Last week, Earth was buzzed by asteroid 3122 Florence, the largest space rock NASA has seen come this close since it began tracking near-Earth objects (NEOs). Using a 70-m (230-ft) antenna at the Goldstone Deep Space Communications Complex, NASA took the opportunity to study Florence close up, confirming its monster size and revealing that it has two tiny moons.

By NASA's count, there are more than 16,000 NEOs whipping around out there, but only 60 of them are known to have their own moons. And of those 60, Florence is just the third "triple asteroid," meaning it has two moons orbiting it.

Radar images taken between August 29 and September 1 revealed that the moons probably measure between 100 and 300 m (330 and 985 ft) across. The outermost moon orbits Florence roughly once a day, while the inner object whips around its parent body every eight hours, making it the shortest orbital period of any asteroid's moon known so far.

As for the asteroid itself, the radar images were able to pin down its size more accurately and reveal some of the topographic features on its face. Florence is slightly bigger than previous estimates: with a diameter of 4.5 km (2.8 mi), the rock would pose a serious threat to life on Earth were it on a collision course. It rotates on its axis once every 2.4 hours, and it's a relatively round rock, with a ridge running around its equator and two large flat areas broken up by at least one large crater.

On September 1, Florence's closest approach to Earth brought it to within a distance of 7 million km (4.4 million mi), or about 18 times the distance between Earth and the Moon. Since then, the asteroid has been drifting away from us, and it won't come this close again for at least another 500 years.

Ongoing analysis of the asteroid will try to examine Florence's terrain in more detail, narrow the window of the orbital periods of its moons, and help astronomers work out the rock's total mass and density.

Source: JPL Center for Near Earth Object Studies

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