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SYFY WIRE Bad Astronomy

No, the Earth (Almost Certainly) Won’t Get Hit by an Asteroid in 2032

By Phil Plait

By now, you may have heard of asteroid 2013 TV135. There are reports going around that it has a chance of hitting us in 2032.

Weâve been through this before. Many times. Many, many times. Like those other times, this one will almost certainly miss us. If you read a report saying otherwise, make sure your skeptical sense is tingling.

Piece of the Rock

2013 TV135 is an asteroid with an estimated size of about 400 meters (a quarter mile), and was just discovered on Oct. 8 at the Crimean Astrophysical Observatory, and several other observatories have confirmed it. Itâs a Near-Earth Object, meaning that as far as we can tell, it has an orbit that brings it near the Earth (hence the term, duh). In this case, it will pass near us in the year 2032.

The question, of course, is how near? That turns out not to have a straightforward answer. Right now we donât know the orbit well, and it may not come within tens of millions of kilometers of us. All we can say for now is what is the likelihood of it impacting the Earth, and the oddsâas it stands today, Oct. 18, 2013âare about 1 in 63,000. If that sounds high to you, think of it this way: it has a 99.998 percent chance of missing us.* Thatâs good enough for me, and thatâs likely to go to a 100 percent safety level in the coming weeks.

Target: TV135

Most stories Iâve seen so far about TV135 are saying the odds of an impact are low, so thatâs good. I also see some that may be overhyping it a bit. One quote making the rounds is from Dmitry Rogozin, who is the Russian deputy prime minister and also in charge of the defense and space industry. He is active in working on an asteroid defense system, which is great. On his Twitter feed, Rogozin quotes the headline from an article about the asteroid (translated by Google from Russian):

Life BBC News, "a 400-meter asteroid threatens to blow up the Earth"  ... Well, that's supergoal for the domestic space

An L.A. Times article wrote that up this way:”

"A 400-meter asteroid is threatening to blow up the Earth,â Russian Vice Premier Dmitry Rogozin, in charge of his nation's space research, wrote Wednesday on his Twitter account. âHere is a super target for the national cosmonautics.â

Hrmph. Iâm not sure I agree with that characterization of what Rogozin said; he was quoting another article. Still, itâs a bit off in its facts anyway. A 400-meter asteroid canât âblow up the Earthââitâs a big planet, and thatâs an exaggeration. (Though Iâll note for humans an impact that size would suck mightily, causing local devastation and serious global problemsâhereâs the output from an impact calculator that will give you the grim details.) Also, I donât think this asteroid makes a good target for us to divert; 19 years lead time isnât enough to do a whole lot. I discuss this in my TEDx talk I gave about preventing asteroid impacts. It might be enough to hit it with a space probe and change its orbit slightly, but for all we know, it could move to an orbit that would hit us later. Again, as I note in my talk, these things take a bit of finesse, and certainly a lot of lead time.

UPDATE, Oct. 18, 2013 at 16:00 UTC: The sadly widely read U.K. tabloid Mail Online put up this headline about TV135:

Yeah, nice. It's technically accurate but definitely plays up the doomsday angle. Note that this is the same news venue that shamefully prints climate change denial articles; funny how they amplify a disaster that almost certainly won't happen and downplay one that is certainly real.

Specifics aside, the whole quote is a bit breathless for me, given the long odds of an impact anyway. I am not at all concerned about an impact from TV135. Hereâs why.

Blip in the Sky

Asteroid 2012 DA14 (center) seen moving relative to background stars in 2012. Photo by La Sagra/ESA.

Asteroids are discovered because they move relative to the background stars. Even over a few hours that motion is apparent in a good telescope. Using some pretty sophisticated math, the orbit of the asteroid can be determined once the motion is observed for long enough, and that orbit can then be projected into the future to see if the rock gets near us.

However, itâs hard to get an accurate orbit thatâs precise enough to project decades in time. There are always some measurement uncertainties, making the calculated orbit fuzzy; what you get is a most probable orbit, which allows you to get a most probable position for the asteroid some time in the future. But really you get a big fuzzy volume of space it could very likely be in, and it gets bigger and fuzzier the further in the future you project it.

The best analogy is to imagine youâre an outfielder in a baseball game. You see the pitcher throw the ball, but as soon as the batter hits it, you have to close your eyes. Now try to catch the ball.

Not so easy, is it? The best thing is to keep observing the ball, moving around as you refine your estimate of where it will be when it gets near you. The longer you keep your eyes on the ball, the better your ability to know exactly where itâll be when it reaches you.

Asteroids are the same. If we only have a few daysâ (well, nights) worth of observations, our prediction for the future is unreliable. But if we keep observing it over time, the orbit gets nailed down, and our statistics get better.

What Does 1 in 63,000 Mean?

Right now, we only have eight days of observations of TV135, which is very short indeed. Trying to project that orbit 19 years into the future makes its position very uncertain. Thatâs why the odds of an impact are so low.

How low? It means that given the most likely position of the asteroid when it gets near Earth, plus the uncertainty in that position, there is only a 1 in 63,000 chance that the asteroid and Earth will occupy the same space at the same time.

Itâs not hard to picture odds like that. Suppose I draw a circle 280 pixels across, and I pick a random pixel inside it.* The odds of you picking that exact same pixel at random are just about the same as the odds of the Earth getting whacked by TV135. Iâd happily bet a nice sum of money that you wouldnât do it.

And it gets worse. Well, better for us, I mean: In reality, as we observe TV135 more, and we get a better orbit for it, that uncertainty in its orbit will shrink. As it happens, the Earth is not all that close to the most probable location of the asteroid when it swings by, so as we refine the orbit, the chances are pretty good that the odds of impact will get even longer.

So, to be clear: In 2032, there is a tiny chance TV135 will hit us, but itâs very unlikely, and as time goes on, the chance will get smaller.

Keep Watching the Skies

The good thing about this is it shows weâre still looking for asteroids, but it also shows that sizeable rocks are still out there we donât know about. In fact, while weâve discovered about 10,000 near-Earth asteroids bigger than roughly a hundred meters in size, there are probably a million or so out there. That means weâve only found 1 percent of the dangerous rocks in space.

This is a serious problem, as the Chelyabinsk impact showed us. Something like that happens maybe once per century, and a Tunguska-sized impact maybe once every 300 years. But itâs not clockwork; it could take longer, or it could be sooner. Even though the odds of TV135 hitting us are very low, it could just as easily have had a much higher chance, and an impact from a 400-meter rock would be very bad. We need to keep our eyes on the sky, and the more the better.

NASA, as well as the B612 Foundation, are working on ways of finding more dangerous asteroids, and we still need more telescopes on the ground looking as well. Of all the natural disasters that can befall us, asteroid impacts are one we can very nearly 100 percent prevent. We just need to choose to do so.

Correction, Oct. 18, 2013 at 17:45 UTC: This post originally stated that the probability of a safe pass is 99.99998 percent. It's 99.998 percent. This is still a pretty safe bet. Also, in the illustration depicting a 1-in-63,000 impact chance, the post orginally had the circle as 500 pixels across. It's 280 pixels across. My apologies; that was a holdover from another analogy I worked on, where the small dot was the Earthâthat is a circle and not a square pixel. My thanks to @ierceg for pointing this out.

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