The asteroid Gaspra has a surface magnetic field as strong as the Earth's, according to measurements made by NASA's Galileo spacecraft as it flew past in October 1991. In fact, the field strength is comparable to that of the most magnetic meteorites found on Earth.

Gaspra's magnetism indicates that the asteroid, which is 14 kilometres in diameter, cooled in a strong magnetic field, says Margaret Kivelson of the University of California at Los Angeles. This field could have been generated by the Sun, by a larger body of which Gaspra was a part, or by impacts on the surface of the asteroid.

One minute before Galileo reached its closest point to Gaspra, 1600 kilometres from the asteroid, the magnetometer the spacecraft was carrying sensed a change in direction of the surrounding magnetic field. Three minutes later, after the spacecraft had travelled 1300 kilometres, the surrounding field returned to its original orientation.

Any body orbiting the Sun will cause changes in the magnetic field around it because it perturbs the solar wind - the stream of charged particles flowing away from the Sun. But the asteroid alone should have disturbed a volume of space only a few tens of kilometres across. Kivelson told the American Geophysical Union in Baltimore last month that Galileo's readings are evidence that something much larger than the asteroid is disturbing the solar wind. She believes it is the magnetosphere produced by the asteroid's magnetic field.

This evidence for Gaspra's strong magnetic field is indirect but, according to Kivelson, 'the timing is very suggestive'. The field rotated 'just in the direction you would expect if something was slowing down the solar wind going by Gaspra', she says. Some stony iron and chondritic meteorites are known to be strongly magnetised. The spectrum of light reflected from Gaspra indicates that the asteroid is rich in metals, which could create a magnetic field.

Gaspra is the first asteroid to be studied magnetically. But at the end of August Galileo will be flying by another one, called Ida, and Kivelson hopes to measure the magnetic field of this asteroid as well.

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