Galileo entered the exploratory phase of its mission well
before its arrival at Jupiter when it flew by the small asteroids,
Gaspra and Ida. Their mean orbital distances from the sun are 2.2
astronomical units (AU) and 2.95 AU, respectively. The closest
approach to Gaspra [ Yeomans et al., 1993], a small,
irregularly shaped body of mean radius 
7 km, took place on
October 29, 1991 at 22:3640 UT at a distance of 1600 km, or about
230 Gaspra radii.
Because the flyby distance was so large, there was little expectation that a Gaspra-related signature would appear in the fields and particles data. Yet, remarkably, the magnetic field rotated through a rather large angle, the initial rotation occurring 1 minute before closest approach to Gaspra and the second 2 minutes after closest approach. Figure 7 shows the field vectors at different times and locations along the flyby trajectory projected into the plane of the upstream solar wind velocity (direction x, positive towards the sun) and the upstream solar wind magnetic field (direction y, negative in the direction of orbital motion). Kivelson et al. [1993b] report that the timing of the field perturbations and their sense are consistent with their being Gaspra-imposed disturbances of the solar wind magnetic field. During the 3 minutes between two rotations, the spacecraft moved 1300 km. The final rotation returned the IMF to a direction close to that in the pre-flyby solar wind.
The spatial extent and the magnitude of the perturbation are hard to explain in terms of any direct interaction between the tiny asteroid and the solar wind. Therefore, Kivelson et al. proposed that the solar wind perturbations must have been imposed by a Gaspra-associated interaction region larger than the asteroid itself. One can discount the possibility that the perturbation was imposed by a cloud of pick-up ions or charged dust grains. The gyroradii of ions picked up with the local solar wind velocity of 600 km/s is 3000 km if they are protons and much larger than that if they are heavy ions [the solar wind velocity was reported in the talk abstracted in Frank et al., 1993b]. With such large gyroradii, the observed sharp onset and termination of the perturbation would be impossible to achieve. Another possibility, which Kivelson et al. favor, is that Gaspra has a strong enough intrinsic magnetic field to stand off the solar wind and produce an asteroid magnetosphere. The model is illustrated schematically in Figure 7. Kivelson et al. find that a field large enough to deflect the solar wind around a magnetosphere of the required spatial extent implies a large asteroid magnetization. Indeed, the implied intrinsic magnetic moment per kg is comparable with that reported as remanent magnetization for some metal-rich chondrites and iron meteorites [ Sugiura and Strangway, 1988]. It is worth noting that Galileo's trajectory did not come close enough to Gaspra to measure its magnetic field directly, so this is an indirect inference. Nonetheless, the evidence that small asteroidal bodies may be highly magnetized has attracted the attention of planetary scientists especially because of its relevance to models of solar system origin.
From the perspective of plasma physics, the asteroid signature provides intriguing insight into a little-studied plasma phenomenon, the development of a wake-like structure in the vicinity of an object whose dimensions are too small to be in the magnetohydrodynamic regime. The asteroid, whose mean radius is intermediate between the gyroradii of electrons and ions, modifies the incident solar wind flow through signals carried in the whistler mode. As the phase and group speeds of whistler waves can equal or exceed the solar wind speed, they affect a spatial region that extends far off the sun-Gaspra line as illustrated in Figure 7. Greenstadt [1971 a and b] had speculated on the possible formation of whistler wings in the vicinity of a sufficiently small body, but before the Galileo flyby, none had been observed in a space plasma situation.