By Alan Boyle Science editor
updated 11/1/2005 8:01:33 PM ET 2005-11-02T01:01:33

Planetary scientist Don Yeomans laughs at the title of “asteroid czar,” but as the manager of NASA’s Near-Earth Object Program, he’s arguably the go-to guy for asteroids and other celestial objects that could blast us back into the Stone Age.

Over the past decade, Yeomans and his colleagues at NASA's Jet Propulsion Laboratory in Pasadena, Calif., have had to cope with a series of asteroid alerts — arising in part because astronomers are getting so much better at spotting space rocks that have a chance of crossing Earth's orbit sometime in the next century or two.

The biggest concern right now has to do with an asteroid called Apophis, which will have a close encounter with Earth in 2029 and might even hit us in 2036, depending on how its orbit changes between now and then. NASA says Apophis will almost certainly miss Earth — but just in case, Yeomans and other at the space agency have sketched out plans for space missions to divert the asteroid if necessary.

NASA issued its plan as a response to concerns expressed by former astronaut Rusty Schweickart, who is calling attention to potential threats from near-Earth objects as the chairman of a California-based group called the B612 Foundation.

Yeoman discussed Schweickart's concerns, NASA's response and the bigger picture behind near-Earth objects on Tuesday in an interview with

During the discussion, he made clear that he doesn't let his status as NASA's chief asteroid-watcher go to his head. For someone who watches out for cosmic doomsdays, Yeomans seems remarkably grounded.

Image: Yeomans
Mike Nelson  /  AFP/Getty Images
Donald Yeomans is the manager for NASA's Near-Earth Object Program at the Jet Propulsion Laboratory.
“Several months ago, the Pasadena newspaper did an article on strange jobs, and I was there with a belly dancer, a tattoo artist and a dog groomer," he said with a laugh. "So that’s the kind of company I keep here."

He also keeps company with a growing array of skywatchers, including researchers at JPL's own Near Earth Asteroid Tracking project in Hawaii, professionals at other facilities such as the Lincoln Near Earth Asteroid Research project in New Mexico — and hundreds of amateur astronomers around the world.

"Back in the early ’90s and before that, this near-Earth object issue was looked upon, even among our scientific colleagues, as sort of a crazy topic," he recalled.

Some even thought NASA was using near-Earth objects as a ploy to get more money, he said. But since then, asteroid-watching has gotten a lot more respectable.

"We shouldn’t be going hard over and devoting a lot more resources than what we have now, perhaps," he said. "But the modest level of spending that NASA is doing now — about $4 million a year — is probably appropriate, at least for the time being, for the insurance that we get as a result of tracking these objects into the future. We don’t get the giggle factor nearly as much as we once did."

Although the study of near-Earth asteroids has come a long way in the past decade, one big question still hangs in the air: Who takes over if Yeomans and his colleagues actually spot an asteroid or comet heading our way?

At least officially, it's not NASA. True, NASA identifies and tracks potentially threatening near-Earth objects, and NASA does have plans to respond to Apophis if necessary. However, dealing with an actual threat is not part of the space agency's job description, Yeomans said.

"NASA does not have the charter to look at that, nor does anyone else," he said. "That’s the point — no one does at the moment."

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

Rusty had said that this is an unusual case, because you do have this scenario where the movement of the course doesn’t have to be all that much. He said that in the case of a larger asteroid, you would have a bigger problem — and we don’t really have a good idea how to deal with that. Is that something you would subscribe to, that more needs to be done to deal with larger asteroids? Or if you had enough notice, could you make that small amount of motion and move the larger asteroids away?

If you have to move something a centimeter a second to move it an Earth diameter away in 10 years, it’s proportionately less if you have 20 years or 30 years, so it gets down to a few millimeters per second. The key is to discover these things early, and that’s exactly the goal of NASA’s search programs.

I’ve been warned, and warned, and warned again by NASA Headquarters not to sign NASA up for any mitigation responsibilities — because NASA does not have that responsibility at the moment. So I’m not to say anything on that.

What would have to happen to make those sorts of studies? Rusty has said that there should be an agency designated to protect Earth from near-Earth objects and address some of these mitigation measures — whether it’s a national or international agency, or set up under an international treaty. Is there a range of political scenarios for that?

What would have to happen, of course, is that the policy makers and Congress would have to direct NASA or the Department of Defense or someone to take responsibility for this, and then they would begin the studies necessary to come up with those options. As I mentioned, NASA does not have the charter to look at that, nor does anyone else. That’s the point — no one does at the moment.

The charter that NASA has is to identify those objects.

Identify and track. But not mitigate, and not deflect. They’re really sensitive about that.

But in the case of Apophis, there is some talk about a deflection scenario …

Right. Well, we did that analysis to directly respond to Rusty’s letter to [NASA Administrator] Mike Griffin. It was a pretty quick and dirty analysis, just using the fact that Deep Impact was so successful. We showed that this could be done more quickly than Rusty thought, so we don’t have to do anything now. We can wait until after 2013, when this is almost certainly going to go away. But even if it doesn’t, we still have options to deal with it. …

In terms of the technical options, hitting an asteroid with something is one alternative. And the other alternative would be to put something that has a thruster on the asteroid, and nudge it into a different course over a long time frame. Does that pretty much cover it?

