Tag: Rusty Schweickart

Asteroid 2011 AG5: a football-stadium-sized rock to watch carefully

By Phil Plait | March 6, 2012 7:00 am

Over the weekend, I posted about asteroid 2012 DA14, which is not the threat some people are claiming it is (at least not right away). And now I have to tell you about an asteroid that might be a threat in the year 2040. Most likely it won’t be, but it’s something we need to look at carefully.

There’s some background I have to give you so this all makes sense, but let me sum up here: the odds of an impact from asteroid 2011 AG5 are low, but not easily dismissed. If it passes us at just the right distance in 2023, it’ll swing back again and impact the Earth in 2040. We don’t know the orbit of it well enough to say either way just yet, and it may be late 2013 before we can be sure. An asteroid expert at NASA says waiting until next year for more observations is not a problem, but another asteroid expert is saying that waiting that long is a bad idea: we should start analyzing a possible deflection campaign for this rock now. I’m personally leaning toward the idea that getting moving on the initial analysis now is not such a bad idea. If you prefer, I have a list of bullet points at the conclusion of this post with summarized information.

[In the interest of full disclosure: Below, I will be talking about Rusty Schweickart and Don Yeomans. I've known Rusty for several years, and Don and I will be on a panel together talking about asteroid impacts at SXSW next week. I honestly like both Rusty and Don. They're good men, very intelligent and honest, and I have a lot of respect for both of them.

Also, because of the length and nature of this post, I strongly urge everyone to read the whole thing, carefully, before commenting. Thank you.]


The rock

The asteroid, called 2011 AG5, was discovered in early 2011 by a telescopic survey of the sky designed to look for asteroids that can get near the Earth. Although its exact size is unknown, it’s roughly 140 meters across — the size of a football stadium. As you can see from this diagram, it orbits the Sun on an elliptical path that brings it out past the orbit of Mars and inside the orbit of Earth. It circles the Sun once every 1.7 years.

As it happens, the orbit of AG5 brings it close to Earth every few orbits. In 2023, it will pass us at a distance of about 1.6 million km (1 million miles). That’s a safe distance, with no chance of it hitting us at all. However, you have to appreciate the gravity of this upcoming situation.


The keyhole

When AG5 passes us in February 2023, the Earth’s gravity will bend its orbit a little bit, changing the path the rock takes. If it passes close to the Earth the orbit changes a lot; if it’s too far the orbit changes only a little. But if AG5 passes us at just the right distance, the orbit will change just the right amount to put it on a collision course with Earth. This region of space is called a “keyhole”, and in this case, should AG5 slip through it, it will hit us 17 years later, in 2040. That collision, though not global in scope, would be catastrophic: equal to about a 100 megaton explosion, twice that of the largest nuclear weapon ever detonated.

The problem is, we don’t know the orbit of AG5 well enough to know if it will travel through the keyhole or not. As I pointed out in the article about the asteroid 2012 DA14, it can be tricky to try to predict asteroid orbits too far into the future. The orbit of an asteroid is determined by making measurements of its position over time, and because of various effects (like blurring due to our atmosphere) it is impossible to get exactly precise positions. They can be good enough to get an accurate orbit for the next few years, but the farther into the future you look, the fuzzier that path gets.

In the case of AG5 we know its orbit well enough to know for sure it’ll miss us by a million miles in 2023, but we don’t have the accuracy yet to know if it will thread the eye of this keyhole, which is very roughly 360 km (240 miles) across. It’s like standing by the side of a road and knowing a car driving down it will safely miss you by 10 meters, but you can’t be sure if that exact distance will be 10.004 meters or 9.996 meters. And that’s the sort of accuracy we need for AG5.


The odds

At the moment, given the observations we have, the odds of AG5 passing through the keyhole in 2023 are about 1 in 625. For an asteroid impact, that’s actually pretty high as these things go, but still pretty low in realistic terms. Let me be clear: any professional poker player will tell you never to bet on an inside straight, and the odds of getting the card you need in that case are only 1 in 13 or so. The odds of AG5 hitting us are much lower than that!

Moreover, since the orbit of the asteroid is uncertain, as we get better observations the predicted path is likely to change, to move. In that case — which is almost certainly the way things will play out — the predicted orbit will move away from the keyhole and we’ll be safe from a 2040 impact. This sort of thing has happened several times before with asteroids as their positions are observed over time, and the orbital paths clarified.

Still, a 1 in 625 chance is high enough that we need to be sure. So how do we do that?

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