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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 9:30

Lecture 42:
Asteroids & Meteorites

Key Ideas:



Discovery of Asteroids

The gap between the orbits of Mars (1.5 AU) and Jupiter (5 AU) with no planets:

Jan 1 1801:

About a hundred more objects smaller than Ceres, but in orbits between Mars and Jupiter, were found by 1872.

The Asteroid Belt

By 2006, telescopic surveys have found:

(Current list of Minor Planet Names at the Harvard-Smithsonian Center for Astrophysics)

Could be as many as 1.1-1.9 Million asteroids larger than 1km in diameter in the Solar System.

90% of asteroids are in the "main belt":

Some Named Asteroids:

1 Ceres
2 Pallas
Asteroids visited by spacecraft:
243 Ida
253 Mathilde
433 Eros (NEAR landed on 2001 Feb 12)
951 Gaspra
25143 Itokawa (Hayabusa sample return mission)
1814 Bach
1815 Beethoven
1818 Brahms
4147 Lennon
4148 McCartney
4149 Harrison
4150 Starr
2620 Santana
4305 Clapton
3834 Zappafrank
4442 Garcia

Kirkwood Gaps

Gaps in the asteroid population are found at particular mean motions resonances with Jupiter:

Also get confining resonances:

Both types of orbit groups are caused by the slow inward migration of Jupiter during the late stages (last 10-100Myr or so) of the formation of the Solar System. Jupiter is estimated to have moved inwards a few tenths of an AU during this time. As Jupiter moved in, asteroids got swept into the resonant orbits and trapped. The gaps we see today in the main belt are thus an artifact of that ancient epoch of planetary migration. We'll see something similar to this in the Kuiper Belt of the outer solar system in the next lecture.

Families of Asteroids

Asteroids are grouped into "Families" distinguished by having similar orbits (semi-major axis, direction of orbit, and inclination). Examples include the resonant families noted above (Hildas, Floras, etc.), and these important families:

Trojan Asteroids:

Earth-crossing Asteroids:

Sizes of Asteroids

Largest asteroid is Ceres:

The rest are much smaller:

Overall, about 50% of the mass of the entire main belt is in the 4 largest asteroids (1 Ceres, 4 Vesta, 2 Pallas, & 10 Hygiea).

Shapes of Asteroids

Asteroids are irregular in shape:

Rotate as they orbit:

Composition of Asteroids

Classify asteroids by their spectral colors:

There are also a few oddball types that fall outside the three main classes (like 4 Vesta).

Monoliths or Rubble Piles?

Some asteroids are clearly solid chunks of rock & metal:

Others appear to be "Rubble Piles"

The rubble pile asteroids were probably formerly solid, but got shattered by impacts.

Examples of rubble pile asteroids with unusually low densities for their composition are 253 Mathilda (~1.3 g/cc) and 25143 Itokawa (~2.3 g/cc).

Clues to Asteroid Origins

Silicate- and Iron-rich asteroids are probably fragments of larger, differentiated bodies:

Carbonaceous asteroids may be the remnants of more primordial material that never got differentiated.


Chunks of rock & iron smaller than asteroids orbiting the Sun.
Sizes range from grains to 100 meters across.
Streak of light when a meteoroid enters the Earth's atmosphere. Most are tiny grains.
Any remnant that reaches the ground intact.

Types of Meteorites:

Stony Meteorite: (92%)
Composed mostly of silicate rock.
Probably fragments of S-type asteroids
Iron Meteorite: (6%)
Composed almost entirely of iron.
Probably fragments of M-type asteroids.
Carbonaceous Chondrites: (rare)
Carbon-rich, with complex carbon compounds.
Probably fragments of C-type asteroids.

Origin of Meteorites

Orbits of some meteors have been traced back after their entry into the atmosphere:

Meteorites are among the oldest rocks in the solar system (radioactive ages of 4.6 Gyr), and are thus the leftovers from the formation of the solar system.

Supplement: Meteor Impacts

I include this because lots of people ask about meteor impacts, especially the big ones that do damage, but the topic is a little more than I can cover in my lecture.

About 100 tons of meteoroids hit the Earth each day:

Rare large meteoroid or asteroid strikes:

Even a modest meteor strike could devastate a large city (although the Earth is mostly covered in oceans and empty areas, so the chances are smaller than getting clobbered by earthquakes, hurricanes, tornados and other more familiar natural disasters). This has led to some concern about tracking near-Earth asteroids and large rocks, with a consequent increase in the number of asteroids cataloged.

How bad could it really be? Visit the Earth Impact Effects Program provided by the University of Arizona Lunar & Planetary Lab for an interesting quantitative analysis. You pick the impact parameters and it calculates the results.

Readings in Universe: Chapter 17, sections 17-1 through 17-6
Return to [ Unit 6 Index | Astronomy 161 Main Page ]
Updated: 2006 November 24
Copyright Richard W. Pogge, All Rights Reserved.