Asteroids, Meteorites, and Comets - Physics & Astronomy

Asteroids, Meteorites, and Comets

The Search for the “Missing” Planet

• Bode’s Law relates the sizes To find the mean distances of the
of planetary orbits planets, beginning with the following
simple sequence of numbers:
• Astronomers noticed a 0 3 6 12 24 48 96 192 384
“missing” planet if this rule
of thumb was correct. Add 4 to each number:
4 7 10 16 28 52 100 196 388
Body Actual Bode's Law
distance Then divide by 10:
Mercury (A.U.) 0.4 0.4 0.7 1.0 1.6 2.8 5.2 10.0 19.6 38.8 The
Venus 0.39 0.7 resulting sequence is very close to the
Earth 0.72 1.0 distribution of mean distances of the planets
Mars 1.00 1.6 from the Sun:
1.52 2.8
Jupiter 5.2
Saturn 5.20 10.0
Uranus 9.54 19.6
19.19

The Discovery of Ceres

• Discovered on 1 January 1801
by Giuseppe Piazzi

– Semi-major axis = 2.7654 AU

– Orbital period = 4.60 yr

– Mass = 0.00015 Earths NASA's Hubble Space Telescope color image of
= 0.0128 Moons Ceres, the largest object in the asteroid belt.

• Now classified as a minor
planet, or asteroid.

• Also classified as a dwarf
planet.
Diameter

comparis

on of the

dwarf

planet–

asteroid

Ceres

with the

Moon

and Earth.

How do we find asteroids?

• Asteroids appear to
move relative to
background stars

Path of 1 Ceres in late 2012
As an asteroid moves along its orbit it will
produce an elongated trail in an image taken
with a telescope following the background of
"fixed" stars, which are not moving in this way.

Small, irregular objects, The Asteroid Belt
mostly in the apparent gap
between the orbits of Mars
and Jupiter.

Thousands of asteroids with
accurately determined orbits
known today.

Asteroids pose virtually no hazard to
space navigation. The average
distance between them is 106 km

Sizes and shapes of the largest
asteroids, compared to the moon

The Asteroid
Belt

As seen from the side

As seen from
above

This is where we find the
majority of asteroids

The Asteroid Belt

• Jupiter’s gravity likely
prevented the formation
of a planet between Mars
and Jupiter.

• Most of the planetesimal
were “kicked” out of this
region, leaving the
objects that now make
up the asteroid belt.

• Jupiter’s gravity also
shapes the asteroid belt
⟶ Kirkwood gaps

Kirkwood Gaps

The distribution of
asteroids in the main belt
is NOT uniform.

Gaps form at specific
regions where there are
whole number ratios (1/3,
2/5, 3/7, etc.) of Jupiter’s
orbital period.

Mean motion resonances

Where else have we seen resonances phenomena?

Picture of Ida (with its tiny moon) from Galileo. Asteroids

Picture of the asteroid Gaspra from the
Galileo spacecraft

Studying Asteroids

• 15 asteroids have by Computer model of asteroid (216) Kleopatra,
visited by spacecraft based on radar analysis.

• Radar can be used to Motion of near-Earth object 2001 FE90 showing
determine the brightness variation over two rotations on
shapes of asteroids 6/26/2009.
that are too small to
resolve with
telescopes

• Optical telesopes can
be used to infer an
asteroid’s shape and
rotation period

Non-Belt Asteroids

Not all asteroids orbit within the asteroid belt!

Asteroids with elliptical Trojans:

orbits, reaching into the Sharing stable
inner solar system. orbits along

the orbit of

Jupiter:

Trapped in
the

Lagrangian
points of
Jupiter.

Some potentially colliding with Mars or Earth.

Near Earth Objects Orbits of near Earth asteroids wider than .88
(NEOs) miles are in yellow; comets in blue.

• Objects that cross Paul Chodas / JPL / NASA
Mars’ orbit or orbit
entirely within the orbit
of Mars are called near-
Earth objects.

