12. Asteroids and meteorites
- There are billions of asteroids
in the main asteroid belt, located between the orbits of Mars and Jupiter.
- The asteroid belt is largely
empty space, and a spacecraft may safely travel through it.
- Hundreds of Trojan asteroids
circle the Sun in the same orbit as Jupiter. These asteroids are located near
the two Lagrangian points where the gravity of the Sun balances that of
- The Earth resides in a swarm of
asteroids. Many of these near-Earth asteroids travel on orbits that intersect
the Earth’s orbit, with the possibility of an eventual devastating collision
with our planet.
- Asteroids can be chaotically
shuffled out of certain orbits in the main belt, and redirected into the inner
- The asteroids are the pulverized
remnants of former worlds that failed to coalesce into a single planet.
- Groups of asteroids, known as
families, have very similar orbits. The members of each family are the
collision fragments of a larger object, which was itself much smaller than a
- The combined mass of billions of
asteroids is less than five percent of the mass of the Earth’s Moon.
- The largest body in the main
asteroid belt, 1 Ceres, and the first to be discovered there, is about 950
kilometers across and contains about one-third of the total mass of all the
- Unlike most, if not all, of the
other asteroids, 1 Ceres has a round shape, suggesting a differentiated
interior with a rocky core, thick mantle of water ice, and a dusty outer crust. Ceres has been designated a dwarf planet.
- The gravity of the large majority
of asteroids is too weak, and the mass too low, to pull them into a round
shape. Their irregular, chipped shapes have been formed by eons of collisions.
- The colors of sunlight reflected
from asteroids indicate that they formed under different conditions prevailing
at varying distances from the Sun.
- Roughly 75 percent of the
main-belt asteroids are the dark, black carbonaceous C-type orbiting the Sun in
the outer half of the belt; about 15 percent of asteroids are bright, red,
silicate S-type, residing on the sunward side of the main belt. Metalic, M, asteroids account for some of the remaining ones, and a few V asteroids are covered with volcanic basalt.
- Asteroids may be mined for
minerals or water.
- Periodic brightness variations
tell us that most asteroids are non-spherical objects spinning with periods of
a few hours.
- Radar images of asteroids reveal
diverse shapes and cratered surfaces, ranging from solid rock to loose rubble, as well as binary asteroids and even a triple one.
- The close-up view, obtained by
passing spacecraft, indicates that asteroids have been battered and broken
apart during catastrophic collisions in years gone by.
- Some asteroids are thought to be
rubble piles, the collected fragments of past collisions held together by
gravity; other asteroids are solid rocks of uniform internal composition.
Asteroids 253 Mathilde and 25143 Itokawa are rubble piles, whereas asteroid 433
Eros is a battered but solid rock.
- The Near Earth Asteroid Rendezvous, abbreviated NEAR, spacecraft was the first to orbit an asteroid and the first
to land on one. NEAR circled the
near-Earth asteroid 433 Eros for a year, examining it dusty, boulder-strewn
landscape in great detail, obtaining an accurate mass for the asteroid, and
showing that much of it is solid throughout.
- The Japanese Hayabusa spacecraft has orbited the small, near-Earth asteroid
25143 Itokawa, whose orbit crosses that of the Earth. This asteroid, just 535
meters in its longest dimension, has a lumpy, oblong shape composed of two
parts with rough terrain joined by a smooth region. It is a loose collection
of rubble held together by its weak gravity, with surface rocks sorted by
shaking and jostling. Hayabusa landed on the asteroid and may return a sample of it to Earth.
- The Dawn spacecraft, launched in 2007, is expected to orbit the
asteroid 4 Vesta from September 2011 to April 2012 and to orbit the dwarf
planet – asteroid 1 Ceres in 2015.
- Asteroid rotation periods range
from a couple of minutes to a few months, and some of them wobble instead of
- Meteorites are rocks from space
that survive their descent to the ground.
- The number of recovered
meteorites more than doubled when scientists discovered a large number of them
on the blue ice of Antarctica.
- A few meteorites may have been
blasted off the Moon or Mars, but most of them are chips off asteroids.
- The organic matter found in
meteorites predates the origin of life on Earth by a billion years; but the
meteoritic hydrocarbons are not of biological origin.
Copyright 2010, Professor Kenneth R. Lang, Tufts University