On September 5th, 2017 we learned about

Oversized asteroid’s unusual flyby reveals two miniature moons in orbit

Two new moons were just discovered in our cosmic backyard, although they won’t be there for long. The two unnamed bits of rock just careened past Earth on September 1st, coming as close as 4.4 million miles away. The reason for their hurry is that, as moons, orbit a larger body, which in this case is the asteroid known as 3122 Florence. That flyby was was probably the best view we’ll get of these new moons for at least 500 years, since even though Florence’s orbit around the Sun is close to our own, we don’t really share the same space all that often.

A close, but comfortable, look

Obviously, 3122 Florence isn’t as big an object as Mars, our Moon, or even the dwarf planet Ceres. Still, at around 2.7 miles across, Florence does stand out as being the biggest rock we’ve ever seen zipping by the Earth like this, although there are at least nine others in the “neighborhood” that we know of around this size.) There’s some comfort to be found in this amount of space between our planet and these neighbors though, as Florence is more than large enough to basically end life as we know it if it were somehow redirected to collide with Earth.

As predicted though, Florence maintained its course around the Sun, never getting closer than 18 times the distance to the Moon. This meant that nobody had to worry about saving the world, and could instead work on learning about this asteroid from the convenience of our own observatories. Before Florence’s arrival, nobody knew exactly what to expect, although a Moon was a definite possibility. The fact that the asteroid arrived as a triplet, that is, with two moons between 300 to 1,000 feet across was a bit of a bonus.

Convenient data collection

Seeing the relatively small objects with optical telescopes was tricky, since at those distances something 2.7 miles across doesn’t reflect all that much light. Instead, astronomers took advantage of the asteroid’s proximity, and collected data with radar imaging. They were able to pick up the orbits of the mini-moons, as well as get a hint of a bulge along Florence’s equator. More analysis is needed, but we already have estimates about the two moons’ orbits, thought to be around eight and 22 hours. Florence itself was seen to rotate rather quickly, finishing a turn ever 2.4 hours.

With the number of instruments pointed at our brief visitor, this encounter may yield more information in the coming months. While NASA is working on missions to catch up to asteroids further out in space, this was an unusual moment when the mission safely delivered itself to us.

Source: Asteroid Florence Has Two Moons by Kelly Beatty, Sky & Telescope

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