Our Solar System

951 Gaspra

951 Gaspra orbits the Sun near the inner edge of the main asteroid belt between Mars and Jupiter.

orbit: 330,000,000 km from the Sun (average)
diameter: 18x11x9 km

Gaspra was named by its discoverer Neujmin for a resort on the Crimean peninsula. Consequently, many of the asteroid's craters have been named for resorts and spas worldwide.

Like 243 Ida, Gaspra is an S-type asteroid, believed to be composed of a mixture of rocky and metallic minerals.

The first of the handfull of asteroids that have so far been observed close-up, Gaspra was encountered Oct 29, 1991 by the Galileo spacecraft on its way to Jupiter (Galileo later visited 243 Ida and NEAR visited 253 Mathilda and 433 Eros).

Gaspra is a member of the Flora family of asteroids.

Gaspra's surface is covered with impact craters. From the number of small craters on its surface, we can estimate that Gaspra is about 200 million years old

243 Ida and Dactyl

243 Ida is a Koronis asteroid orbiting the Sun between Mars and Jupiter:

orbit: 428,000,000 km from the Sun (average)
size: 58x23 km

Ida was a nymph who raised the infant Zeus (Jupiter). Ida is also the name of a mountain on the island of Crete, the site of a classic shrine and the cave where Zeus was said to have been reared.

The second of the small number asteroids that have so far been observed close-up, Ida was encountered Aug. 28, 1993, by the Galileo spacecraft on its way to Jupiter.

Ida has a satellite! (It's the small spot to the right in the picture above.) It is the first natural satellite of an asteroid ever discovered. Provisionally designated "1993 (243) 1", it received the name Dactyl (and the permanent designation "(243) Ida I") in September 1994. The name is derived from the Dactyli, a group of mythological beings who lived on Mt. Ida and protected the infant Zeus. Other accounts are that the Dactyli are the children of the nymph Ida and Zeus.



Dactyl is about 1.6 x 1.2 km, surprisingly round for such a small body. It orbits Ida at approximately 90 km.

The discovery that one out of two asteroids observed up close is in fact a binary system has reinvigorated an old debate about the frequency of binary asteroids. But more data is needed before the controversy can really be resolved.

The application of Kepler's third law to Dactyl's orbit gives a rough estimate of Ida's mass and therefore its density. That value is somewhere between 2.2 and 2.9 grams/cm3 (or perhaps a bit higher), a loose range because Dactyl's orbit is only crudely known.

Ida was originally thought to be an S-type asteroid, like Gaspra, composed of nickel-iron and some silicates. But a density of 2.9 is too low for that. Instead, Ida could well have a composition like that of ordinary chondrite meteorites, which are primitive and largely unaltered.

Interestingly, while the spectra of Ida and Dactyl are very similar they are nevertheless distinctly different; Dactyl is not simply a chunk of Ida. It is thought that the binary system may have formed during the collision and breakup that created the Koronis family.

The surfaces of Ida and Dactyl are heavily cratered and therefore apparently quite old. But dynamical calculations indicate that the whole Koronis family is relatively young. Such calculations also indicate that objects the size of Dactyl may not be to survive for more than 100 million years or so. Perhaps the heavy cratering took place at the time of the breakup that created the Koronis family rather than the 4 billion years ago as is usually the case for such surfaces.

Galileo measured variations in the solar magnetic field as it passed by Ida (a similar effect was found at Gaspra). This indicates the Ida must contain some magnetic material, though its density is far too low for it to be similar in composition to an iron or stony-iron meteorite.

It seems that many other asteroids are also accompanied by tiny moons. 3671 Dionysus also apparently has a moon as does 45 Eugenia and 762 Pulcova as well as many smaller near-Earth asteroids.

253 Mathilde

253 Mathilde is a main belt asteroid with a relatively small perihelion (1.94 AU)

orbit: 394,000,000 km from the Sun (average)
size: 59 x 47 km
Mathilde was discovered in 1885 by Johann Palisa. The name is thought to honor the wife of astronomer Moritz Loewy, then the vice director of the Paris Observatory.

The spacecraft NEAR made a flyby of Mathilde on 27 June 1997; NEAR's main mission to orbit the asteroid 433 Eros.

The most of the asteroids that have so far been encountered by spacecraft such as 433 Eros, 243 Ida and Gaspra are S-type asteroids; Mathilde is our first look at a C-type. C-type asteroids are believed to be the source of carbonaceous chondrite meteorites.

Mathilde has at least 5 craters larger than 20 kilometers in diameter (and we only got to see a little more than half its surface). Ida and Gaspra do not have such large craters. It is not clear how such large craters can be produced on such a small body.

Mathilde's density is only 1.4 gm/cm^3. It is probably very porous, somewhat like styrofoam. This may help account for the large craters.

