human’s
ability to perceive an odor — called olfactory acuity
— is dependent on factors such as age, smoking
habits, gender, and nasal allergies. Plus, there are
genetic factors that come into play, like the previ-
ously mentioned inability of some to smell hydrogen
cyanide. Ultimately, the Eau de Titan will be affect-
ed by the individual astronaut’s olfactory setup.

So, what does Titan smell like? For the average
astronaut, my best guess would be: sweet and musky
(ethylene), with fishy overtones (methylamine), and a
hint of bitter almonds (hydrogen cyanide) accented
by gasoline (benzene).

But will we ever know for sure? That’s the plan.
NASA’s recently selected Dragonfly mission
aims to send a small rotorcraft to touch down on
Titan in 2034. The craft will then fly above the
surface to explore a larger swath of the saturnian
moon. During its mission, Dragonfly will utilize
its state-of-the-art suite of instruments to directly
detect molecules present both in moon’s atmosphere
and on its surface.
And, I’d imagine, as Dragonfly uses its eight sets
of helicopter blades to take off, it might just kick up a
cloud of Titan dust with a very interesting — if not
slightly deadly — olfactory bouquet.

Morgan L. Cable is a scientist and supervisor of the Titan is home to many hydrocarbon lakes and seas (blue and black) that are prime targets for
Astrobiology and Ocean Worlds Group at NASA-JPL. future exploration missions. This map of Titan’s northern polar region was constructed from
radar data Cassini collected during 127 flybys of the intriguing moon. NASA/JPL-CALTECH

WWW.ASTRONOMY.COM 35

SKY THIS MONTH Visible to the naked eye
Visible with binoculars
THE SOLAR SYSTEM’S CHANGING LANDSCAPE AS IT APPEARS IN EARTH’S SKY. Visible with a telescope
BY MARTIN RATCLIFFE AND ALISTER LING

SEPTEMBER 2020 Neptune reaches
opposition this
Catch the month on
outer planets September 11.
This image, taken
in September
2010, shows the
blue-colored ice
giant (center) and
its largest moon,
Triton. DAMIAN PEACH

Mars is nearing its On September 21 and 22, high and the sky is darkening. 8° east of Jupiter, manages
highly anticipated Mercury passes close to Spica, They set 20 minutes later. On a fine magnitude 0.3, also
peak this month and is a few Virgo’s brightest star. The pair September 30, Mercury sets 50 dipping two-tenths of a
weeks away from its 2020 will be difficult to see due to minutes after the Sun. There’s magnitude by September 30.
opposition — a very favorable bright twilight. They’re side by a slim window to catch it. A waxing gibbous Moon lies
one for Northern Hemisphere side September 21, standing in their vicinity September 24
observers. The Red Planet is 0.7° apart. Spica shines at A stunning planetary duo and 25.
best viewed after midnight, magnitude 1 and Mercury is attracts telescopic observers
once it reaches higher elevation. magnitude 0. Try spotting after dark. Jupiter shines Let’s look at Jupiter first. It
Early evenings are dominated them 20 minutes after sunset, at magnitude –2.6 in the wanders westward until
by Jupiter and Saturn, whose when the pair are 5° high. Ten southern sky. It dims two- September 13, when it halts at
spectacular presence beckons minutes later, they’re only 3° tenths of a magnitude by the its stationary point and returns
you to get your scope. Mercury end of the month. Saturn, only to direct motion, shaving off a
makes a brief and challenging degree in the gap between itself
appearance shortly after sunset Mars and the Moon meet in Pisces and Saturn by the end of the
late in the month, while Venus month. The ringed planet
dazzles in the predawn sky. Algol Hamal PISCES reaches its stationary point
Walk out any September PERSEUS two weeks later, on the 29th.
evening an hour after sunset
and you’ll find two planets ARIES Moon Mars Jupiter’s disk spans 44" and
already up in the southern sky. presents a wealth of detail.
Jupiter is the brightest, nearly Uranus Two dark equatorial belts are
30° above the southern horizon. most obvious, and more
Saturn lies to its east (left). Pleiades CETUS delicate structures lie between
They’re situated in eastern them and at higher latitudes.
Sagittarius, and observers at Menkar The dusky features are best
dark sites have a spectacular seen during moments of steady
view of the Milky Way along TAU R U S seeing, when patience will
with these planets. provide the reward of a
Before we turn to this bright Aldebaran stunning view. Occasionally,
pair, let’s first get a glimpse of the Great Red Spot makes a
Mercury. It’s difficult to spot as 10° commanding appearance,
it approaches its greatest east- scooting across the disk in
ern elongation October 1, as the September 6, 12:30 A.M. a matter of hours.
ecliptic lies so low to the hori- Looking east
zon that it’s not too favorable. Your scope will also show
Mars and the Moon share the sky after midnight in early September. Return the four famed Galilean
to the same field with binoculars on a moonless night from a dark site to spot moons, first spotted in 1610.
Uranus, sitting between Hamal and Menkar. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY Io, Europa, Ganymede, and