Oh, no. Those are just two of many options. You could put up a sun-focusing mirror nearby, and focus sunlight on one side of the asteroid, and the ablation of material on the front side would introduce some thrust on the asteroid. You could mount a shuttle engine on one of these things, but then you’d have to worry about the rotation. You’d have to pulse every time it came back to the same location. Or you could have mass drivers. That has been suggested.

The weapons lab folks suggested buried nuclear devices for standoff nuclear blasts that would ablate the front side and introduce a thrust. Or blow it up soon enough so that the shrapnel would miss the earth. You don’t want to blow this thing up just before impact, because then you’ve got a shotgun effect that would be worse. Not too many people are comfortable with this nuclear option, but the Lawrence Livermore folks were sort of keen on that one, as was [nuclear-energy pioneer] Edward Teller in his day. …

There are a lot of techniques that could be used, assuming you have the time. The key is, you have to find them early, and then things are tractable. If you find it too late, within a year of impact, there’s not a whole lot you can do. You just evacuate, if it’s small enough to cause only local damage. And if it’s not, then … Well, the big ones, the ones that cause global problems, are the easiest ones to find. And that is NASA’s goal now, to find the ones that are larger than a kilometer. We’re doing well, actually: We’re up to 807 out of a total population of about 1,100 asteroids of that scale.

Once we find them, they’re no longer a problem, because we can track them for 100 or 200 years in the future, and keep an eye on them. None of them are a problem, but it’s the smaller ones — and there are hundreds of thousands of them  that we haven’t been able to deal with yet, because we don’t have the telescope apertures yet.

And I suppose there’s also an issue with long-period comets, as well as asteroids that spend almost all their time inside Earth’s orbit. Those are some of the things that are mentioned as “out-of-the-blue” threats.

That’s true. There are the Atens, the objects that spend most of the time interior to Earth’s orbit. They are difficult to find, because they don’t appear in darkness very often. There is at least one NASA program that is looking as close to the sun as possible to find those, and they’ve found a few.

Interactive: Below the belt You mentioned the comets. Now, they’re the wild cards. The bad news is, if they come in from the outer solar system, they don’t start outgassing until they get well inside the orbit of Jupiter, and thus don’t show themselves. It only takes 9 months to get from Jupiter’s orbit to Earth’s orbit. So that’s the bad news.

The good news is, in terms of numbers, there are 100 times more asteroids than comets in the near-Earth space. So asteroids are really the major problem, and we can deal with them. Comets are 100 times less of a problem, in terms of numbers, and we can’t really deal with them anyway at the moment. So we’re concentrating on the asteroids at the moment.

It would take a lot more highly developed technologies to discover comets early enough to do something about them. We’d have to have sensors out beyond the asteroid belt, near Jupiter’s orbit, looking for these things. And even if we found them, the orbit determination at that distance would be so poor, we couldn’t predict precisely whether they would be a threat until they got in a lot closer anyway. So at the moment there’s no clear path for dealing with long-period comets. But they’re a 1 percent problem, compared to the 99 percent problem of the asteroids.

Could you refresh my memory on missions to near-Earth objects that are in the works? I know that Japan’s Hayabusa mission to bring a sample back from an asteroid is nearing its climax, and then there’s the European Rosetta mission.

Hayabusa is an interesting mission. Their target body, Itokawa, is a near-Earth asteroid that’s representative of the type of asteroid that would most likely be a problem. The structure of the object looks like a rubble pile, but the bulk density is not unlike Eros, which means it’s not completely all rubble held together by its own self-gravity.

You mentioned Rosetta, which will set down on the surface of a comet in another few years. I think that’s about it, unless Deep Impact gets a follow-on to another comet. And of course there are several Discovery proposals in the works for comets and asteroids, but they haven’t been selected as yet.

It sounds as if Apophis itself might be a candidate for a Discovery mission someday.

Well, that’s the thing with near-Earth objects: Many of the ones that represent the largest potential threats are also the easiest ones to get to, because they’re in very Earthlike orbits. So the rule of thumb is that the more Earthlike an object’s orbit is, the easier it is to get to. So it’s one of the easiest objects to reach in terms of a rendezvous mission. It does make sense, to me anyway, that somebody would suggest this as a Discovery mission. We could put sensors on board the asteroid, and watch the asteroid perhaps readjust itself as it flies by in 2029.

It’s going to be a naked-eye object. It’s going to go underneath geosynchronous satellites. I mean, it’s going to be quite a show. You could engage the guy on the street rather well with this mission. … That object, to my mind, is less of a threat and much more of interest for scientific purposes and engaging the public in this issue. So I think this is an opportunity that we shouldn’t pass up.

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