Asteroid ace keeps NASA grounded

Here's the edited Q&A from Tuesday's interview, beginning with a discussion of NASA's plans to divert Apophis if necessary:

MSNBC.com: It’s interesting that there is a timetable for dealing with this particular asteroid. Could you talk about the thinking that goes into how you approach something like this, which is a very low-probability but high-impact event?

Yeomans: The first point to be made is that the whole point of NASA’s near-Earth object search is to find these objects well in advance of any threatening encounter. Once you have a couple of decades to work with, this problem becomes tractable. So the object was discovered as part of the program here, and we ran the orbit calculations forward and found this close approach in 2029.

Then we discovered that depending on the circumstances of that close approach in 2029, there could be what we call “resonant returns.” In 2036, seven years after the 2029 return, the earth has gone around the sun seven times, of course, and the asteroid would have gone around six times. And they arrive at the same place at the same time, if the object passes through a 600-meter [2,000-foot] “keyhole” in 2029. Now the chances of that happening are pretty minuscule, but it hasn’t been ruled out yet.

The plan is to continue observations of this object, both optically and with radar, and according to our statistical analysis, there’s an opportunity next May for radar observations, and at that time there’s a 50-50 chance that this remaining 2036 possibility will disappear, because the orbit gets that much better. And then in 2013, there’s yet another opportunity. If the risk doesn’t go away next year, there’s a 96 percent chance that it will go away in 2013.

And that’s based on the fact that as you make more observations, you know the orbit more precisely?

That’s right.

Isn’t there also something called the Yarkovsky effect that you would be looking for in 2013?

Sure. The Yarkovsky effect is one of the largest contributors to our uncertainty about this object.  As an asteroid is heated by the sun — at asteroid noon, for example — you can imagine that if the asteroid were not rotating, the heat of the sun would simply heat it up and it would reradiate some of that heat back toward the sun. But the asteroid is rotating, of course. Just as the earth does not get hottest at noon but gets hottest at 3 o’clock in the afternoon, there’s a thermal lag. The asteroid actually reradiates much of its energy not directly back toward the sun, but there’s a component that’s in the direction of the asteroid’s motion or counter to it, depending on which way it’s rotating. That introduces a rocketlike thrust on the asteroid itself … and that affects the orbit and the position of the asteroid as a function of time. It’s a very small effect, but over a few decades, it’s quite important.

Typically, in order to completely understand the Yarkovsky effect, you have to understand the surface. Is it solid rock? Is it jumbled rock? … If you understood the surface completely, as well as the rotation pole and the direction, you’d be able to model this rather well. But even without that, we can simply look at the difference between our predictions of where the object should be, not including the Yarkovsky effect, and the actual position in a few years, and say, “All right, the difference is due to the Yarkovsky effect.”.

We’re confident that we can model that Yarkovsky effect, given additional observations.

And that explains why you would be so much surer in 2013 than you would be in 2006?

That’s right. And in 2021, there’s yet another radar opportunity. By then we will know the Yarkovsky effect rather well. Even without a mission, we would know with a precision of a few hundred meters. So we could almost certainly tell whether the asteroid will pass through this keyhole in 2029. Again, it’s far more likely that it will not than that it will.

[B612 Foundation Chairman Rusty Schweickart's] point was that you won’t know whether it will definitely miss this keyhole until later on, after the 2013 encounter, so you’d better mount a mission now. He’s thinking in terms of a rendezvous, so you’d have to have a lot of time. Our point is that there’s no hurry — yet. If this thing doesn’t go away in 2013, there’s still time to mount a precursor mission that might drop off a transponder to track this thing to an accuracy of a few meters. In the unlikely event that that still doesn’t remove the threat in 2036, there’s time for a subsequent deflection mission along the lines of the Deep Impact comet mission.

That’s a much faster mission, if you run into it rather than making a rendezvous and nudging it. That was the point: He maintained initially that we needed a lot of time and we had to start now. … What we’re saying is that there are faster and less expensive ways to attack this problem, should it still be a problem in 2013 — which is frankly quite unlikely.

In this case, would hitting the object make enough of a difference to alter the asteroid’s course?

Any change in velocity that you give the asteroid before the 2029 encounter is dramatically magnified as a result of that encounter. For example, the analysis that we did showed that if you give the asteroid a modest tenth of a millimeter per second change in velocity, over three years that amounts to a 25-kilometer [15-mile] change of position — which would move it out of the 600-meter keyhole. It takes very little to get it out of this keyhole. We’re not trying to move it kilometers, or hundreds of kilometers, or thousands of kilometers. We just need to move it a few hundred meters and get it out of that keyhole. And if we do that, it misses completely in 2036.

In terms of the dynamic analysis, would a projectile on the scale of Deep Impact’s impactor make that much of a difference for a 400-meter-wide object, or would you have to scale up the projectile?

Actually, scale it down. A 1,000-kilogram [2,200-pound] spacecraft would do that. Deep Impact’s mothership and impactor together were more than that. A tenth of a millimeter per second is next to nothing, and the keyhole is next to nothing. That’s what allows us to get away with this. Normally, if you had an Earth-threatening encounter, you would want to move the asteroid at least an Earth diameter. Now you’d be talking about thousands of kilometers. But because of this keyhole, we only have to do a few hundred meters.

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