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Astronomy, p386, 1-20
1. What are the Trojan, Apollo, and Amor asteroids?
TROJAN asteroids reside at Sun-Jupiter L4 & L5
APOLLO asteroids: Asteroids whose paths cross Earth's orbit & whose semi-major axes exceed 1 AU
AMOR: Asteroids whose paths cross only Mars’s orbit
ATEN asteroids: Asteroids whose paths cross Earth's orbit & whose semi-major axes are 1 AU or less
(See Figure 14.1 on p360)
2. How are asteroid masses measured?
Scientists use spectroscopy to see what materials are on an asteroid’s surface, they measure the albedo of an
asteroid and how much heat it radiates. Also they note how the asteroid’s gravity affects other objects. These all
give clues to the asteroid’s size and mass.
3. How have the best photographs of asteroids been obtained?
Probe flybys are the source of our best images of asteroids. The Galileo probe went on a CRAZY ride before
reaching its intended destination, Jupiter. (After launch it passed Venus for a gravity boost, then got halfway to
Mars and came back to flyby earth for a gravity boost, then went out beyond Mars to visit/photograph asteroid
Gaspra, then came back to earth for a final gravity boost and was flung out to visit Ida & Dactyl in the Asteroid
Belt before continuing on to Jupiter.)
The NEAR (Near Earth Asteroid Rendezvous) probe orbited asteroid Eros for a year then actually landed on it
despite not being designed as a lander and having no landing gear.
4. What are the Kirkwood gaps? How did they form?
Areas in the Asteroid Belt that have fewer asteroids, caused by resonances with Jupiter's orbital motion. (The
Kirkwood gaps in the Asteroid belt are similar to the “gaps” in Saturn’s ring system in that they’re not totally
empty, just areas that have far fewer objects)
5. How do the C-type and S-type asteroids differ?
C-type are Carbonaceous (low reflectivity/albedo), and S-type are Silicate (rocky, more reflective).
As you move away from the Sun in the Asteroid belt the prevalence of C-type asteroids increases. (M-type
refers to metallic asteroids)
6. Are all asteroids found in the asteroid belt?
Most but not all. Some NEOs (Near Earth Asteroids) cross earth’s orbit and even Venus’s and Mercury’s.
Asteroids lying beyond Neptune’s orbit tend to be more icy and less rocky and are called Kuiper Belt Objects
(KBOs)
7. What are comets like when they are far from the Sun? What happens when they enter the inner solar
system?
- During most of a comet's orbit, far from the Sun, only its frozen nucleus exists. (It’s like one of Uranus or
Neptune’s small icy moons, only it’s orbiting the Sun instead of a planet. These moons would grow tails if they
ever got as close to the sun as comets get!)
- When a comet is 3-4 AU from the Sun, however, its icy surface becomes too warm to remain stable. Part of it
becomes gaseous and expands into space, forming a diffuse coma ("halo") of dust and evaporated gas around
the nucleus. The coma becomes larger and brighter as the comet nears the Sun. At maximum size, the coma can
measure 100,000 km in diameter—almost as large as Saturn or Jupiter.
- An ion tail and a dust tail can then become visible as the comet nears perihelion. These tails can be VERY
long, sometimes reaching 1AU or so in length!
8. Where in the solar system do most comets reside?
Short period comets (orbit Sun in < 200 years, aphelion is roughly at Neptune’s orbit) originate in the
Kuiper Belt, but most long period comets never reach the inner solar system and scientists think they reside in
an enormous spherical “cloud” surrounding the solar system 100,000 AU in diameter. (NOTE: this is 1.5 LY
and the nearest star to the sun is 4.3 LY away!) They call this the Oort Cloud.
9. Describe the various parts of a comet while it is near the Sun.
(see question 7)
10. What are the typical ingredients of a comet nucleus?
Mostly water ice, some ammonia and/or methane ice, CO2, and dust particles (“dirty snowball!”)
11. How do we know what comets are made of?
Scientists use spectroscopy to analyze a comet’s coma and its tail(s).
12. What are some possible fates of comets?
Sublimate totally (evaporate!); Crash into a planet or moon, pass too close to the sun and disintegrate or
evaporate.
13. What are two ways a comet’s orbit can change?
A comet can pass another comet, or also be deflected by a planet’s gravity.
14. Compare & contrast the Kuiper Belt and the Asteroid Belt.
SIMILAR:
- both lie in the plane of the Solar System (the ecliptic)
- Both contain small objects.
DIFFERENT:
Asteroid belt lies 2 – 3.3 AU from the Sun while the Kuiper Belt lies from 30-55 AU, well
beyond Neptune’s orbit.
KBOs tend to be icy, asteroid belt objects tend to be rocky.
15. Why has the number of planets in the Solar System recently decreased?
As Ceres was “demoted” from planet to asteroid as other asteroids were discovered, Pluto has been demoted
now that other “plutinos” and other Kuiper Belt objects are being discovered.
16. Explain the difference between a meteor, a meteoroid, and a meteorite.
METEOROID – object travelling though space
METEOR – object glowing as it enters earth’s atmosphere at high speed. (aka “shooting star”)
METEORITE – Meteor that reaches and impacts earth’s surface.
17. What causes a meteor shower?
When earth passes though a comet’s debris trail and more particles than usual interact with the earth’s
atmosphere.
18. What do meteorites reveal about the age of the solar system?
Using radiometric dating, we know that almost all meteorites are between 4.4 and 4.6 billion years old.
These numbers are nearly identical to the age of the oldest rocks found on earth.
19. Why can comets approach the Sun from any direction, whereas asteroids generally orbit close to the
plane of the ecliptic?
The Oort Comet cloud is spherical, while the asteroid belt and the Kuiper belt are each lie in the plane of
the solar system.
20. What might be the consequences if a 10-km-diameter meteorite struck Earth today?
- With an object that size we’d probably call it an asteroid impact instead of a meteorite impact
- Unprecedented splash-generated tsunami. (remember: average ocean depth is ≈ 4km)
- Mass extinctions if sunlight is obscured by ejecta circulating in the atmosphere.
- Beautiful, multicolored sunrises and sunsets if sunlight is only partially obscured by ejecta circulating
in the atmosphere.