"(45) Eugenia" is a main belt asteroid about 215 km in diameter, and is only the second asteroid found to have its own moon. The moon, temporarily known as S/1998(45)1, is about 13 km wide, and has an almost circular orbit 1190 kilometers out from Eugenia. This orbit takes 4.7 days to complete. From the orbit of the moon, Eugenia's mass has been calculated, and found to be only about 20% greater than water.

Two theories have been proposed to account for this low density. One is that Eugenia is an extremely porous conglomeration of rocks. It would have to be very porous indeed to have such a low density. The other theory is that the asteroid is mostly H2O ice, with only a light dusting of rock. When more evidence is found to prove one of these theories, it may give us information about the frequency of collisions in the asteroid belt, and hopefully some insight into the processes that shaped our solar system.

Eugenia's moon was discovered in 1999 by the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii, using an adaptive optics system to compensate for the blurring of Earth's atmosphere.

More information and graphics can be found at:
  • http://www.boulder.swri.edu/~merline/press_release
  • http://nssdc.gsfc.nasa.gov/planetary/factsheet/asteroidfact.html

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