36 ASTRONOMY • SEPTEMBER 2020

RISING MOON I Beyond the edge

THERE’S SOMETHING In between the maria is the Seldom-seen maria
UNCOMMON going on prominent crater Neper, with
this month. Starting on the its lava-filled basin and nota- N
19th, look at all that terrain ble central mountains. Beyond
OBSERVING between Mare Crisium and the it lies the smaller Jansky and, if E CONSOLIDATED LUNAR ATLAS/UA/LPL. INSET: CLEMENTINE (NASA/JPL/USGS)
HIGHLIGHT limb! Mare Marginis and Mare you can see one more, Jansky F
Smythii are huge splotches of at longitude 92 east. On the Goddard
MERCURY stands side by side dark instead of thin channels. northern flank of Marginis, the Mare Marginis
This is about the best perspec- rugged walls of Goddard ring
with the bright star Spica on tive of the eastern zone you a flat lake of lava that flooded Neper
September 21. The pair is only can get — thanks to lunar higher than the crater’s peak.
visible for about an hour after libration, we see “around” the For a few evenings, the lumpy Mare Smythii
side of the Moon a bit more rim of Al-Biruni should poke
sunset, sinking in the west. than we access with our nor- up from the limb just beyond Given lighting conditions
mal face-on view. Goddard. Observe these and and libration, portions of the
Callisto orbit the planet every you’re on your way to Moon’s eastern limb that aren’t
1.8, 3.6, 7.2, and 16.7 days, The two large dark zones becoming a veteran normally visible will be on display.
respectively. The inner three are huge ponds of lava that selenophile.
moons are in a 4:2:1 orbital welled up from below and Moon will be up all evening, a
resonance, meaning every few froze billions of years ago. Each night great time to try to spot the
days they return to approxi- The northern one is Marginis, after First effects of libration for yourself.
mately the same location literally the “sea at the mar- Quarter, the
relative to Jupiter. And all gin,” while the southern one is combined lunar By the time we get to Full, all
four moons undergo eclipses, Smythii, honoring British ama- orbit and spin of the features past Crisium will
transits, and occultations. teur astronomer William Henry gives the impression have spun beyond the limb. The
Here are a few examples this Smyth. The white wall on the that this eastern flank of the sequence repeats the next couple
month to watch for; note there limb beyond Smythii is made Moon is rolling away from us. of months, but less prominently.
are more than just those listed up of a series of crater rims, Watch for Goddard’s rim and
here during the month. the largest being Hirayama, Neper’s peak to stick up off
Purkyne, and Babcock. Is the the limb when they are in pro-
Io and Ganymede pass each libration enough for us to see file. September 26 is this year’s
other on September 15. From inside them? Fire up the mag- International Observe the
the Midwest and eastern U.S., nification and take a look. Moon Night; an 80-percent-lit
Io and its shadow are already
transiting Jupiter as darkness METEOR WATCH I Dust on display
falls. Io leaves the disk at
9:51 P.M. EDT and wanders Scattered light METEOR RATES WANE after
west, while Ganymede heads their August peak. There are no
the other way. The pair is clos- Early morning after midmonth is an ideal time to search for the major showers in September. The
est (10") around 10:30 P.M. EDT, cone-shaped glow of the zodiacal light. STEPHEN RAHN background or sporadic rate of about
then Ganymede continues seven meteors per hour continues,
toward the planet’s limb. and you might even be lucky to catch
Io’s shadow leaves Jupiter at an errant fireball.
11:03 P.M. EDT and Ganymede
is occulted by the planet start- If you’re up observing Mars in
ing at 11:43 P.M. EDT. the early hours — say between 4:30
and 5:30 A.M. local time — be on the
On September 30, Callisto’s lookout for the zodiacal light. This
shadow begins creeping onto cone-shaped brightening occurs
Jupiter starting at 9:04 P.M. along the steeply inclined ecliptic,
EDT; it takes 10 minutes to extending from Leo through Cancer
fully appear. How soon do you and into Gemini before twilight. Venus
notice it on the northeastern will be embedded in the middle of
limb? Io reappears from eclipse the faint light. You’ll need moonless
22" off the eastern limb of the conditions, which occur in the last two
planet at 12:03 A.M. EDT on weeks of the month. Higher-altitude
October 1, while Callisto’s observing sites improve your chances
shadow continues across of viewing the zodiacal light, which is
Jupiter’s disk. a persistent faint glow from meteoritic
dust that pervades the solar system.
— Continued on page 42
WWW.ASTRONOMY.COM 37

STAR DOME N

HOW TO USE THIS MAP C A M E L O PA R DA L I S M82
M81
This map portrays the sky as seen
near 35° north latitude. Located NE E IA CNAGS SCI8O6P9E
inside the border are the cardinal NGC
directions and their intermediate E S R 884 Polaris
points. To find stars, hold the map P
overhead and orient it so one of S U URSA
the labels matches the direction MINOR
you’re facing. The stars above CEPHEUS
the map’s horizon now match NCP
what’s in the sky.
T ANDROMEDA
The all-sky map shows
how the sky looks at: R

10 P.M. September 1 I
9 P.M. September 15
8 P.M. September 30 A

Planets are shown N
at midmonth
G DRACO

U

L

U

M33 M31
M

PISCES L A C E RTA CYGNUS
Deneb
PEGASUS M57
MAP SYMBOLS E LY R A Vega

Open cluster Path of the Sun (ecliptic) DE M27 V U L P E C U L A
Globular cluster M15 SAGIT TA
Diffuse nebula
Planetary nebula Enif Altair
Galaxy
L
STAR
MAGNITUDES CETUS PHINUS
EQUU
Sirius
0.0 3.0 L
1.0 4.0
2.0 5.0 EU

S

AQUARIUS AQUILA

M11 M16
SCUTUM M17

Fomalhaut AUPSI CA P R I C Saturn Jupiter M22 M20
SCU STCRIISN U S M8
STAR COLORS O R N

A star’s color depends U S
on its surface temperature.
L
The hottest stars shine blue
SE P
•• Slightly cooler stars appear white
• Intermediate stars (like the Sun) glow yellow TO
• Lower-temperature stars appear orange
• The coolest stars glow red R M
• Fainter stars can’t excite our eyes’ color
MICROSCOPIUM SAGIT TARIUS
receptors, so they appear white unless you INDUS
use optical aid to gather more light CORO NA I S
AUST RAL
BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT
www.Astronomy.com/starchart. GRUS TELESCOPIUM

S

SEPTEMBER 2020 SAT.

