1997

253 Mathilde

The NASA Galileo mission’s flybys of the main belt asteroids 951 Gaspra and 243 Ida in 1991 and 1993 revealed that even small planetary bodies can have interesting surface features (and moons of their own), spurring interest in a dedicated space mission to study asteroids up close. The first such project, NASA’s Near Earth Asteroid Rendezvous (NEAR) mission, was launched in 1996 to conduct a study of the Earth-approaching asteroid 433 Eros. Along the orbital path toward its encounter with Eros, NEAR acquired some “bonus” science in 1997 by flying past the large main belt asteroid 253 Mathilde.

Mathilde, discovered in 1885 by the prolific Austrian astronomer and asteroid hunter Johann Palisa, orbits along an elliptical path between Mars and Jupiter. More recent telescopic observations show that Mathilde is quite dark—almost black as coal—reflecting only about 4 percent of the incident sunlight. This low albedo (ratio of reflected light to incident sunlight), along with its relatively gray color and featureless spectrum, led astronomers to classify Mathilde as a C-type (comparable to carbon-bearing meteorites) asteroid, in a different class from the S-type (more like stony meteorites) asteroids Gaspra and Ida. Mathilde’s different spectral class and the fact that it rotates very slowly, taking more than 17 days to spin once on its axis, created some excitement about what NEAR might see.

The NEAR flyby of 253 Mathilde confirmed the asteroid’s low albedo and also revealed a number of surprises. Even though only about 60 percent of the asteroid was photographed, a good estimate of the asteroid’s volume combined with an estimate of its mass allowed scientists to estimate Mathilde’s density at about 1.3 grams per cubic centimeter. This surprisingly low value is much less than typical rock densities. Since the asteroid is likely too close to the Sun (too warm) to contain ice, the best alternative was to deduce that there is up to 50 percent empty space inside Mathilde, between large chunks of rocky material. Mathilde seems to be a porous, rubble-pile asteroid.

NEAR’s discovery of half a dozen giant impact craters on Mathilde support this idea. If the asteroid were a coherent rocky body, then any one of those impacts should have destroyed it; however, interior pore space absorbs impact energy, enabling the asteroid to remain intact in the face of otherwise devastating blows.

Ghostly view of the large C-type main belt asteroid 253 Mathilde, photographed by the NASA Near Earth Asteroid Rendezvous (NEAR) spacecraft during a June 1997 flyby. Mathilde is about 36 miles (60 kilometers) across, with a dark surface dominated by many large impact craters.