Asteroids (Minor Planets)

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

Discovering asteroids is out of control (in a good way). The first four asteroids were discovered in 1801, 1802, 1804 and 1807, then no more until 1845. Since 1847, there has not been a year without a discovery.

There were 100 by 1868, 200 by 1879, 500 by 1903, 1000 by 1923, and 2000 by 1960.

By then, the counting gets murky because asteroids are not assigned a number until their orbits are well determined. Asteroids 1998-2001 were discovered in 1938, 1973, 1960 and 1973, respectively. By 1981 the count was 3000; it was 4000 by 1989 and 5000 by 1987. Interspersed among these are often clusters of discoveries decades old that were finally rediscovered and tracked conclusively.

By 1997 the count was 10,000, 20,000 by 2000, and 30,000 the next year. By the end of 2002 it was over 65,000. Improved automated search techniques and satellite imagery, and just a touch of beginning to take the impact hazard seriously, account for the explosive rate of discovery. The count topped 100,000 in early 2005. As of 2008 it had passed 150,000.

The Asteroid Belt

asterid belt

A snapshot of the inner solar system.

Most asteroids really do form a well defined belt. The belt is densest from just beyond Mars to halfway between Mars and Jupiter, roughly 250 to 500 million kilometers. The orbits, for the most part are not extremely inclined to the ecliptic, though of course there are exceptions. We can estimate a thickness of about 100 million kilometers. So the volume of the asteroid belt is

Now, how far apart are the asteroids? If we estimate there are a million, that's 589,000 x 1012 cubic kilometers per asteroid. If we assume they're uniformly spaced (they're not), we can assume each asteroid occupies a cube. The cube root of 589,000 x 1012 cubic kilometers is 84 x 104 kilometers or 840,000 kilometers. If there are a million asteroids, they are still almost a million kilometers apart on the average. If there are ten million asteroids, they will still be almost as far apart as the earth and moon. The seven planetary missions that have passed through the asteroid belts (Pioneer 10 and 11, Voyager 1 and 2, Galileo, Cassini and New Horizons, plus the missions that have been sent specifically to comets and asteroids, have seen only a handful of asteroids, and then only by careful planning. From the surface of any given asteroid, you couldn't even see the next one.

The fact that the outer edge and inner edge are both fairly sharp is a good illustration of what "clearing" the vicinity of a planet means. While there are transient objects that pass close by major planets, orbits that are permanently too close are unstable.

Spacecraft Imagery

Gaspra was the first asteroid ever photographed close-up by the Galileo spacecraft in 1991. It measures 20 x 12 x 11 km (12.5 x 7.5 x 7 miles).
Ida was the first asteroid discovered to have a satellite, a 1-km object given the name Dactyl. It was viewed by the Galileo spacecraft in 1993, and measures 56 x 24 x 21 kilometers (35 x 15 x 13 miles)
Mathildewas the largest asteroid directly imaged befopre the Dawn mission, measuring 52 kilometers (33 miles) in diameter. It was viewed by the Near Earth Asteroid Rendezvous mission in 1997
Comparison of Mathilde, Gaspra and Ida
Comparison of a number of asteroids and comets.

Hubble Space Telescope Imagery

Vesta is oval with a large bump at the south pole, apparently the central peak of a huge impact crater.
These images show the largest asteroid, Ceres, at left and Vesta at right. Ceres at 1000 km in diameter is large enough to have assumed a spherical shape and qualifies as a dwarf planet. It is far enough from the Sun that it contains ice.

Radar Imagery

Geographos is the most elongated object yet observed in the solar system.
Castalia looks like two dinner rolls stuck together
Toutatis appears to be a double object, possibly a contact binary, a pair of asteroids held loosely in contact by gravity, perhaps with a layer of rubble in between.
Another view of Toutatis from a different angle.


Eros is one of the best-known asteroids that passes close to earth. NEAR (Near-Earth Asteroid Rendezvous) was designed to intercept and orbit Eros.
A series of pictures showing Eros' rotation.
Orbiting Eros involved some space firsts. Not only was it the first targeted asteroid encounter, it was the first orbiting of an irregular object, and the first of an object whose mass was not well known. The spacecraft handled much of the orbital maneuvering automatically.
End-on view of Eros
Once orbit was achieved, the spacecraft was maneuvered into progressively closer orbits.
Left and below: Eros' small gravity made it possible to direct the spacecraft to land gently even though it was not originally designed to do so. The picture at right below was taken from an altitude of 250 meters.

This was the first landing on an asteroid.


Asteroid 25143 Itokawa is only about 300 meters long, the size of an aircraft carrier. In 2005 it was visited by the Japanese spacecraft Hayabusa. Itokawa appears to be a loosely compacted mass of rubble.
Hayabusa landed on the asteroid twice in an effort to collect samples. The sample capsule is scheduled to land in Australia in 2010.
Hayabusa photographs its own shadow. The bright halo around it is called backscatter, and is seen whenever sunlight reflects from rough surfaces.


Anne Frank (Asteroid)

5535 Annefrank, named for the World War II memoir writer, was viewed by the Stardust mission in 2002. It is about 8 kilometers long.

Vesta from Dawn

The Dawn mission was the first mission to major asteroids, the first to use ion propulsion, and the first to orbit two separate objects. Vesta looks a lot like its Hubble images: oval with a large peak at one pole.

Here's a polar view of Vesta showing the large central peak. There are actually two overlapping craters. Also shown are some of the other asteroids imaged by spacecraft.

Image result for vesta

Ceres from Dawn

For all the technical virtuosity of the Dawn mission, Ceres is perhaps the most excruciatingly dull object ever imaged. Apart from a few craters with bright areas, probably salt, there's no stratigraphy, no tectonics. Maybe Saturn's Mimas rivals it, excapt Mimas has that giant crater. (Note the background stars are artistic license.)

Related image


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Created 13 January 1998, Last Update 11 January 2020