Comets, Asteroids &
Meteoroids
Ch 21.5
Comets
• Small, icy body that orbits the Sun.
• Parts of a comet:
– Nucleus
– Coma
– Tails
– http://www.classzone.com/books/earth_scienc
e/terc/content/visualizations/es2706/es2706p
age01.cfm
Anatomy of a comet
• Nucleus
– Frozen water, frozen CO, frozen CO2, formaldehyde
& dust
– Evaporates as it nears the sun & flows outward
• Coma
– Outflowing gas & dust
– Appears bright because of gases & dust reflecting the
sunlight
• Tails
– Dust tail = dust swept from nucleus
– Plasma tail = solar wind sweeps it outward, so it
always points away from sun
Comet orbits
• Long-period orbits
– Longer than 200 years
– Comet Hale-Bopp (2400 years; passed by in
1997)
• Short-period orbits
– Shorter than 200 years
– Comet Halley (76 year orbit)
What happens to comets?
• 3 things that destroy them:
– Collision with planet or sun
– Erosion (loses ice & dust with each orbit)
– Breaking into pieces
Asteroids
•
•
•
•
•
Orbiting rocks
Minor planets
250,000 asteroids whose orbit we know
Orbits vary in shape & size
Most are in asteroid belt at 2.1-3.3 AU
from sun
Asteroids
• Largest & also first discovered = Ceres
• 1000 km diameter
Trojan asteroids
• In Jupiter’s orbit around Sun
• Leading or trailing Jupiter
Meteors
• Meteor = bright streak of light
• Meteoroid = the debris itself
• Meteorite = piece that reaches the ground
• http://www.classzone.com/books/earth_sci
ence/terc/content/visualizations/es2707/es
2707page01.cfm
Awesome meteor facts:
• Traveling at 11 km/sec
• Heats up, melts & vaporizes = glow that
we see
• 8 – 10 meteors can be seen each hour on
a regular night
Meteor Showers
• Times when the numbers of meteors rises
to 15-100 /hour
• These are predictable ~ 10x/year as Earth
crosses orbit of meteoroids
Meteorites
• Rare
• Chances of being struck by a meteorite
more massive than 0.1 kg are 1:10 billion *
• * p 346 Astronomy by John D. Fix
Types of meteorites
• Stony
– Rocky material
• Metallic
– Iron & nickel
• Stony-iron
– Rocky material, iron & nickel