SUN. MON. TUES. WED. THURS. FRI.

M51 Mizar C AVNE ENSAT I C I 1 2345 ILLUSTRATIONS BY ASTRONOMY: ROEN KELLY
URSA MAJOR
NW 6 7 8 9 10 11 12

CBOEMRAE N I C E S 13 14 15 16 17 18 19

BOÖTES Arcturus 20 21 22 23 24 25 26

S 27 28 29 30

M13 A Note: Moon phases in the calendar vary in size due to the distance
HERCULES LI from Earth and are shown at 0h Universal Time.

N CALENDAR OF EVENTS
A
1 Venus passes 9° south of Pollux, 1 P.M. EDT
CORO 2 Full Moon occurs at 1:22 A.M. EDT
BORE
Asteroid Pallas is stationary, 9 A.M. EDT
SERPENS VIRGO The Moon passes 4° south of Neptune, 5 P.M. EDT
CAPUT 6 The Moon passes 0.03° north of Mars, 1 A.M. EDT
W The Moon is at apogee (252,032 miles from Earth), 2:29 A.M. EDT
The Moon passes 3° south of Uranus, midnight EDT
M5 9 Mars is stationary, 2 P.M. EDT
10 Last Quarter Moon occurs at 5:26 A.M. EDT
OPHIUCHUS 11 Asteroid Fortuna is at opposition, 3 A.M. EDT
Neptune is at opposition, 4 P.M. EDT
LIBRA 12 Jupiter is stationary, 8 P.M. EDT
14 The Moon passes 4° north of Venus, 1 A.M. EDT
S ECRAPUEDNAS 17 New Moon occurs at 7:00 A.M. EDT
18 The Moon is at perigee (223,123 miles from Earth), 9:48 A.M. EDT
8 Antares M4 The Moon passes 6° north of Mercury, 6 P.M. EDT
LUPUS 22 Mercury passes 0.3° north of Spica, 5 A.M. EDT
M6 SW Autumnal equinox occurs at 9:31 A.M. EDT
M7 23 First Quarter Moon occurs at 9:55 P.M. EDT
25 The Moon passes 1.6° south of Jupiter, 3 A.M. EDT
O R P IUS The Moon passes 2° south of Saturn, 5 P.M. EDT
N6G2C31 28 Saturn is stationary, 11 P.M. EDT
S C 29 The Moon passes 4° south of Neptune, 10 P.M. EDT
Asteroid Leto is at opposition, 11 P.M. EDT

WWW.ASTRONOMY.COM 39

PATHS OF THE PLANETS

LYN PER AND LAC

C Vn LMi AUR
COM
CNC GEM The Moon passes 0.03° north
Sun of Mars on September 5/6
CRV CRT Path of the Moon

LEO Vesta Uranus PEG
PSC
Venus The Moon passes 4° north ORI Asteroid Fortuna reaches
Mars opposition September 10/11

of Venus on September 14 TAU Flora Parthenope
Celestial equator
SEX

MON Papagena AQR CAP
CMa Massalia
ERI
H YA Asteroid Leto reaches

LEP opposition September 29 Ceres

FOR Neptune appears at its PsA
best during September
ANT PYX COL

PUP CAE PHE

VEL MIC

HOR

Moon phases Dawn Midnight

18 17 16 15 14 13 12 11 10 9 8 7 6 54 3 2 1

To locate the Moon in the sky, draw a line from the phase shown for the day 30 29 28 27
straight up to the curved blue line.

THE PLANETS Uranus THE PLANETS IN THE SKY
IN THEIR ORBITS
Jupiter These illustrations show the size, phase,
Arrows show the inner Saturn and orientation of each planet and the two
planets’ monthly motions brightest dwarf planets at 0h UT for the dates
and dots depict the in the data table at bottom. South is at the top
outer planets’ positions to match the view through a telescope.
at midmonth from high
above their orbits. Neptune Mars
Opposition is
September 11 Venus

Mercury

Pluto

Venus

Mars Earth PLANETS MERCURY VENUS
Ceres Autumnal equinox
is September 22 Date Sept. 30 Sept. 15
Magnitude 0.0 –4.2
Mercury Angular size 6.6" 17.4"
Illumination 63% 66%
Jupiter Distance (AU) from Earth 1.018 0.959
Distance (AU) from Sun 0.722
40 ASTRONOMY • SEPTEMBER 2020 Right ascension (2000.0) 0.450
Declination (2000.0) 13h58.0m 8h46.0m
17°07'
–14°54'

This map unfolds the entire night sky from sunset (at right) until sunrise (at left). Arrows SEPTEMBER 2020

JULYand colored dots show motions and locations of solar system objects during the month.

