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William J. Merline (Southwest Research Institute) has reported in the International Astronomical Union Circular the discovery of a companion to main belt asteroid 121 Hermione. The object, discovered with the 10-meter Keck II telescope in Mauna Kea, Hawaii, appears to be about 13 kilometers across and has a projected separation from Hermione of 630 km. Hermione has a diameter of 209 km and is located at 3.4 times the distance from the Sun as the Earth. The discovery that Hermione has a large companion brings the number of binary asteroids close to two dozen which includes a number of Near Earth Asteroids.
Jean Luc Margot, an astronomer at the California Institute of Technology has suggested that binary Near Earth Asteroids, such as 2000 DP107 pictured opposite, form by disruption of asteroids due to spin. Many binary Near Earth asteroids spin at high speeds, close to those in which they would fly apart. Margot suggests that close encounters with planets could explain the high speed rotation and formation of binary asteroids.
Hermione, however, is a main belt asteroid which orbits between the planets Mars and Jupiter. Because of the large size of Hermione’s newly discovered companion it is unlikely, although possible, that it was captured as it passed by. It might have formed, however, due to a large collision since binary asteroids don’t form by magic, whatever their names.
More info: Asteroid 2000 DP107
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