Be glad that the asteroid dubbed “Florence” won't revisit Earth for many, many years: observations during its weekend fly-by revealed that the space-rock is so big it's captured two moons.
Sky-watchers have been waiting for Florence (officially 3122 Florence) for some time: it was first spotted in 1981 by astronomer Schelte Bus using Australia's Siding Spring observatory and named after the nurse Florence Nightingale. The asteroid is big and bright enough that even though it missed us by several million kilometres over the weekend, backyard astronomers geared up to photograph it.
At around 4.5km in size, NASA explained last week, Florence is the largest object to pass this close since the agency began its work detecting and tracking near-Earth asteroids.
And it's also got a high albedo, with Sky & Telescope saying it reflects around 20 per cent of the sunlight that strikes it.
To track the flyby, NASA used the 70-metre antenna at its Goldstone Deep Space Communications Complex and found an unexpected pair of moons, both estimated to be between 100 and 300 metres across. That makes the asteroid just the third discovered to have a three-object system.
The inner moon's orbit is estimated at around eight hours, and between 22 and 27 hours for the outer moon.
The radar images also provide our first close-up view of Florence itself. Although the asteroid is fairly round, it has a ridge along its equator, at least one large crater, two large flat regions, and numerous other small-scale topographic features. The images also confirm that Florence rotates once every 2.4 hours, a result that was determined previously from optical measurements of the asteroid’s brightness variations.
Further observations using Goldstone's Solar System Radar and the NSF Aricebo Observatory through September 8 will help estimate the asteroid's total mass and density, NASA says.
NASA reckons Florence won't come this close again until some time after 2500. ®