CYG HER UMa Callisto 1
OPH LMi
CVn Europa 2 Jupiter
LYR BOÖ COM
CrB Io 3
4 Ganymede
Ganymede 5
VUL Pallas LEO 6
SGE JUPITER’S 7
Sun MOONS
AQL SER SER Comet SEX 8 Europa
PANSTARRS Dots display
Sun positions of 9
V I RMercury Galilean satellites
(tehceliSputinc) CRV CRT HYA at 11 P.M. EDT on 10 Io
P SCT LIB the date shown.
Saturn Path of South is at the 11
Jupiter top to match the 12
view through a 13
Pluto SGR Comet telescope. 14
88P/Howell 15
The Moon passes 1.6° south of ANT 16
Jupiter on September 24/25 LUP
17 Callisto
CEN
18
Early evening 19
20
26 25 24 23 22 21 20 19 18 17 16 21
22
Jupiter S 23
WE 24
Saturn 25
N 26
27
10" 28
29
Ceres Uranus Neptune Pluto 30

MARS CERES JUPITER SATURN URANUS NEPTUNE PLUTO

Sept. 15 Sept. 15 Sept. 15 Sept. 15 Sept. 15 Sept. 15 Sept. 15
–2.1 7.9 –2.5 0.4 5.7 7.8 14.7
20.9" 0.7" 42.6" 17.6" 3.7" 2.4" 0.1"
95% 99% 99% 100% 100%
0.447 4.630 9.421 100% 100%
1.394 2.042 5.133 19.095 28.925 33.607
2.981 10.003 19.786 29.929 34.120
1h48.8m 22h38.5m 19h14.5m 19h48.9m 2h31.1m 23h21.9m 19h37.1m
6°44' –25°03' –22°45' 14°23' –5°19' –22°40'
–21°23'

WHEN TO

SKY THIS MONTH — Continued from page 37 VIEW THE

A close pass Peak Mars observing season PLANETS

S begins in earnest as we near EVENING SKY
next month’s opposition. The Mercury (west)
Ganymede Jupiter Red Planet is less than half an Jupiter (south)
astronomical unit from Earth Saturn (south)
W Io and its disk spans 19". (One Neptune (east)

Io’s shadow astronomical unit is the aver- MIDNIGHT

age Earth-Sun distance.) Mars’ Mars (east)

30" apparent size will grow to 22" Saturn (southwest)
September 15, 10:30 P.M. EDT by the end of September, just Jupiter (southwest)
1" shy of its diameter at opposi-
Io and Ganymede pass each other while moving in opposite directions on tion. It’s located in Pisces, Uranus (east)
September 15. Europa and Callisto are also visible, farther to the west. nestled in the V of this faint Neptune (south)

MORNING SKY

Saturn’s disk is a mellow east soon after sunset on constellation. The planet’s Mars (southwest)
yellow with few significant fea- September 1. Binoculars will eastward drift slows to a halt Venus (east)
tures, although keen eyes will show the magnitude 7.9 planet September 9, after which it
begins a retrograde loop, track- Uranus (southwest)
Neptune (west)

spot some belt structure on the near 4th-magnitude Phi (φ) ing quickly west. Starting the

disk, which spans 18". But the Aquarii. Scan 2.5° east of the month at magnitude –1.9, its

planet’s major attraction is its star to find Neptune, where it orange glow is unmistakable in occults the Red Planet, visible

rings: Three major rings extend forms a triangle with a pair a region devoid of bright stars. from Central and South

outward 40", and their narrow of 6th-magnitude field stars. It brightens to –2.4 this month America, North Africa, and

axis is 15.5" — just shy of the Neptune lies midway between — comparable to Jupiter. southern Europe.

planet’s polar diameter of 16". them on September 30, now A bright waning gibbous The next three months are

The shadow of the planet is only 1.5° east of Phi. A tele- Moon lies 1.5 Moon-widths the best time to swing your

noticeable on the east side of scope reveals its bluish disk, south of Mars on September 5. scope toward Mars. Your best

the rings. spanning 2.3". The same night, our satellite views will be after midnight,

You’ll find 8th-magnitude

Titan, Saturn’s brightest moon, COMET SEARCH I Come again?
due north of Saturn on

September 1 and 17 and due

south on September 9 and 25. WITH A SHORT PERIOD of Comet 88P/Howell
Visible in small scopes are 5.5 years, Comet 88P/Howell has

Tethys, Dione, and Rhea, all returned to keep us company N
roughly 10th magnitude. Their through early winter. You may not a

relative positions change over a remember its name because that q e LIBRA i
few hours. Twelfth-magnitude extra half-year placed Earth on r i` NGC 5897 Sept 1
Enceladus is near the bright the other side of the solar system 5
edge of Ring A, where it can during its last passage. Only every E OPHIUCHUS M80 b 15 m
be lost in ring’s brilliance, so 11 years are we in a good position NGC 6144 20 10
you’ll need to look carefully. to view it.
30 25 Path of
Iapetus reaches superior The comet’s closest approach Antares M4 Comet Howell
conjunction September 7, when to the Sun occurs on the 26th,
it lies 63" due north of Saturn and it should glow around 9th p
near magnitude 11. The moon’s magnitude all month. A green
two-toned surface causes its halo will be easy to capture for a o l
magnitude to vary from 10.5 small telephoto lens on a tracking SCORPIUS o
down to 11.7 as it orbits the mount. Under country skies, a
planet. At superior conjunction, LUPUS 5°
50 percent of each hemisphere
faces Earth. The moon contin- 4-inch scope will visually pick up
ues toward eastern elongation,
reaching it September 25. this small gray fuzz. Unlike most comets, whose paths sharply incline to the nearly flat plane of
On September 4 and 5, Howell the planets, Howell’s is close to the ecliptic. As a result, it tends to pass the
Neptune reaches opposition same deep-sky objects upon each return at the same time of year.
shares a medium-power field

with the loose globular cluster

NGC 5897 in Libra. Leverage the brief period of darkness at dusk before moonrise. Much better will be

the nights of September 22 through 25, when Howell threads the gap between Messier globulars 4 and

on September 11 in Aquarius. 80, nestled close to brilliant Antares. Unfortunately for the “comet ferret” Charles Messier, his scope was

It rises with a Full Moon in the just a bit too small, leaving the comet’s discovery to Ellen Howell from Palomar Mountain in 1981.

