The asteroid Gaspra has a surface magnetic field as strong as the Earth鈥檚,
according to measurements made by NASA鈥檚 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鈥檚 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.
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Any body orbiting the Sun will cause changes in the magnetic field around it
because it perturbs the solar wind 鈥� 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鈥檚 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鈥檚
magnetic field.
This evidence for Gaspra鈥檚 strong magnetic field is indirect but, according
to Kivelson, 鈥榯he timing is very suggestive鈥�. The field rotated 鈥榡ust in the
direction you would expect if something was slowing down the solar wind
going by Gaspra鈥�, 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.
