Asteroids

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There are thousands, even millions, of small rocks that orbit the Sun, most of them between the orbits of Mars and Jupiter. A plot of the known asteroids is available at the Minor Planet Center. About one million of them are larger than 1 kilometer across. Those smaller than about 300 kilometers across have irregular shapes because their internal gravity is not strong enough to compress the rock into a spherical shape. The largest asteroid is Ceres with a diameter of 1000 kilometers. Pallas and Vesta have diameters of about 500 kilometers and about 15 others have diameters larger than 250 kilometers. The number of asteroids shoots up with decreasing size. The combined mass of all of the asteroids is less than the Moon's mass. Very likely the asteroids are pieces that would have formed a planet if Jupiter's strong gravity had not stirred up the material between Mars and Jupiter. The rocky chunks collided at speeds too high to stick together and grow into a planet.

Though there are over a million asteroids, the volume of space they inhabit is very large, so they are far apart from one another. Unlike the movie The Empire Strikes Back, where the spacecrafts flying through an asteroid belt could not avoid crashing into them, real asteroids are at least tens of thousands of kilometers apart from each other. Several spacecraft sent to the outer planets have travelled through the asteroid belt with no problems.

There are three basic types of asteroids:

1. C: they are carbonaceous---made of silicate materials with a lot of carbon compounds so they appear very dark. They reflect only 3 to 4% of the sunlight hitting them. You can tell what they are made of by analyzing the spectra of sunlight reflecting off of them. This reflectance spectra shows that they are primitive, unchanged since they first solidified about 4.6 billion years ago. A sizable fraction of the asteroids are of this type. The asteroid called Mathilde, explored by the NEAR spacecraft is an example of this type (see picture below).
2. S: they are made of silicate materials without the dark carbon compounds so they appear brighter than the C types. They reflect about 15 to 20% of the sunlight hitting them. Most of them appear to be primitive and they make up a smaller fraction of the asteroids than the C types. Gaspra and Ida, explored by the Galileo spacecraft on its way to Jupiter, are examples of this type (see picture below).
3. M: they are made of metals like iron and nickel. These rare type of asteroids are brighter than the S and C types. We think they are the remains of the cores of differentiated objects. Large objects were hot enough in the early solar system so that they were liquid. This allowed the dense materials like iron and nickel to sink to the center while the lighter material like ordinary silicate rock floated up to the top. Smaller objects cooled off quicker than larger objects, so they underwent less differentiation. In the early solar system, collisions were much more common and some of the smaller differentiated large asteroids collided with one another, breaking them apart and exposing their metallic cores.

Note their irregular shapes! Small bodies can have irregular shapes because their gravity is too weak to crush the material into the most compact shape possible: a sphere. Depending on the strength of the material of which they are made, the largest non-spherical asteroids (and moons) can have diameters of roughly 360 to 600 kilometers. Planets are much too large (have too much gravity) to be anything but round. Ceres, the largest asteroid, is large enough to be round and is now re-classified as a "dwarf planet" (along with Pluto, Charon, and Eris).

The Japanese Aerospace Exploration Agency has a spacecraft called Hayabusa enroute back to Earth after orbiting and landing on Itokawa, a small near-Earth asteroid only half a kilometer in length. Hayabusa collected at least one sample from the asteroid's surface and will return to Earth in June 2010. Below are images of Itokawa from Hayabusa when it was just 7 kilometers from the asteroid. It has a rough surface but very few impact craters. Itokawa is basically a rubble pile formed by the ejecta from a large impact on a larger object coming back together gravitationally.

Itokawa + 270 deg surface

Itokawa + 90 deg surface

In late September 2007, NASA launched the DAWN spacecraft to explore the two largest asteroids, Ceres (about 960 km in diameter) and Vesta (520 km in diameter), for about six months at each asteroid. Vesta will be explored from August 2011 to May 2012 and Ceres will be explored from February 2015 to July 2015. Below are the best pictures we have of these asteroids (the Vesta image is a 3D computer model derived from Hubble Space Telescope data) and how they compare to the much smaller Eros asteroid that was explored by the NEAR Shoemaker spacecraft. Its primary goal is to help us figure out the role of size and water in determining the evolution of the planets. Ceres is a primitive and relatively wet protoplanet while Vesta has changed since its formed and is now very dry. At nearly the same distance from the Sun, why did these two bodies become very different?

A few other asteroids have surfaces made of basalt from volcanic lava flows. When asteroids collide with one another, they can chip off pieces from each other. Some of those pieces, called meteoroids, can get close to the Earth and be pulled toward the Earth by its gravity.

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last updated: October 25, 2007