• These objects
occasionally pass very
close to Earth, and can
potentially pose a
threat.

NEOs

• As of February 21, 2013, 9738 Near-Earth objects have
been discovered. Some 863 of these NEOs are asteroids
with a diameter of approximately 1 kilometer or larger.
Also, 1379 of these NEOs have been classified as Potentially
Hazardous Asteroids (PHAs).

Evidence of Impacts on Earth

Meteor Crater, Arizona Wolfe creek crater, Western Australia

Pingualuit Crater, Quebec Nördlinger Ries, Germany

The K-T event 10 km,

• Sixty-five million years
ago about 70% of all
species then living on
Earth disappeared
(including dinosaurs)
within a very short
period.

• K-T boundary clay
contains iridium, an
element common in
iron-rich minerals like
asteroids and meteorites

Impacts in Modern Times 2013 Russian meteor event

Tunguska River in Siberia, Russia in
1908
2150 square kilometers affected
Airburst of large meteoroid or comet
(100 meters in size) 5–10 kilometers
above the surface of the Earth.
largest impact event on or near Earth in
modern times.

• Meteoroid = small body Some Nomenclature
in space
• Sizes from microscopic
• Meteor = meteoroid dust to a few centimeters.
colliding with Earth and
producing a visible light • Estimated 300 tons of
trace in the sky meteoritic material falls
on Earth each day.
• Meteorite = meteor that
survives the plunge • Typically impact onto
through the atmosphere the atmosphere with 10 –
to strike the ground... 30 km/s (≈ 30 times faster
than a rifle bullet).

Analysis of Meteorites 3 broad categories:

• Iron • Stony
meteorites meteorites(95%)

(4%)

• Stony-Iron
meteorites

(1%)

What Does a Meteorite Look Like?

Every year between 30,000 and 80,000
meteorites larger than 20g in mass fall from
space to Earth.

Willamette
Meteorite

Goose Lake meteorite (iron)

Selection bias:

Iron meteorites are easy to recognize
as meteorites (heavy, dense lumps of
iron-nickel steel) – thus, more likely to
be found and collected.

The Origins of Meteorites

Probably formed in the solar
nebula, ~ 4.6 billion years ago.

Almost certainly not from comets (in
contrast to meteors in meteor
showers!).

Probably fragments of stony-iron Allende meteorite
planetesimals

Some melted by heat produced by 26Al decay (half-life ~ 715,000 yr).

26Al possibly provided by a nearby supernova, just a few 100,000
years before formation of the solar system (triggering formation
of our sun?)

The Origins of Meteorites (2)

Planetesimals cool and differentiate
(if they are large enough)

Collisions eject material from different depths with
different compositions and temperatures.

Meteorites can not have been broken up from
planetesimals very long ago

 so remains of planetesimals should
still exist.

 Asteroids

Asteroid Collisions

Carbonaceous Chondrites

• A rare type of stony meteorite
which contains large amounts of
the magnesium-rich minerals
and a variety of organic
compounds, including amino
acids.

• No evidence of melting (many
were never heated above 50° C)

• Primitive and undifferentiated
meteorites

• Most of them contain water or
minerals that have been altered
in the presence of water, and
some of them contain larger
amounts of carbon as well as
organic compounds.

Comets

Comet Halley, 1986

Comet McNaught C/2009 R1 was visible on June 6, 2010.
CREDIT: Michael Jäger

Comet Holmes, 2007 Hale-Bopp, 1997

Comets

• Small objects made of frozen Halley's Comet becomes visible to
gases, rock and dust, "dirty the unaided eye about every 76
snowballs“ – the nucleus years as it nears the sun. Image
credit: Lick Observatory
• When close enough to the Sun,
displays a visible coma (a thin,
fuzzy, temporary atmosphere) and
sometimes also a tail.

• Coma is surrounded by a giant
hydrogen envelope.

• Breakup of comets lead to meteor
showers on Earth.

