Enhanced colour Galileo image of asteroid 951 Gaspra.
ASTEROID GASPRA
 
951 Gaspra was discovered by Grigoriy Neujmin in 1916 and is named after a holiday resort on the Black Sea. Gaspra's craters are named after famous Spa towns and health resorts around the world like Bath in England and Aix in France.
 
Orbit
Gaspra is a member of the Flora family, found in the inner part of the main asteroid belt. Gaspra's perihelion, or closest approach to the Sun, is 272 million kilometres, which is fairly close for a main belt asteroid. Its orbit is elliptical (0.17) and inclined slightly (4.1o) to the plane of the ecliptic. Travelling with an average orbital velocity of 20 kilometres per second, it takes Gaspra 3.28 years to complete one orbit of the Sun. Gaspra spins counterclockwise, and has a short rotation period - only 7 hours. Being a member of the inner main belt, Gaspra is most likely doomed. The crowded belt means Gaspra will one day collide with another asteroid, and will be broken into smaller fragments.
 

The orbit of Gaspra.
 
Physical properties
Gaspra is a small, angular, irregular-shaped asteroid. It measures 19 kilometres x 11 kilometres, and has a surface area of about 525 square kilometres. The mass, and density, of Gaspra is unknown.
 

Comparison between Gaspra and Earth.
 
Atmosphere
No atmosphere has been detected - Gaspra is unlikely to exhibit any outgassing and lacks the gravity to keep an atmosphere.
 
Surface
The Galileo spacecraft, en route to Jupiter, flew by Gaspra on October 29th 1991. It was the first spacecraft encounter with an asteroid. Galileo made a second asteroid rendezvous on August 28th 1993 with 243 Ida. Galileo's trajectory was specially designed to take it past these bodies and obtain close-up pictures of the surface
 

Galileo's first image of Gaspra was the first spacecraft image of an asteroid.
 
Galileo passed Gaspra at a distance of 1600 kilometres and imaged the asteroid in black and white and at a variety of wavelengths beyond human vision. The resulting composite images enable differences in surface colour to be seen not visible at optical wavelengths. Gaspra's rapid 7 hour rotation period meant that the whole surface could be imaged.
 

Images of Gaspra as Galileo approaches the asteroid.
 
Images from Galileo indicate that Gaspra is a fairly typical S-type asteroid. It is fairly bright, but in visible light Gaspra appears grey.
 

Gaspra appears grey in visible light.
 
As well as being colourless there is hardly any tonal variation. Enhanced colour images were made of Gaspra using high resolution greyscale images combined with lower resolution images taken at violet, green, and near-infrared wavelengths.The enhanced colour images reveal a reddish coloured surface, with blue ridges and crater rims, similar in appearance to Ida's. The reddish materials are thought to be due to weathering processes (solar radiation and micrometeorite bombardment) altering the physical properties of the surface and the way it reflects light. The blue regions are fresh rocks exposed by downslope movement of loose regolith. Larger craters overturn the uppermost layers depositing deeper materials on the surface. The reflectance spectra (amount of reflected light) and absorption at different wavelengths by these materials show that they are metal-rich silicate rocks like pyroxene and olivine.
 
Gaspra does not have any very large craters, but has many small ones. More than 600 have been counted. They typically measure one or two kilometres across. The number of small craters compared to larger ones is much greater for Gaspra than for other objects of similar size.
 

High resolution enhanced colour Galileo image of Gaspra.
 
Gaspra was probably once part of a much larger Solar System object, but broke off in a collision. The lack of very large craters indicates that the surface may be quite young, perhaps no older than 400 million years.Gaspra has grooves in its surface, the longest of which measure about 4 kilometres. The grooves are 100 to 300 metres wide and have a depth of tens of metres. The grooves appear to form two groups that cross each other. Prior to the Galileo flyby of Gaspra, such grooves had only be seen on the Martian moon Phobos.
 

The cratered surface of Gaspra as seen by Galileo 10 minutes before closest approach.
 
There are also shorter cracks in the surface which are about 100 metres long. They are too indistinct to determine their origin with any certainty. Some researchers suggest they are caused by tidal forces, but they may equally be the result of stresses set up by meteorite impacts or the remnants of structures formed on its parent body.
 

Highest resolution image of Gaspra showing evidence of grooves.