42 ASTRONOMY • SEPTEMBER

LOCATING ASTEROIDS I

Nothing fishy about it

Venus buzzes the Beehive GEMINI THE GOOD TIMES FOR ASTEROID TRACKING are back
with the rise of dwarf planet 1 Ceres to binocular visibility. All
Pollux this month it slides one field of view above the yellow luminary
Fomalhaut, plying the southern sky after dusk. Unless you live in
CANCER CANIS MINOR the southern U.S., where Piscis Austrinus gets high enough, you’ll
M44 Procyon need those binoculars to trace out the rest of the constellation,
whose second-brightest star is magnitude 4.3.
Moon Venus
Wait until late evening for Ceres to climb above the horizon’s
LEO haze, then nudge your view upward to its path. Our line of sight is
far from the Milky Way’s central bulge, so we won’t be bothered
Regulus HYDRA by a forest of background stars, as in previous months. In fact, at
magnitude 7.7, Ceres will be one of the brightest dots in the eye-
Alphard 10° piece of a telescope. Find a notable pattern of at least three stars,
plop them into your logbook, and come back a night or two later
September 14, 1 hour before sunrise to confirm which one shifted. Avoid the nights of the 1st and
Looking east 28th, when the Harvest Moon skims to the north.

By coincidence, Ceres just hit its least bright opposition on
August 28, lying at the distant end of its egg-shaped orbit when we
passed by. Because the next “farthest close approach” will occur in
2043, Ceres will get easier for many observing windows to come.

Venus and the crescent Moon rendezvous before sunrise near the Beehive Ceres goes for a swim
Cluster (M44) on September 14, offering early risers a morning treat.
AQUARIUS N
when the planet rises to nearly The dramatic features of Sinus
60° altitude for midlatitudes Sabaeus and Sinus Meridiani Sept 1 Path of Ceres
in the U.S. This is a far better appear during the last week of 5 10 15 20 25 30
setup than the perihelic opposi- the month.
tion of 2018, when the planet c
remained very low for Northern Uranus lies 13° east of Mars E¡
Hemisphere observers. on September 1. If you have a
go-to mount, it’s easy to find. PISCIS 1°
Telescopes will show the disk But there are no bright stars in AUSTRINUS
92 percent lit on September 1 the region, so star-hoppers have
and almost fully illuminated a challenge. After midnight — Fomalhaut
(99 percent) by the end of the once Uranus reaches a decent
month. Major features visible altitude in the eastern sky — Ceres dips between Aquarius and Piscis Austrinus this month. A few
on the martian disk at 2 A.M. scan with your favorite binocu- degrees away, you’ll find the bright star Fomalhaut.
local time in early September lars between Hamal (Alpha [α]
include Syrtis Major and the Arietis), Aries’ brightest star, located south of the bright stars floats nearby, only 5° from
bright Hellas basin. Watch for and Menkar (Alpha Ceti) in Castor and Pollux in Gemini the both Venus and M44.
brightening in some regions Cetus. The two are 23.5° apart; Twins. Shining at magnitude
that could be indicative of the Uranus lies between them just –4.3, it’s unmistakable. A tele- Venus crosses into Leo in
onset of a dust storm. under 11° from Hamal and scope reveals a gibbous phase of late September. The planet
shines at magnitude 5.7. The 60 percent in early September, ends the month 3° west of
In the second week of 6th-magnitude star 29 Arietis which grows to 71 percent by Regulus, Leo’s 1st-magnitude
September, the dark Mare lies near the planet. The ice the end of the month. Venus is alpha star, located at the base
Sirenum and Mare Cimmerium giant is 0.6° southwest of the receding from Earth and conse- of the sickle asterism.
span the martian disk. They’re star on September 1; the gap quently its disk shrinks from 19"
followed in mid-September by widens to just over 1° by to 16" over the same period. Martin Ratcliffe provides
the brighter volcanic Tharsis September 30. Uranus’ greenish planetarium development for
Ridge and Olympus Mons near disk is visible in a telescope. A spectacular scene occurs Sky-Skan, Inc., from his home
the terminator. Also watch for High magnification reveals a on September 14, when Venus in Wichita, Kansas. Alister
bright features along the termi- 3.5"-wide disk. lies 2.5° south of the Beehive Ling, who lives in Edmonton,
nator that could be clouds. By Cluster (M44) in Cancer the Alberta, has watched the skies
September 21, the Mariner Venus rises around 3 A.M. Crab. A waning crescent Moon since 1975.
Valley rotates onto the disk. local time on September 1,

GET DAILY UPDATES ON YOUR NIGHT SKY AT
www.Astronomy.com/skythisweek.

WWW.ASTRONOMY.COM 43

The great
ASTERISM HUNT

You can find lots of unofficial star patterns

using binoculars, a telescope, or just your eyes.

BY MICHAEL E. BAKICH

TTHE BIG DIPPER IS PROBABLY Before this publication, asterisms and
the Northern Hemisphere’s best-known constellations were coequal. Delporte
constellation, right? Wrong. The Big himself begins the introduction of the
Dipper isn’t a constellation at all, but an report with this sentence: “In their
asterism. An asterism is an unofficial descriptions, the ancients left us a star-
group of stars recognized by (generally studded sky divided into asterisms, but
amateur) astronomers or the public. without any clear-cut boundaries.”
They have been in a separate category
than constellations since 1928. Asterisms may belong to a single con-
In that year, Belgian astronomer stellation, or they may use stars from sev-
Eugène Joseph Delporte, the head of eral. As you’ll also discover when you
a commission of the International head outdoors to find these patterns,
Astronomical Union, formalized the many asterisms are easier to find than
number of constellations in the sky (88), most of the constellations. My list
as well as their boundaries. His report includes naked-eye, binocular, and
was published in 1930 as Délimitation telescopic asterisms visible from the
Scientifique des Constellations (Scientific Northern Hemisphere. To make things
Demarcation of the Constellations). easier, I’ve divided them into seasonal
groupings, beginning with fall.