It's albedo is only 4%. Furthermore, its surface color is very uniform despite the deep craters. This indicates that its interior is homogeneous, perhaps a pristine sample of the early solar system.

Another oddity is that Mathilde's rotation rate is very slow, 17.4 days. Perhaps this is somehow due to the many large impacts it obviously suffered

433 Eros

433 Eros is an S-type asteroid orbiting the Sun mostly between the orbits of Earth and Mars.


orbit: 172,800,000 km from the Sun (average)
size: 33x13x13 km


Eros was the Greek god of love and desire.

At a press conference on February 17, 2000, mission scientists for the Near Earth Asteroid Rendezvous mission exuded the air of kids in a candy shop as they discussed the latest results from asteroid Eros. After less than a week in orbit, NEAR has already returned dazzling pictures that have surprised and delighted researchers.

"At first I was stunned speechless by the beauty of this asteroidal landscape," said Mark Robinson, a member of the NEAR imaging team from Northwestern University. "Once I got over that, the geology took over." The first images from NEAR showed that Eros has an ancient surface covered with craters, grooves, layers, house-sized boulders and other complex features. "This is not just another rock floating out in space," continued Robinson. "There's a lot of neat geology going on." "There are tantalizing hints that the asteroid has a layered structure, like a sheet of plywood." said Andrew Cheng, of the Applied Physics Laboratory at Johns Hopkins University, who serves as the NEAR mission's lead scientist. "These layers appear to be very flat and appear to run end-to-end. This could come about if Eros was once part of a larger body, perhaps a fragment of a planet."

This idea fits the general picture that scientists have of asteroids. Most are concentrated in a belt between the orbits of Mars and Jupiter. Asteroids are likely to be leftover pieces of a planet that tried to form 4.6 billion years ago when the solar system was young, but couldn't because of nearby Jupiter's disruptive gravitational field. Eros might be a fragment from a planetoid that coalesced long ago and later broke apart as a result of collisions with other asteroids. With NEAR in orbit, scientists now know that Eros's density is 2.4 grams per cubic centimeter -- about the same as the density of Earth's crust.

"With this new data, it now looks like we have a fairly solid object," says radio science team leader Dr. Donald Yeomans of NASA's Jet Propulsion Laboratory in Pasadena, CA. "There is no strong evidence that it's a rubble pile like Mathilde," the large asteroid NEAR passed and photographed in 1997, and which we just talked about.

Eros has a giant gouge which was of interest. Here's a close-up image.

Inside Eros' Giant Gouge This picture was taken from NEAR on February 15, 2000, while the spacecraft was passing directly over the large gouge that creates Eros's characteristic peanut shape. It is a mosaic of individual images showing features as small as 120 feet (35 meters) across. Although most of the asteroid is in shadow, we are able to see inside the gouge. Many narrow parallel troughs closely follow the shape of the gouge. Although they appear curvilinear from this view, they are most likely oriented parallel to the length of the asteroid. The strong lighting contrast along the terminator (the line separating day from night on Eros) makes it easy to see that most of the surface is saturated with impact craters. Inside the gouge, however, only smaller craters are present, indicating that the area within the gouge is younger than the surface along the terminator. This implies that the event that caused the gouge must have happened more recently than the formation of the rest of the surface of Eros.

But that was not the end of the NEAR mission. NEAR, then renamed NEAR-Shoemaker (after Gene Shoemaker) On Monday, 12 February 2001, the NEAR spacecraft touched down (yes, landed) on asteroid Eros, after transmitting 69 close-up images of the surface during its final descent. This was the first time humans landed a spacecraft on an asteroid! Watching that event was the most exciting experience of several asteronomer's lives.

Here is one of the final images:

The landing was a gentle one, cruising to the asteroid's surface at less than 4 mph. Afterwards the NEAR Shoemaker spacecraft was still communicating with the NEAR team at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md.

And here is the final image:

This last image snapped by NEAR Shoemaker was only 394 feet (120 meters) from the asteroid's surface and covered a 20-foot (6-meter) area. NEAR Shoemaker continued to send a signal to Earth, assuring the team that it had landed gently. The signal was identified by radar science data, and about an hour later was locked onto by NASA's Deep Space Network antennas, which continued to monitor the spacecraft 196 million miles from Earth. Later, NASA has given the go-ahead for the NEAR mission to collect data from the surface of Eros through Feb. 28 2001 tacking four days onto an extension granted after the NEAR Shoemaker spacecraft's historic landing on the asteroid last week. The extension gives NEAR Shoemaker's gamma-ray spectrometer additional time to observe the elemental composition on and below Eros' surface, and the NEAR team at least two more opportunities to download this information through NASA's heavily used Deep Space Network of antennas