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SYFY WIRE Bad Astronomy

Close encounter by the (big!) asteroid Florence

By Phil Plait

On Friday (September 1, 2017), an asteroid named 3122 Florence sailed past Earth, coming pretty close on a cosmic scale: Its orbit brought it to a mere 7 million kilometers from us.

Don’t panic! It can’t hit us; while it does get cosmically close to us, its orbit is tipped in such a way that it doesn’t physically intersect our orbit. So, it can get near us, but it can’t hit us. Current predictions show Friday’s encounter was the closest pass it’ll make to us for 500 years!

When I wrote about this a few weeks ago, I said that the science from this event should be pretty exciting. Guess what: I was right.

Why? Because radar images of Florence made as it passed show that the asteroid has two moons! We’ve known for a long time that asteroids can have moons, but more than one is rare. It’s not clear just how many asteroids are trinary, but only a handful are known (asteroids with one moon are kinda common; about 16% of asteroids bigger than 200 meters in diameter that pass near Earth have a single moon).

The radar animation is very cool. Watch:


(Note: The original animation was portrait mode, so I rotated it before uploading it to YouTube; that makes it easier to watch but the annotations are sideways. C’est la vie.)

The radar animation is composed of images made starting a few days before the pass using NASA’s Goldstone Deep Space Communications Complex. The two moons are indicated (they’re probably 100 – 200 meters in diameter; Florence, itself, is about 4.5 kilometer across), with the inner one moving much more rapidly than the outer one. You can even see the inner moon get eclipsed as it passes behind the main body of Florence as seen from Earth!

This animation actually needs a bit of explanation. Unlike a video you take with your camera, it’s not showing the usual two dimensional view of the asteroid changing in time. The horizontal axis in the YouTube video represents distance to the asteroid; if it has a hill or a crater you’d see it in that dimension. But the vertical axis represents the velocity at which the object is moving! The faster something spins, the more it gets smeared out in the vertical direction.

The asteroid looks round in this animation, but what you’re really seeing is its rotation (it spins once every 2.36 hours) creating a Doppler shift in the radar return, so it spreads out in the vertical axis. If it weren’t rotating, it would look like a single horizontal line. Weird, I know, but that’s the way this stuff works (my friend Emily Lakdawalla has a wonderful essay giving more info on this and I highly recommend reading it).

Another weird thing is that, the way it’s displayed, the Earth is off to the right in the video. The radio telescope sends radar pulses at the asteroid, which then reflect off it, so, in a sense, the telescope is illuminating the asteroid as well as seeing it (like a flash on a camera). That’s why the limb of the asteroid to the right looks bright; that’s where the radar pulses are reflected the strongest. Also, that’s why you see the inner get eclipsed near the end of the video; it’s not out of sunlight, it’s on the far side of Florence as seen by the radar, so it’s not visible.

Florence was caught by the Virtual Telescope on August 28, 2017 when it was about 9 million km from Earth. The ‘scope tracked the asteroid, so stars appear as short streaks; the long streak is the AMC-14 satellite orbiting Earth.


The presence of these moons is interesting. It’s likely moons around asteroids can form in different ways. The most obvious way is from a relatively low speed impact from another asteroid. Some pieces fly off and go into orbit. The physics is actually far more complicated than that (it usually is), but that’s the gist. The orbits, sizes, and compositions of the moons will depend a lot on the impact characteristics, so they tell us something of the rock’s history*.

Given that astronomers classify Florence as a Potentially Hazardous Asteroid, knowing more about it is fine with me. It’s called that not just because it gets close, but also because it’s big. At 4.5 km across it would make a decent sized Rocky Mountain! If it hit us, it would be very, very bad. Happily, as I pointed out, it can’t, at least not for a long, long time. But others (smaller ones) can, so the more we know about any of these beasts, the better.

If you’re wondering what the pass looked like through more conventional telescopes, my friend and astronomer Adam Block took this nice series of images which he turned into an animation:

Named after Florence Nightingale (English nurse, Crimean War 1854), asteroid 3122 (Florence) is a large 5km asteroid...

Posted by Adam Block on Friday, September 1, 2017


Gianluca Masi used the Virtual Telescope to take a slightly different approach, following the asteroid, itself, and letting the stars move by. It’s like you’re flying along with it:

WOW! the Potentially Hazardous Asteroid (3122) Florence imaged on 30 Aug. 2017 by Gianluca Masi / Virtual Telescope...

Posted by Gianluca Masi on Thursday, August 31, 2017


In both videos, the moons are far too faint and to close to the main body to see.

If you want to see Florence for yourself, it’s still possible, but you’ll need a decent telescope and some experience to find it. If you want to give it a shot, Sky and Telescope has you covered with maps. It’s fading as it pulls away though, so it may be a challenging target — by September 6, it’ll be 10th magnitude, about 40 times fainter than the faintest star you can see with your naked eye. But you won’t get another chance to see it until 2024 when it passes again, and even then it’ll be over 50 million km away and probably won’t be visible except to much bigger telescopes (if you’re more of an advanced observer, JPL can generate an ephemeris for you).

But don’t fret! Another asteroid is on its way: 2012 TC4 will pass us on October 12, and this will be a close shave indeed: It’ll get about 44,000 km from the Earth’s surface. That’s a really tight pass, and all sorts of observations are planned for it. TC4 is much smaller than Florence, only about 20 meters across, but such a close encounter should provide all kinds of interesting info about it as well. Stay tuned!

*As another example, if the two components are about equal in size, then it’s possible they formed when an asteroid started spinning faster and faster until it broke apart. The spin can be accelerated by sunlight, of all things!