The Water Jar This is easy observing, the kind that
you can share with non-astro family
TONY HALLAS members and friends. Not only will they
immediately see the patterns (especially
the naked-eye ones), they’ll also enjoy
hearing any stories you tell about them.
So, head outside the next clear night and
have some fun.

In the fall sky

The first asterism is an easy one to find.
It’s the Great Square of Pegasus.
That’s kind of a strange name consider-
ing only three of its stars — Markab
(Alpha [α] Peg), Scheat (Beta [β] Peg),
and Algenib (Gamma [γ] Peg) — belong
to the constellation Pegasus. The
remaining star, Alpheratz, is the Alpha

The Great Square of Pegasus JOHN CHUMACK

star in Andromeda. Still, the four do The Coathanger JOSÉ J. CHAMBÓ
make a striking square.
WWW.ASTRONOMY.COM 45
If you have a problem calling this
asterism “of Pegasus,” then picture it as
the Baseball Diamond. In this case, Beta
Peg becomes home plate, Alpha And is
first base, and so on. You can even imag-
ine a pitcher, catcher, many outfielders,
and stands full of fans — every one a star!

To find the next asterism, head due
south. Although the Circlet of Pisces
falls within the naked-eye category, its
stars are much fainter than the ones in
the Great Square. They are Gamma, 7,
Theta (θ), Iota (ι), 19, Lambda (λ), and
Kappa (κ) Piscium. Of these, Gamma is
the brightest, shining at magnitude 3.7.

The Circlet marks the head of the

JOHN CHUMACK
MARTIN C. GERMANO
Kemble’s Cascade The 37

western fish in this constellation. A cord in a catalog of 471 open clusters he com- from the Pleiades (M45) to Hamal (Alpha
leads east to Alrescha (Alpha Piscium), piled. For that reason, the Coathanger Arietis). Its three stars weren’t assigned
and another heads northward to the carries the designation Collinder 399. Greek letters. Instead, they carry the
eastern fish, which, unfortunately, isn’t Flamsteed numbers 35, 39, and 41 Arietis.
nearly as easy to spot as the one marked Another easy-to-find asterism in the
by the Circlet. fall sky lies near Capella (Alpha Aurigae), The brightest of the three is 41 Ari
the sky’s sixth-brightest star. It’s the (magnitude 3.6), followed by 39 Ari
Proceed west about twice the diameter Kids, named so because of their near- (magnitude 4.5) and 35 Ari (magnitude
of the Circlet, and you’ll arrive at the ness to Capella, a Latin word that can 4.7). In the early 17th century, Dutch car-
Water Jar in the constellation Aquarius. translate to “mother goat.” tographer Petrus Plancius added the
It appears as a triangle with a central magnitude 5.3 star 33 Ari to this trio and
point. The four stars are Gamma, Eta (η), Unlike the Coathanger’s stars, the formed the now-defunct constellation
Pi (π), and Zeta (ζ) Aquarii. The central three in the Kids stand out nicely. In Apes the Bees. Other celestial mapmak-
star, Zeta, is a double star made of Zeta1 order of brightness, they are Almaaz ers reinvented it as Musca Borealis the
Aqr (magnitude 4.4) and Zeta2 Aqr (Epsilon [ε] Aur) at magnitude 3.0, Northern Fly (from which the asterism’s
(magnitude 4.6), which combine to make Haedus (Eta Aur) at magnitude 3.2, and name derives) and Lilium the Lily.
the star’s apparent magnitude 3.7. Many Sadatoni (Zeta Aur) at magnitude 3.8.
early celestial cartographers pictured The next fall asterism, Kemble’s
a stream of water flowing out of the Look southwest of the Kids in the Cascade, lies north of Perseus in the
Water Jar and into the mouth of Piscis northern part of the constellation Aries constellation Camelopardalis. Because
Austrinus the Southern Fish. the Ram for a much fainter triangle- it’s 2.5° long, it looks best through bin-
shaped asterism, the Northern Fly. It lies oculars — that’s how its discoverer,
Now head northwest about one- north of the midway point along a line
quarter of the way across the sky and The Spring Triangle
find Albireo (Beta Cygni). It’s the 3rd- JOHN CHUMACK
magnitude double star that’s the head of
Cygnus the Swan. From it, draw a line 3°
to the south to Anser (Alpha Vulpeculae),
which glows at magnitude 4.4. Continue
that line another 4½°, and you’ll find the
Coathanger. Although you can spot
this stellar gathering with just your eyes,
it looks best through binoculars.

Ten of the Coathanger’s stars rise
above 7th magnitude. The brightest are
4 Vulpeculae (magnitude 5.1), 5 Vul
(magnitude 5.6), and 7 Vul (magnitude
6.3). The combined magnitude of all the
Coathanger’s stars is 3.6, but it’s not bright
because the light is spread across an area
1° wide. Swedish astronomer Per Arne
Collinder included this group as No. 399

46 ASTRONOMY • SEPTEMBER 2020

amateur astronomer Lucian Kemble,
found it.

