The stone seen in the photograph taken by the Galileo space is not any stone, but a meteorite that almost touched the Earth.
This asteroid is called Gaspra and the photo is taken at 16,200 km. As you will remember, last year the asteroid became a reader, very close to the Earth, 400 million km.
The size of this asteroid (20 x 13 km) is not very small and if it had touched the Earth would have caused both ecological damage and incredible materials.
To finish it must be said that Galileo's mission was to take pictures of the satellites of Jupiter, but as the asteroid reached half way...