Gaspra, the first asteroid encounter
Galileo spacecraft, en route to Jupiter, flew by the asteroid (951) Gaspra at 22:36:46 UTC on October 29, 1991. The closest approach was 1,601 km, and the relative speed was about 8 kilometers per second. It was the first ever asteroid encounter by a spacecraft. In all, 57 images of Gaspra were taken by Galileo's SSI camera, covering about 80% of the asteroid, the closest taken from a distance of 5,300 km. The best images have a resolution of 54 meters per pixel.
The objectives of the encounter included determining the size, shape, cratering characteristics, and composition of the asteroid, as well as surveying the surrounding environment. Due to the failure of the spacecraft's main antenna, the results of the observations were recorded on the magnetic tape of the on-board recorder and most of images taken were not transmitted to Earth until November 1992, when Galileo was once again nearing Earth.
The imagery revealed a cratered and irregular body, measuring about 18.2 km × 10.5 km × 8.9 km. Its shape was not remarkable for an asteroid of its size. While Gaspra has plenty of small craters — over 600 of them ranging in size from 100 to 500 meters — it lacks large ones, hinting at a relatively recent origin. However, it is possible that some of the depressions were eroded craters. Perhaps the most surprising feature was several relatively flat planar areas. Measurements of the solar wind in the vicinity of the asteroid showed it changing direction a few hundred kilometers from Gaspra, which hinted that it might have a magnetic field, but this was not certain.
Gaspra is an S-type asteroid that orbits very close to the inner edge of the asteroid belt and is a member of the Flora family. It was discovered in 1916 by Russian astronomer Grigory N. Neujmin (1886–1946) in Crimea, and is named after Gaspra, a Black Sea retreat. Gaspra orbits the Sun at the distance of approximately 2.21 au, with a perihelion 1.83 au and an aphelion 2.59 au. The orbital period of Gaspra is 3.29 years. Gaspra's irregular shape and the prominence of grooves, linear depressions 100-300 m wide and tens of meters deep, suggest that the asteroid was derived from a larger body by catastrophic collision. Analysis of cratering rates suggests the age of the surface is between about 20 to 300 million years.
This montage of 11 images taken by the Galileo spacecraft as it flew by the asteroid Gaspra on October 29, 1991, shows Gaspra growing progressively larger in the field of view of Galileo's solid-state imaging camera as the spacecraft approached the asteroid. The earliest view (upper left) was taken 5 3/4 hours before closest approach when the spacecraft was 164,000 kilometers from Gaspra, the last (lower right) at a range of 16,000 kilometers, 30 minutes before closest approach. Credit: NASA/JPL
Galileo spacecraft was launched aboard Space Shuttle Atlantis, mission STS-34, on October 18, 1989. The mission consisted of two spacecraft – the orbiter became the first spacecraft to be placed into orbit around Jupiter, and the atmospheric probe was the first to enter the planet's atmosphere. Galileo was the first spacecraft to approach asteroids – (951) Gaspra and (243) Ida. During its mission, Galileo also made observations of Venus, Earth, the Moon, and comet Shoemaker-Levy 9. After gravitational assist flybys of Venus and Earth, Galileo arrived at Jupiter on December 7, 1995.
References:
Meltzer, Michael (2007). Mission to Jupiter: A History of the Galileo Project (PDF). The NASA History Series. Washington, DC. NASA
© 2025, Andrew Mirecki
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