A magnification of 15x works best for
framing the 15 stars in this starry chain.
The easiest way to find the Cascade is to
locate its brightest star, 5th-magnitude
SAO 12969, that sits in the center. The
other stars glow between 7th and 9th
magnitudes on either side of it.

If you see a subtle glow at the south-
east end of the Cascade, you’ve found the
Jolly Roger Cluster (NGC 1502), an open
cluster that glows at magnitude 5.7.

Our last fall asterism is big and bright:
the Segment of Perseus. Various
sources include different stars, but the
most frequently mentioned are, in order,
Eta, Gamma, Alpha (named Mirfak),
Psi (ψ), and Delta (δ) Persei. Some writ-
ers state that the Segment was Perseus’
sword, which he used to decapitate the
Gorgon Medusa. Most depictions of the
constellation, however, place the sword
well above the Segment, which falls
within the hero’s body.

In the winter sky The V JOHN CHUMACK

Because Orion the Hunter is the most In the spring sky

prominent constellation in the sky, it con- The first vernal asterism that you
should probably find is the Spring
tains lots of asterisms originating from Triangle, a giant figure pulled straight
out of a geometry class. It’s visible in the
all over the world. Many use the same When you’re done viewing the 37, Northern Hemisphere all through spring,
and three bright stars make up its points.
three bright stars in the Belt. Starting at step away from your scope and draw
First is Arcturus (Alpha Boötis), the
the northernmost one, they are Mintaka a line a bit more than 20° long up (north- brightest star in the northern sky and
fourth-brightest overall. It shines with
(Delta Orionis, magnitude 2.3), Alnilam west) from the Belt, where you’ll encoun- an orange hue at magnitude –0.04. From
(Epsilon Ori, magnitude 1.7), and Alnitak ter the V of Taurus. This letter’s eastern Arcturus, move to sapphire-blue Spica
(Alpha Virginis), which represents a
(Zeta Ori, magnitude 1.7). tip is Aldebaran (Alpha Tauri), the ruddy sheaf of grain (probably wheat) in Virgo
the Maiden’s hand.
This straight-line trio is also known eye of Taurus the Bull. The other stars
as the Rake and the Three Kings. If are Theta, Gamma, Delta, and Epsilon Spica varies in brightness between
magnitude 0.92 and 1.04, with a period
you include the stars Eta and Iota Tauri. of a bit more than four days. Finally,
complete the Spring Triangle with
Orionis, the four-sided asterism called The fainter stars of this asterism Denebola (Beta Leonis), the tail of Leo
Venus’ Mirror appears. are part of the Hyades star cluster. the Lion. Denebola, at magnitude 2.1, is
the sky’s 59th-brightest star, which is
Orion also holds a fun telescopic aster- Aldebaran, which lies some 90 light-years fairly notable considering human eyes
ism: 37. It’s the brightest stars of the open closer to us, is not. can detect roughly 6,000 stars.

cluster NGC 2169, which is the southern Our final winter-sky asterism Next up is the Head of Hydra,
which lies on the western end of the sky’s
tip of an isosceles triangle with Xi (ξ) and is Pakan’s 3 in the constellation largest constellation. To find it, look 2°
south of the midpoint of a line that joins
Nu (ν) Orionis as the other points. NGC Monoceros. You’ll find it about 3° south-
WWW.ASTRONOMY.COM 47
2169 lies less than 1° from each star. If west of the open cluster M50, or 2° north

your observing site is dark, you might of 4th-magnitude Theta Canis Majoris. I

spot NGC 2169 as a faint have been able to identify

glow with your naked eyes. Because Orion this asterism through 18x50
Point a telescope at the the Hunter is the binoculars, but a telescope
will guarantee success.
cluster and use low power to

see the number 37. Actually, most prominent The 11 stars of Pakan’s 3
the letters LE are a bit easier constellation in range from magnitude 8.5
to visualize. To successfully the sky, it contains (SAO 133838) to magnitude
see the 37, your field of view lots of asterisms 9.5 (SAO 152034). From
should have north up and top to bottom, it spans

east to the left. originating from all exactly 0.5°.

over the world.

TONY HALLAS The False Comet be through 10x or higher binoculars, or
The Keystone through telescopes with eyepieces that
TONY HALLAS provide a field of view a bit wider than
a quarter degree.

Our next spring sky asterism, the Y
of Virgo, ranks as the largest in a single
constellation — but just barely. Its longest
dimension beats that of the Big Dipper
by 1°. The Y contains six stars, and they
all have common names, which indicates
that they’re fairly bright. Indeed, the
faintest glows at magnitude 3.9.

But we begin the Y at its bottom with
its brightest star, Spica (Alpha Virginis),
a blue-white sparkler. From it, move 15°
northwest to Porrima (Gamma Vir).
There, the Y branches in two directions,
toward the north-northeast and the
west-northwest. The north-northeast
leg contains Minelauva (Delta Vir) and
Vindemiatrix (Epsilon Vir). The west-
northwest branch contains Zaniah (Eta
Vir) and Zavijava (Beta Vir).

Moving from the very large to the
very small, point your telescope 12°
due west of Spica to find the Stargate
in the constellation Corvus. It lies mid-
way between Delta Corvi and Chi (χ)
Virginis, and it consists of two triangles
of stars, one within the other. The outer
one measures roughly 5' on a side, while
the sides of the small one each span less
than 1'.

You can spot the outer triangle
through binoculars — well, two of its
stars, at least. They glow at magnitudes
6.6 and 6.7, and the third is much fainter,
at magnitude 9.9. The three inner stars
are more subtle: magnitudes 8.0, 9.7, and
10.6. The best views come through a
4-inch or larger scope with an eyepiece
that gives a magnification of about 50x.

