A typical asteroid discovery photo. The long streak is the path made by asteroid OPO9810b. If the image had been taken from the ground, the path of the asteroid would be a straight line, relative to the background stars; but because it was taken by the Hubble Space Telescope, the orbital motion of the HST caused the path to be curved. The curvature of the path can be used to estimate the distance of the asteroid (the more curved, the closer it is to the Earth). Based on the curvature shown here, OPO9810b was about 87 million miles from the Earth, and 156 million miles from the Sun, at the time the image was taken. Assuming an albedo, or reflectivity, similar to that of other asteroids, its brightness (magnitude 18.7) can then be used to estimate its size -- about one and a quarter miles; however, the albedos of asteroids vary considerably, so the object could be considerably larger (if darker) or smaller (if brighter). Note: The numerous short red or blue streaks in the image are due to cosmic rays striking the detector, while a red or blue filter was in use. (Image Credit: R. Evans & K. Stapelfeldt (JPL), WFPC2, HST, NASA, apod050417)

     A comet-like asteroid, P/2010 A2, as observed by the Hubble Space Telescope on January 29, 2010. When discovered by the MIT LINEAR asteroid project on January 6, it was thought that this object was a comet (hence its designation as P/2010 xx, meaning a periodic comet discovered in 2010). However, subsequent study showed that it had a point-like nucleus (about 150 yards in diameter, based on its brightness) off to one side of its "coma" (as shown in the inset) and no gas. It is now believed that this is one of two asteroids which recently had a "hypervelocity" impact, and the "tail" is dust scattered along its orbital path by light pressure (sunlight bouncing off the particles), after the collision. Such a collision typically involves an impact velocity of more than 10 thousand miles an hour, and produces an impact energy at least thirty times greater than that of a rifle bullet. Filamentary structures in the "tail" are probably caused by clumps of gravel-sized debris falling apart, and dust from those clumps being blown backwards by the light pressure which created the tail (larger pieces, such as gravel-sized objects, are not significantly affected by light pressure).
     It has long been supposed that (rare) collisions between smaller asteroids are gradually grinding them down to smaller sizes (collisions involving larger asteroids may build them up, as their gravity causes much of the debris to fall back onto their surfaces), but this is the first time that an object has been observed which may be the result of such a collision. Early in the history of the solar system, collisions between small asteroidal bodies would have been at very low velocities, as they were all moving around the Sun in nearly the same direction, at nearly the same speed (as particles in the rings of Saturn do). But in the eons since then, gravitational perturbations by the planets (especially Jupiter) have given different asteroids very different orbits, so that instead of colliding at low velocities, they have high-velocity impacts, as did P/2010 A2 and its unknown impactor. (NASA, ESA, D. Jewitt (UCLA), apod100203)

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)

A closeup of (21) Lutetia, taken July 10, 2010 by the Rosetta spacecraft, en route to comet Churymov-Gerasimenko, which the spacecraft will reach in 2014. Lutetia has been a bit of a mystery compared to other asteroids in its size range (about 60 miles diameter). It has a reflection spectrum similar to that of metallic asteroids, but also shows characteristics of carbonaceous bodies. It is the largest asteroid which has been visited by a spacecraft to date, and detailed analysis of the Rosetta images and data should considerably increase our knowledge of this object. (Credit: ESA OSIRIS Team MPS/ UPD/ LAM/ IAA/ RSSD/ INTA/ UPM/ DASP/ IDA The Planetary Society)

Two years earlier (in 2008), Rosetta passed four mile wide asteroid (2867) Stein. The image above shows opposite sides of the small asteroid. (Credit: ESA OSIRIS Team MPS/ UPD/ LAM/ IAA/ RSSD/ INTA/ UPM/ DASP/ IDA, as shown on NASA's Planetary Remote Sensing page)

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)