A typical asteroid discovery photo. The long streak is the path made by asteroid 2002MN. From the brightness of its trail, this asteroid is estimated to be 100 meters (about 300 feet) across. The asteroid passed within the orbit of the Moon in 1998, when this HST photo was taken, but its track wasn't noticed until 2002 (hence the start of its identification number). (R. Evans & K. Stapelfeldt (JPL), WFPC2, HST, NASA, apod050417)

One of the strangest-looking celestial objects ever observed, the Earth-orbit-crossing asteroid Itokawa, as imaged by the Japanese-made Kayabusa spacecraft in November, 2005. Most small solar system bodies are covered with ancient craters, or at least a smattering of craters; but virtually no craters are visible on one-third-mile long Itokawa. This leads to the idea that the asteroid may be a very loosely-agglomerated rubble-pile, perhaps the result of a "soft" collision between two even smaller rubble-piles, which is so sensitive to gravitational perturbations and collisions that material collapses into any depressions, removing any trace of their existence. Itokawa is one of a group of asteroids, the Apollo asteroids (named after the first such asteroid discovered), whose orbits cross those of the Earth and thus pose a threat of collision with the Earth, despite having orbits whose orbital semi-major axes are larger than that of the Earth. Earth-orbit-crossing asteroids whose orbital semi-major axes are smaller than that of the Earth are called Aten asteroids (also after the first such asteroid discovered). (ISIS, JAXA, apod051121)

Another view of asteroid Itokawa, showing a strangely smooth area, or "sea", perhaps due to settling of rubble during a collision that combined two smaller asteroids into a larger rubble pile. The Kayabusa spacecraft landed on one such area, and may have collected soil samples. Unfortunately, it is very uncertain whether the spacecraft worked properly, and it is currently experiencing communications and control problems which may prevent its return to Earth, with or without samples. (ISAS, JAXA, apod070422)

The asteroid 243 Ida, and its satellite, Dactyl, in a picture taken by the Galileo spacecraft, enroute to Jupiter, on August 28, 1993.. Dactyl is about one mile across, while Ida is about 35 miles long by 15 miles wide. Completely unknown and unsuspected a few years ago, asteroid moons are apparently quite common. Over half a dozen are already known, despite the difficulties involved in observing them. (NASA, JPL, Galileo Project, apod020630)

Pictures of Ida taken during Galileo's approach, showing its rotation. The asteroid rotates once every 4 hours 39 minutes. Time runs forward from right to left, and bottom to top (from small to large images). (NASA, JPL, Galileo Project)

The most detailed picture of Dactyl taken by Galileo. Taken about 4 minutes from closest approach to Ida, at a distance of 2400 miles. The moon is less than a mile across. The largest crater shown is almost 1/5 mile across, and more than a dozen large craters are visible. (NASA, JPL, Galileo Project)

A false-color image of asteroid 951 Gaspra. Colors have been greatly exaggerated to show small variations in appearance. Picture taken by the Galileo spacecraft, enroute to Jupiter, in 1991. Gaspra is about 12 miles long. (The Galileo Project, NASA, apod021027)

Radar images showing the shape and rotation of asteroid 216 Kleopatra. A relatively large main-belt asteroid, Kleopatra was discovered in 1880, but its shape remained unknown until these images were obtained in 2000. Approximately 60 miles wide and 135 miles long, Kleopatra has a very strange shape, suggestive of a violent collision history. Its surface reflects radar as though somewhat porous, suggesting that it may consist of a solid core overlain by a considerable amount of rubbleized material. (Stephen Ostro et al. (JPL), Arecibo Radio Telescope, NSF, NASA, apod000510)

     Four views of Ceres, obtained in 2004 with the Hubble Space Telescope. For a short time in the very early 1800's Ceres was thought to be a planet, despite its small size; but with the discovery of numerous other objects in the region between Mars and Jupiter, it gradually became clear that Ceres was merely the first (and largest, at 600 miles diameter) of a new class of small, or minor planets. Since they are so small that until recently, they could only be seen as dots, they eventually came to be called asteroids, meaning small objects that seem similar to stars, but are not.
     As it happens, Ceres is once again under consideration for planetary status, as the current IAU draft proposal for defining a planet would include any round object that orbits the Sun, and is 500 miles or larger in diameter. If all goes well, Ceres and Vesta (the fourth asteroid discovered) will be orbited by the Dawn spacecraft (in 2015 and 2011, respectively). (NASA, ESA, J. Parker (SwRI) et al., apod060821)

     A topographic map of Vesta, created with the Hubble Space Telescope, in 1997. As on topographic maps of the Earth, blue represents low areas, and red higher areas. The map shows a relatively rugged surface, highlighted by a single crater (bottom center) spanning the width of the asteroid. The central peak is a common feature in large craters.
     Vesta is the fourth asteroid discovered, and the largest in the inner portion of the asteroid belt. Its nearest rival, Pallas, is about the same size, but significantly less massive. It appears that Vesta is primarily made of rocks similar to those in the interior of the Earth, while Pallas has large amounts of carbon compounds, as well as rocky material.
     Vesta is thought to have had a tremendous collision about a billion years ago, which ejected large portions of its surface into space. Many smaller asteroids which have orbits similar to Vesta, and many meteorites, have optical properties similar to that of Vesta, suggesting that they are pieces of that asteroid. The HED meteorites are mostly pyroxene, which is a mineral commonly found in basaltic lava flows on Earth, and suggests the presence of volcanic activity on Vesta's surface prior to the impact which broke so many pieces off of it. No other asteroid is currently known to have had geological activity. Between that, and its relatively large size (a little over 300 miles diameter), Vesta has been considered as a planet candidate at various times; but the most recent IAU guidelines (still awaiting approval at this posting) would require objects to be considerably more spherical, and at least half again as large as Vesta, to qualify for planet status. (B. Zellner (GSU), P. Thomas (Cornell), et al., WFPC2, HST, NASA, apod060820)