Procyon (Alpha Canis Minoris) and Broken Engagement Ring. It lies 1.5° In the summer sky
Regulus (Alpha Leonis). west of Merak (Beta Ursae Majoris), one
of the Pointer stars of the Big Dipper. We’ll start our final season’s asterisms
Six stars form this asterism. The with a telescopic one in the northern sky
brightest is magnitude 3.1 Zeta Hydrae. The Ring’s brightest star, SAO 27788, — so far north, in fact, that it’s visible in
From there, move west to Epsilon and which glows at magnitude 7.5, lies at its every season. It’s the Mini Coathanger
Delta Hydrae. Then, swing back east to northern end. It’s the diamond in the in Ursa Minor. Astronomy Contributing
Rho (ρ) Hydrae. Drop 3.5° southwest to ring. Unfortunately, the other stars, Editor Phil Harrington gave it this name
Sigma (σ) Hydrae. At magnitude 4.4, this which lie south and west, don’t give off because it looks like the more famous
is the faintest star in the asterism. Head that much light. The second-brightest Coathanger, which I described earlier.
east again to the sixth and final star, Eta glows at magnitude 9.1, and the faintest
Hydrae, then finish the figure by return- is magnitude 9.9, which makes it only The Mini Coathanger is made of 10
ing to Zeta. one-ninth as bright as SAO 27788. faint stars that lie 2° south-southwest of
Epsilon Ursae Minoris. The brightest is
Now turn your gaze a third of the way The Broken Engagement Ring isn’t magnitude 9.2 SAO 2721 (magnitude 9.2),
across the sky to the north. There, tiny. It spans 16', which is half the diam- while the faintest is GSC 4574:802 (mag-
through binoculars, you’ll find the eter of the Full Moon. The best views will nitude 10.8). From its hook to its base, the

48 ASTRONOMY • SEPTEMBER 2020

The Small Sagittarius Cloud CHRIS SCHUR

Mini Coathanger measures 9'. The base, that mark the Keystone. Traditionally, Four stars form the Lozenge, which
looks less like a trapezoid and more like
at 17', is nearly twice that length. a keystone is the wedge-shaped piece that a crooked rectangle. The brightest is
Eltanin (Gamma Draconis) at magnitude
Our next asterism is a naked-eye locks an arch into place. This one lies 2.2. From it, move west to find Rastaban
(Beta Dra), then head north for Kuma
object called the False Comet. It lies two-thirds of the way from Arcturus to (Nu Dra), and finally northeast for
Grumium (Xi Dra).
near the bottom of the tail of Scorpius. To Vega (Alpha Lyrae).
The final target in our asterism hunt,
see it well, you’ll need a dark sky. British The Keystone is big: 7.5° by 5.5°. The the Small Sagittarius Star Cloud
(M24), is kind of an odd duck because it’s
astronomer Sir John Herschel christened brightest of its stars, magnitude 2.8 Zeta a Messier object that shouldn’t really be
one. Why Charles Messier put this star
this region the False Comet during his Herculis, sits at the southwestern corner. cloud (one of many in this rich region of
the Milky Way) on his list remains a
stay in South Africa in the 1830s. He Head 7° north to find magnitude 3.5 Eta mystery to this day.

could have named it that because, with a Herculis at the northwestern corner. M24 lies 3° due north of Mu
Sagittarii, and it’s not small or faint. It
quick glance, it resembles a comet. Continue eastward 6.5°, where magni- measures 95' by 35' and glows at magni-
tude 2.5. Without question, 10x and
However, I think he named this asterism tude 3.2 Pi Herculis marks the north- higher binoculars provide the best views.

after False Bay, the place where his ship eastern corner. Lastly, you’ll find Finding asterisms in the night sky is
fun and easy. And it offers you some cre-
first touched land in South Africa. magnitude 3.9 Epsilon Herculis at the ativity. Don’t like the Great Square?
Make it a Great X. Not happy calling
To find the False Comet, start 0.5° southeastern corner. Draco’s head the Lozenge? Turn it into a
Klingon battle cruiser. No asterism is an
north of Zeta Scorpii. There, you’ll spot For an added bonus, if your site is official pattern, so you’re free to invent
your own. Good luck!
the Northern Jewel Box (NGC 6231). At dark, look one-third of the way from Eta
Michael E. Bakich is a contributing editor
magnitude 2.6, this is the sky’s sixth- to Zeta for what might appear to be an of Astronomy who observes asterisms from
his home in Tucson, Arizona.
brightest open cluster. Now just make a out-of-focus star. That is the Hercules
WWW.ASTRONOMY.COM 49
4º-long line north to the double star Mu Cluster (M13), one of the northern sky’s

(μ) Scorpii. Open clusters other than best globulars. Binoculars will show it

NGC 6231 along that line are magnitude better than the naked eye, but the best

3.4 Collinder 316, magnitude 8.6 views are through telescopes of all sizes.

Trumpler 24, and magnitude 6.4 NGC If you start at the middle of the

6242. Taken together, these clusters and Keystone and head 20° north and just a

the stars around them give the impres- bit east, you’ll encounter an easy-to-see

sion of a comet. asterism called the

When you’re done with Finding asterisms Lozenge, which marks the
the False Comet, head due head of Draco the Dragon.

north through the massive in the night sky It lies at the midway point

constellation of Ophiuchus is fun and easy. on a line between Vega and
to Hercules. There you’ll find And it offers you Kochab (Beta Ursae
four medium-bright stars some creativity. Minoris).