An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 9:30
Asteroids & Meteorites
- 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
- 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
By 2006, telescopic surveys have found:
(Current list of Minor Planet
Names at the Harvard-Smithsonian Center for Astrophysics)
- ~330,000 asteroidal objects
- ~134,000 with good enough orbits to give official numbers
- 13,200 asteroids with official manes
- When you know its orbit, you can name it.
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:
- 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
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:
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.
- Asteroids confined to specific orbits.
- Occur at the 3:2 (Hildas), 7:2 (Floras), and 1:1
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:
- 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.
- 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 MEarth
- 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 MEarth
- 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
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?
Some asteroids are clearly solid chunks of rock & metal:
- Densities of 3-5 g/cc, like solid rock & metals
- Heavily cratered surfaces and dusty regoliths
Others appear to be "Rubble Piles"
The rubble pile asteroids were probably formerly solid, but got
shattered by impacts.
- 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.
- Chunks of rock & iron smaller than asteroids orbiting
- 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:
- 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.
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:
- Most are no bigger than grains of sand or smaller.
- Stony meteoroids <100 meters burn up before reaching
- Iron meteoroids <40 meters burn up.
Rare large meteoroid or asteroid strikes:
- Carve a crater out of the ground.
- Large asteroid strikes could disrupt climates and trigger
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
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Updated: 2006 November 24
Copyright © Richard W. Pogge,
All Rights Reserved.