Pluto and Comets - Stevens Point

1

Lecture 16
Dwarf Planets and Comets
January 8a, 2014

2

Pluto -- Basic
Information

• Discovered by Clyde
Tombaugh in 1930

• Period: Porb = 248 years
• Distance: a = 39.5 AU
• 3 moons (Charon, Nix, Hydra)
• Demoted to Dwarf Planet in 2006

3

General Characteristics

• Mass = 0.0025 times the Earth

– Determined by using General form of Kepler’s 3rd Law

• Radius = 0.2 Earth

– Determined from eclipses of Charon

•  = 2300 kg/m3

ice and rock

• Little known due to its large distance from the
Sun.

• Pluto is tilted on its side.

4

Spin and Orbit

• Highly elliptical orbit (e = 0.25)

– Pluto is sometimes closer to the Sun than
Neptune

– Orbit is tipped 17° from ecliptic
– Aphelion = 49.3 AU
– Perihelion = 29.7 AU

• Both Pluto and Charon are tidally locked in
synchronous rotation.

– Pspin= 6.4 days (Pluto and Charon)
– Porb = 6.4 days (Charon)

5

Surface Properties

• Planet predominantly water ice
• Frozen methane detected on surface
• May have thin methane atmosphere

• May be similar to Triton

6

Surface Features

Pluto has never been visited by a spacecraft (the New Horizons probe is on its way and
will arrive in 2015) so there are no clear images of its surface. At left are Hubble Space
Telescope global maps of Pluto (smaller insets are actual images) that show bright and
dark areas visible as the dwarf planet rotates. At right is a composite image in true
color that is derived from eclipses by Charon.

7 If Pluto is sometimes closer to the Sun than
Neptune, why doesn’t it ever collide with
Neptune?

A. They do collide every few thousand years.

B. Neptune is primarily made of gases, so Pluto
would pass right through it.

C. Pluto’s orbit is steeply tilted with respect to
Neptune’s, so they never actually cross.

D. The synchronized timing of their orbit periods
ensures a collision never occurs.

8 If Pluto is sometimes closer to the Sun than
Neptune, why doesn’t it ever collide with
Neptune?

A. They do collide every few thousand years.

B. Neptune is primarily made of gases, so Pluto
would pass right through it.

C. Pluto’s orbit is steeply tilted with respect to
Neptune’s, so they never actually cross.

D. The synchronized timing of their orbit periods
ensures a collision never occurs.

9 http://photojournal.jpl.nasa.gov/catalog/PIA05567

Origins of Pluto

• Composition much more
like a moon

• Other objects similar to
Pluto (such as Sedna,
below) are being found in
the Kuiper Belt

http://photojournal.jpl.nasa.gov/catalog/PIA05568

10

Comparison of distant planets

Object Year Diameter Perhelion Aphelion
discovered (km) (AU) (AU)
Pluto
1930 2380 29.7 49.4
1250 41.9 44.9
Quaoar 2002 1800 76.1 942
2860 38.2 97.6
Sedna 2003 1400? 38.7 52.6
1500? 35.2 51.5
Eris 2005

2005 FY9 2005

2003EL61 2005

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Solar System Debris

• After formation of the Solar System, some
material was left over.

• Asteroids, comets, and meteoroids give
clues to composition of early solar system.

– Have undergone little processing (heating,
weathering).

12

Comets

• Made of ices and some rocky material
• Travel in very elliptical orbits about the Sun.

Comet
McNaught,
January
2007. Click
here for more
info.

13

• Long period comets

– May orbit once every million
years

– Origin in Oort Cloud --
spherical cloud up to
100,000 AU from Sun

• Short period comets

– Periods < 200 years
– Origin in Kuiper Belt -- disk

shape 30-100 AU from Sun.

14

Anatomy of a Comet

• Nucleus

– few km in diameter
– ices and rocky material (“dirty ice ball”)
– Only part of a comet that

exists away from the
Sun.

• Coma -- Gases
evaporated off of
surface of nucleus as
Sun heats it.

15 • Hydrogen envelope
• Tails -- Always point away from the Sun

– Dust tail -- small dust particles, slightly curved
in direction of orbit.

– Ion tail -- ionized molecules pushed straight
back by solar wind

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Neither tail trails
the comet like a jet
contrail. In fact,
the tail sometimes
leads the comet!

17

Suppose the density of a comet’s tail is 8.5×10−17
kg/m3 and it’s a cylinder 5×105 km in radius and
1.0 AU (1.5×108 km) long. What is the total mass of
the tail ?

A. 1.0×1013 kg

B. 5.0×1010 kg

C. 1.0×106 kg

D. 20,000 kg

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Suppose the density of a comet’s tail is 8.5×10−17
kg/m3 and it’s a cylinder 5×105 km in radius and

1.0 AU (1.5×108 km) long. What is the total mass of

the tail ?

A. 1.0×1013 kgm  V    r2L
B. 5.0×1010 kg   8.51017 kg/m3  

C. 1.0×106 kg 5.0 108  m 2 1.51011 m

D. 20,000 kg m  1.01013 kg

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Comet NEAT

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Comet Tempel 1 – Deep Impact

http://www.nasa.gov/mission_pages/deepimpact/multimedia/HRI-937.html

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Comet Tempel 1

• Deep Impact revealed the composition of the
comet Tempel 1

• Some of the expected constituents: silicates
(sand), water ice

• …and some surprises:

– Clay, and carbonates (how did these form without
liquid water?)

– iron compounds
– aromatic hydrocarbons

22

Halley’s Comet

•Nucleus almost
completely dark
•Period: 76 years
•Next Visit: 2061

23

Halley’s Comet

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Comet Shoemaker-Levy 9

• Comet struck Jupiter in
July 1994

• Original comet ~2-10
km in diameter

• Before impact it broke
into many small pieces

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Astronomers think that most comets come
from

A. interstellar space.

B. a region in the extreme outer parts of the
Solar System.

C. condensation of gas in the Sun’s hot
outer atmosphere.

D. material ejected by volcanic eruptions on
the moons of the outer planets.