spaceSpace and Physics

Huge Asteroid That Flew Past Earth Had Two Hidden Moons


Jonathan O'Callaghan

Senior Staff Writer

Graphic of Florence flying past Earth on September 1. NASA/JPL-Caltech

A few days ago, the largest asteroid in more than a century flew past Earth. During the event, astronomers made a surprising discovery – the asteroid had two previously unseen moons.

Called 3122 Florence, named after Florence Nightingale, the asteroid measured about 4.5 kilometers (2.8 miles) across. Although many asteroids fly past Earth every year, this was the biggest to fly near Earth since 1890.


Florence flew past at a distance of about 7 million kilometers (4.4 million miles). That’s about 18 times the distance between Earth and the Moon, so thankfully it posed no threat to us. It’s a reminder, though, of the number of potentially devastating asteroids that pass our planet every year.

Using radar images captured by NASA’s Goldstone Deep Space Communications Complex, scientists found two extra objects orbiting with the asteroid as it swung past. Their sizes aren’t certain, but it’s thought they’re both about 100 to 300 meters (300 to 1,000 feet) across.

The asteroid and its two moons. NASA/JPL-Caltech

We also don’t yet know how long it takes each moon to orbit Florence, but it looks like the inner moon takes about eight hours, and the outer moon takes about 22 to 27 hours. The former is the shortest orbiting moon in the 60 near-Earth asteroids known to have moons.

This is only the third asteroid with two moons we’ve found in the 16,400 near-Earth asteroids so far. The last “triple asteroid” was seen back in June 2009, called asteroid 1994 CC.


“The radar images also provide our first close-up view of Florence itself,” said NASA in a statement.

“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.”

Florence, discovered in 1981, is now continuing its journey away from our planet. It’s hoped that radar data from other observations may reveal more about the moons. The asteroid completes an orbit around the Sun every two years and four months, traveling out to a distance beyond Mars. But it won't fly past Earth again until 2050. So it's a good thing we spotted its moons this time around.


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