Magazine article Science News

Three's Company: Asteroid 87 Sylvia and Her Two Moons

Magazine article Science News

Three's Company: Asteroid 87 Sylvia and Her Two Moons

Article excerpt

Among the thousands of asteroids roaming the inner solar system, 87 Sylvia stands out. New observations reveal that two smaller asteroids orbit this 280-kilometer-wide rock. It's the first asteroid found to be accompanied by two moons.

More than just a curiosity, the tiny moons have enabled researchers to determine the mass and density of 87 Sylvia. Like some other asteroids, the rock turns out to be extraordinarily porous, with up to 60 percent of its interior composed of empty space, report Franck Marchis of the University of California, Berkeley and his colleagues in the Aug. 11 Nature. This suggests that 87 Sylvia formed when two much larger asteroids smacked into each other and broke apart.

Most of the fragments from the proposed breakup reassembled into a loose agglomeration held together only weakly by gravity. Planetary scientists call such a fragile coalescence a rubble pile (SN: 7/28/01, p. 61). The two moons are probably leftover debris from the same collision that produced 87 Sylvia, Marchis' team suggests. The astronomers also described their findings this week at the Asteroid Comet Meteor conference in Armacao dos Buzios, Brazil.

Marchis and his colleagues became interested in 87 Sylvia, one of the largest rocks in the asteroid belt between Mars and Jupiter, after another team announced in 2001 that it had discovered a single moon circling the asteroid. That moon measures about 18 km across and orbits 1,360 km from 87 Sylvia.

Scientists know of 60 or so moon-bearing asteroids. …

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