Astronomy 161 An Introduction to Solar System Astronomy Prof. Scott Gaudi

Lecture 42:
Asteroids, Meteoroids, Meteors, and Meteorites

Key Ideas:

• Small bodies in the inner solar system
• Asteroid Belt between Mars & Jupiter.
• Orbits are strongly influenced by Jupiter.
• Made of rock, metal, or a mix of the two.
• Monoliths & Rubble Piles

Meteoroids:

• Tiny bits of rock and/or metal orbiting the Sun.
• Seen as meteors, or collected as meteorites.

Discovery of Asteroids

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

• Searches started in earnest in the late 1700s to find a planet in this region.

Jan 1 1801:

• Giuseppe Piazzi discovered Ceres at 2.8 AU
• Too small to be a planet (914 km diameter)

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

The Asteroid Belt

Over 300,000 asteroids have been found. Roughly 130,000 of these have good enough orbits to get official numbers. When you know the orbit, you can name it pretty much whatever you want within reason. There are over 10,000 asteroids with official names.

(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":

• Lies between about 2.1 and 3.2 AU
• Orbits can be tilted up to 15º (a few up to 30º)
• Some orbits are fairly eccentric (e=0.15)
• ~5 million km apart on average.

Some Named Asteroids:

Mythological:

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)

 

Musicians:

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:

• Example, gap at a=2.5 AU is the 3:1 resonance (3 asteroid orbits for every 1 Jupiter orbit)
• The Main Belt is between the 2:1 & 4:1 resonances.

Also get confining resonances:

• Asteroids confined to specific orbits.
• Occur at the 3:2 (Hildas), 7:2 (Floras), and 1:1 (Trojans) resonances.

Families of Asteroids

Trojan Asteroids:

• Lead & Follow Jupiter in its orbit by 60° (1:1 orbital resonance)
• Trojans are confined into stable orbits by the combined gravity of the Sun & Jupiter.

Earth-crossing Asteroids:

• Apollo & Aten groups
• Source of asteroids that could potentially hit the Earth.

Sizes of Asteroids

Largest asteroid is Ceres:

• 914 km across
• mass of 0.0002 M_Earth
• Large enough to be designated a "Dwarf Planet"

The rest are much smaller:

• Only ~100 are >140 km across.
• About 1.2 Million are >1 km across.
• Total mass in asteroids is only ~0.0006 M_Earth
• Enough for a small rocky body ~5% mass of our Moon.

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:

• Too small for gravity to make them spherical
• Even the largest, Ceres, is only semi-round.
• Have heavily cratered surfaces and a regolith of pulverized material.

Rotate as they orbit:

• Most have rotation periods of ~9 hours.
• Extreme range from <3 hours to many weeks, probably reflecting different collision histories.

Composition of Asteroids

Classify asteroids by their spectral colors:

• C-type: "Carbonaceous" - dark in color, composed mostly of carbon-bearing materials. ~75% of all asteroids.
• S-type: "Silicaceous" - reddish in color, composed mostly of silicates (stony or stony iron). 17% of all astroids. (~16% are S-type)
• M-type: "Metallic" - bluer than S-type, probably iron-rich.

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

Monoliths or Rubble Piles?

• Densities of 3-5 g/cc, like solid rock & metals
• Heavily cratered surfaces and dusty regoliths

Others appear to be "Rubble Piles"

• Lower in density (1-2 g/cc), but clearly composed or rock
• Loose aggregates of rock held together by mutual gravity

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:

• Parents were hot enough to differentiate into silicate mantles & iron cores.
• Got shattered by collisions into smaller pieces

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

Meteoroids

Meteoroids:

Chunks of rock & iron smaller than asteroids orbiting the Sun.

Sizes range from grains to 100 meters across.

 

Meteor:

Streak of light when a meteoroid enters the Earth's atmosphere. Most are tiny grains.

 

Meteorite:

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:

• Some originate in the main asteroid belt.
• Those making "meteor showers" are trails of debris left behind by passing comets.
• Rare meteors have been found that have been knocked off the Moon or Mars.

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.