• Orbital periods from 20 years to
100,000 years or more

– Short period
– Intermediate period
– Long period

Eccentric orbits

The Comet Nucleus

Composite image of the nucleus of
Comet Halley produced from 68
original photographs taken by the
Halley Multicolour Camera on board
the Giotto spacecraft on March 13
and 14, 1986.

Comet Hartley, as imaged by NASA's EPOXI
spacecraft. (Credit: NASA)

Structure of a Comet

The Hydrogen Envelope

• Surrounding every moderately The hydrogen cloud surrounding comet Hale
active comet is a sparse but Bopp in 1997 far exceeds the comet’s visible
extensive envelope of neutral tail (inset). Although not visible from the Earth,
hydrogen atoms. The hydrogen the hydrogen envelope is enormous,
is liberated when ultraviolet completely dwarfing the Sun which is shown as
radiation from the Sun splits the yellow dot in the lower right corner.
the water vapor molecules Credit: SOHO/SWAN (ESA & NASA) & J.T.T.
released from the nucleus of Mdkinen et al.
the comet into the constituent
components, oxygen and
hydrogen.

• Up to 10 million km is size
• Not visible to the human eye
• Visible in UV light

Two Types of Tails

Ion tail: Ionized gas The ion tail glows blue by emitting light when
pushed away from the elections re-combine with electrically charged
comet by the solar wind. ions to make uncharged molecules.
Pointing straight away
from the sun.

Dust tail: Dust set free The dust tail of a comet appears whitish-
from vaporizing ice in the yellow, because its microscopic dust particles
comet; carried away from reflect sunlight.
the comet by the sun’s
radiation pressure.
Lagging behind the comet
along its trajectory

Evolution of Comets

• Comets lose 0.5% to 1% of
its ice each perihelion
passage
• Over time comets lose all A NASA Hubble Space Telescope (HST) image
of comet Shoemaker-Levy 9, taken on May 17,

their ice leaving nothing 1994, with the Wide Field Planetary Camera 2

but rubble and dust called a (WFPC2) in wide field mode.
meteoritic swarm

• These particles orbit in the
comet’s orbit but now
spread out along it.

• Gravitational interactions
with planets can tidally
disrupt comets
(Shoemaker-Levy 9)

Short-Period Comets

• Orbital periods of less than 20
years, also called Jupiter-family
comets

• Orbit more-or-less in the ecliptic
plane in the same direction as the
planets.

• Example: Comet Tempel 1 and
Hartley 2

• These comets could not have
formed here… Where do they
come from? Kuiper Belt?

Intermediate-Period
Comets

• Orbital periods of The orbit of Halley's Comet is pretty typical. It has an
between 20 and 200 eccentricity of .967, meaning an extremely elongated
years and inclinations ellipse with the Sun very close to one end.
extending from zero to
more than 90 degrees

• As of 2012, only 64
Halley-type comets
have been observed

• Probably originate in an
outer icy asteroid belt,
the Kuiper Belt, beyond
Neptune.

The Origin of Long-Period Comets

Long period comets are believed to originate in the Oort cloud:

Spherical cloud of several trillion icy bodies, ~
10,000 – 100,000 AU from the sun.

Highly Gravitational influence
eccentric of occasional passing
orbits and stars may perturb some
periods orbits and draw them
ranging from towards the inner solar
200 years to system.
thousands or
even millions Interactions with
of years planets may perturb
orbits further,
Oort Cloud capturing comets in
short-period orbits.

Meteoroid Orbits

• Meteoroids
contributing to a
meteor shower are
debris particles, orbiting
in the path of a comet.
• Spread out all along
the orbit of the comet.
• Comet may still exist
or have been destroyed.

Only a few sporadic meteors are not associated with comet orbits.

Meteor Showers

• Meteor showers are
mostly caused by the
trails of dust and debris
left in the wake of a
comet. When Earth
passes through this
material we see a meteor
shower.

• Meteors are observed to
radiate from one point in
the night sky called the
radiant.

• Zenith hourly rate varies
between several meteors
per hour to over 100 per
hour depending on the
shower.