There are millions of boulder-size and larger rocks that orbit the Sun, most of them between the orbits of Mars and Jupiter. Some asteroids called Trojan asteroids travel in or near Jupiter's orbit about 60 degrees ahead of Jupiter and 60 degrees behind Jupiter (gravity balance points between Jupiter and the Sun). Some asteroids have orbits that bring them close to Earth's orbit, some even crossing the Earth's orbit. These are called Near-Earth Asteroids (NEAs) and include some 1368 (as of January 2013) "Potentially Hazardous Asteroids" with the greatest potential of very close approaches to Earth. Comets that get near the Earth and NEAs are lumped together in Near-Earth Objects (NEOs). 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 about 25 times less than the Moon's mass (with Ceres making up over a third of the total). 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 and other space movies, 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. Spacecraft sent to the outer planets have travelled through the asteroid belt with no problems (and no swerving about).
There are several possible ways of grouping asteroids. Astronomers use schemes of varying complexities based on composition, orbit characteristics or some combination of both. Here, we will group asteroids into three basic types based on their composition:
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, Eris, Haumea, and Makemake).
The Japanese Aerospace Exploration Agency launched a spacecraft called Hayabusa that returned to Earth after orbiting and landing on Itokawa, a small near-Earth S-type asteroid only half a kilometer in length (535 x 294 x 209 meters). Hayabusa collected at least one sample from the asteroid's surface and returned 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. Its composition is very much like the chondrite meteorites with little iron or other metals.
Itokawa + 270 deg surface
Itokawa + 90 deg surface
Above is the near-Earth asteroid Toutatis imaged by the Chinese lunar probe Chang'e 2 from a distance of just 93 kilometers away, though the spacecraft got to within 3.2 kilometers at closest approach. Toutatis was about 6.9 million kilometers from the Earth when it passed by us in December 2012. Toutatis is 4.5 x 2.4 x 1.9 kilometers in size and like Itokawa, it is probably a rubble pile from the merging of two bodies. Toutatis passes close to the Earth approximately every four years. The next time it will pass even closer will be in November 2069 but at a still safe distance of 3 million kilometers (7.7 lunar distances).
In late September 2007, NASA launched the Dawn spacecraft to explore the two largest asteroids, Ceres (about 960 km in diameter) and Vesta (530 km in diameter), originally for about six months at each asteroid (Dawn's time at Vesta was extended to a full year). Vesta was explored from August 2011 to August 2012 and Ceres will be explored from February 2015 to July 2015. Below left are the best pictures we have of these asteroids (the Vesta image is from the Dawn spacecraft and the Ceres color image is from the Hubble Space Telescope) and how they compare to the much smaller Eros asteroid that was explored by the NEAR Shoemaker spacecraft. Below right is a comparison of the two asteroids with the two smallest planets in our solar system. The Dawn spacecraft's 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?
Some initial findings from Dawn about Vesta include: Vesta once had a subsurface magma ocean—when Vesta was almost completely melted. It has a differentiated interior or a crust, mantle, and core. Dawn confirmed that three classes of meteorites used by meteorite scientists (howardite, eucrite and diogenite meteorites) did in fact come from Vesta. Those types of meteorites account for about 6 percent of all meteorites falling on Earth, making Vesta one of the largest single sources for Earth's meteorites. Vesta does have some minerals that are chemically bound with water ("hydrated minerals") and much of the water was originally delivered as part of Vesta's formation over a small time-frame, rather than primarily as a result with comet collisions later in its history as what produced the water ice found in sheltered craters at the poles of the Moon and Mercury always shut off from sunlight. Vesta also has some very dark patches from impacts with carbon-rich asteroids.
Vesta's topography is steeper and more varied than originally expected. Some of the crater walls are almost vertical instead of sloped like those on the Earth, Moon, and other worlds. It has a huge central peak in one of its impact basins that is much higher and wider, relative to crater size, than the central peaks of craters found on other bodies in the solar system. Two of the largest impact basins on Vesta are much younger than the impact basins found on the Moon. That's geologically speaking of course. The youngest impact basin on Vesta is about a billion years old while the Moon's impact basins, including the ones making the dark, smooth maria are about 3.8 billion years old. Some of the craters have short, wide, straight gullies in the crater walls formed from flows of sand-like material and some gullies that are much longer, narrower, and curvier than would be formed from sand-like material flowing downward. Such sinuous gullies on the Earth are carved by liquid water and those on Mars are from water, carbon dioxide, or other mechanism. Clearly, the geology of Vesta is more complex than originally thought! More discoveries will happen as scientists pore over all of the data beamed back to Earth by Dawn.
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 if they are less than a meter in size, can get close to the Earth and be pulled toward the Earth by its gravity.
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last updated: March 25, 2013