Online Astronomy eText: Asteroids, Comets, and Interplanetary Debris
Pictures of Asteroids
(also see Ceres and Vesta)
Asteroid Toutatis near closest approach
Asteroid 4179 Toutatis passes less than a million miles from the Earth (Sep 29, 2004) (Image credit ESO)
A typical asteroid image shows a starry background, along with the trail made by an asteroid passing through the field of view. Usually, the asteroid is a hundred million miles or more away in the asteroid belt between Mars and Jupiter, and the asteroid trail is very short unless the exposure is relatively long. In this case the exposure was only a minute -- so short that the trail would have have looked just like a faint star save for the fact that the asteroid in question, 4179 Toutatis, happened to be passing very close to the Earth. The asteroid passed less than a million miles from the Earth (less than 4 lunar distances), a relatively close encounter for an object of its size (about 3 miles wide along the longest axis of its very irregular shape).

Discovery image of OPO9810b, with cosmic ray tracks
Asteroid OPO9810b as observed from a moving platform, at a distance of 155 million miles
An atypical asteroid discovery photo. The long curved arc 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 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). Over one hundred asteroids have been discovered using HST images such as this one. Note: The numerous short red or blue streaks in the image above are due to cosmic rays striking the detector while a red or blue filter was in use. The image below shows the original full-field image, with cosmic ray tracks removed. (Image Credit above, R. Evans & K. Stapelfeldt (JPL), WFPC2, HST, NASA; for the "cleaned-up" image below, credit is the same but the original is here)
Discovery image of OPO9810b without cosmic ray tracks

Asteroid P/2010 A2 (Image Credit NASA, ESA, D. Jewitt (UCLA), apod100203)
 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 (probably about a year before the object was discovered) had a "hypervelocity" impact, and the "tail" is dust scattered along its orbital path by light pressure (sunlight bouncing off dust particles in the same way that the dust tail of a comet is created), 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.

Asteroid Itokawa (Image Credit & © ISIS, JAXA)
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. Instead, the surface appears to consist of a mixture of dust, dirt and rugged boulders (note the long shadow cast by a large shard sticking out of the asteroid near bottom center of the image). 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). Note: Often, images posted on this site are slightly altered to enhance their web visibility, but ISAS/JAXA policy forbids such alterations, even by non-commercial sites such as this one. So the image above is posted exactly as it was shown on the Astronomy Picture of the Day site on Nov 21, 2005, the only change being to reduce its size so it fits the smaller "window" on this page.

Another view of asteroid Itokawa (Image Credit & © ISAS,
Another view of asteroid Itokawa, showing a strangely smooth area or "sea", probably due to settling of rubble during a collision that combined two smaller asteroids into a larger rubble pile, and subsequent jostling of the surface by smaller impacts (such jostling tends to separate small particles such as dust and microscopic particles from larger ones such as boulders and building-size chunks). The Kayabusa spacecraft landed on one of the relatively smooth areas, and "kicked" the surface to stir up some dust and hopefully collect soil samples in an interior compartment. The process slightly damaged the spacecraft, making it impossible to follow a relatively direct return path to Earth, as originally planned; but fortunately, a long-term orbital maneuver was accomplished that successfully returned the spacecraft and about 1500 microscopic grains of the asteroid to the Earth, in June 2010. As of this date (early 2011), analysis of the grains is still ongoing, but they are rich in olivine and pyroxenes and other minerals commonly found in meteorites, and not at all like typical Earth rocks, so they are definitely part of the asteroid. Final release of the analysis of the grains is expected to take place in 2012. Note: Often, images posted on this site are slightly altered to enhance their web visibility, but ISAS/JAXA policy forbids such alterations, even by non-commercial sites such as this one. So the image above is posted exactly as it was shown on the Astronomy Picture of the Day site on April 22, 2007, the only change being to reduce its size so it fits the smaller "window" on this page.

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)

A digital representation of a 3D model of the asteroid Eros, using a mosaic of images from the NEAR spacecraft (NEAR Project, NLR, JHUAPL, Goddard SVS, NASA, apod090607)

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 that 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)

Asteroid 5535 Annefrank. Photographed in early November, 2002, by the STARDUST probe enroute to Comet Wild 2. A relatively dark main belt asteroid, about 3 1/2 miles along its greatest diameter. (STARDUST Team, JPL, NASA, apod021113)

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)