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 Jupiter.

    • 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 solar system.

    • 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 major planet.

    • 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 asteroids.

    • 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 rotating uniformly.

    • 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