Another tiny rock will whiz past us tomorrow

By Phil Plait | February 8, 2011 10:04 am

On February 9 at around 19:30 UT, a small asteroid the size of a car will cruise past the Earth, missing us by a distance of about 100,000 km (60,000 miles). By astronomical standards that’s pretty close (about a quarter of the way to the Moon) but it’ll miss us for sure.

Called 2011 CA7, it was discovered earlier this month. The short notice is not too surprising, given that it’s at a very faint magnitude of 20 — you’d need a big telescope to be able to see it at all! Even at closest approach it’ll be magnitude 17, less than a ten-thousandth as bright as the faintest star you can see with your unaided eye.

There is some small uncertainty by exactly how much it’ll miss us; the orbit isn’t precisely determined yet. According to the JPL small-body data browser, the minimum distance 2011 CA7 will pass us is 93,000 km (58,000 miles), and the maximum is 114,000 km (71,000 miles), but the most likely distance is 103,000 km (64,000 miles). These numbers may change as time goes on, but the important thing to note is that it will miss us.

Given the brightness of the object, it’s obviously pretty small, probably around 3 meters in diameter, or about 10 feet. That’s a little bit bigger than 2011 CQ11 which blew past us last week, but not significantly so. Even if it were to hit us it would certainly disintegrate high up in the Earth’s atmosphere and at worst rain down a few rocks; that sort of event happens several times a year somewhere on Earth, if that makes you feel any better. In other words, it’s relatively common and presents very little danger. Interestingly, this little guy has probably passed us many times in the past, but this is the closest encounter yet (which makes sense; it’s so small and faint it’s only when it gets close that it can be seen at all).

This is an interesting rock. Its orbit goes from roughly the orbit of Venus out to that of Mars, and that means it spends most of its time in near-Earth space. I imagine over a long period of time, maybe millions of years, an impact is inevitable — but again, something this small will make a pretty light show but present very little real danger. In that picture above you can see the orbit in teal; the Earth and the asteroid are to the lower left. On this scale — hundreds of millions of kilometers across! — the rock looks like it’s right on top of us, but 100,000 kilometers is a long way off in real terms. A miss is a miss.

I haven’t seen any imaging of it yet but I’m keeping my eyes open, and hopefully we’ll get some nice pictures of this. Stay tuned!


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