Asteroid Florence, which recently passed quite close to Earth, was so large that it had two moons of its own.
Image by NASA/JPL-Caltech
On September 1, the asteroid came within 7 million kilometers of Earth. That may seem like a great distance, but relatively, that's quite close. Astronomers first discovered the asteroid in 1981 and named it after Florence Nightingale. According to calculations, this is the closest that Florence has passed by Earth since 1890. The asteroid is also the biggest asteroid that has passed this close to Earth since 1995, when astronomers first began watching for asteroids we have close encounters with.
Florence is about 4.5 kilometers in diameter. Amazingly, the asteroid also came with a couple of friends—two small moons that were 100 to 300 meters across. To compare, our moon is 3,475 kilometers in diameter. Thus, Florence's moons were quite tiny, at least as far as moons go.
Florence and its two moons [Image by JPL-NASA Center for Near Earth Studies]
However small the moons were, their presence definitely raised a question or two. For one thing, is it normal for asteroids to have moons? And if so, how big should asteroids be in order to have moons?
Asteroids with moons aren't actually unheard of. There are about 16,400 known near-Earth asteroids, and about 60 of them have a moon. However, Florence has two moons, which makes it a triple asteroid. Triple asteroids are much more rare—so rare, that we only know of three near-Earth triple asteroids so far. One of them is Asteroid Florence. The others are Asteroid 1994 CC, which revealed its moons in 2009, and Asteroid 2001 SN263, which revealed its moons in 2008.
What makes Florence unique, however, is that it flew close to Earth. The other two triple asteroids haven't come this close, though they are near-Earth asteroids as well.
When should we expect Asteroid Florence again? The last time it came this close was a century ago, but we'll have to wait until the year 2500 for it to make a close shave again.
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