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No. 86: Mar-Apr 1993

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Cosmic Snowballs And Magnetic Asteroids

Genesis of a cosmic 'dust bunny'
The genesis of a cosmic "dust bunny". The nebulous meteor of July 29, 1970, as observed over Dover, England.
The great diversity of the debris swirling around the solar system is making life difficult for scientists trying to reconstruct solar-system history. At the high end of the density spectrum, we now have an asteroid that seems to be mostly metal (probably iron). This is the asteroid Gaspra, some 13 kilometers across, that the Galileo spacecraft encountered in August 1992 on its way to Jupiter. Scientists had not expected Galileo's magnetometer to flicker as it passed Gaspra at a distance of 1600 kilometers -- but it did. In fact, considering the inverse square law and Gaspra's small size, it was a magnetic wallop. Thus, Gaspra is the first known magnetic asteroid; and it is probably mostly metal.

(Kerr, Richard A.; "Magnetic Ripple Hints Gaspra Is Metallic," Science, 259: 176, 1993.)

At the low end of the density spectrum, we now find that Pluto's moon, Charon, and some of Saturn's moons have very low densities (1.2-1.4), meaning they are probably mostly water ice. Such density figures come from direct observation of these objects' volumes combined with mass estimates from their orbital dynamics.

(Crosswell, Ken; "Pluto's Moon Is a Giant Snowball," New Scientist, p. 16, November 21, 1992.)

Comment. How did this curious mix of ice and iron objects originate? Did some ancient collision demolish a planet with an iron core (like the earth"s) and an icy exterior?

From Science Frontiers #86, MAR-APR 1993. 1993-2000 William R. Corliss