Studying asteroids and comets
Don Yeomans probes the benign, anticipates the possibly lethal.
Don Yeomans is manager of NASA’s Near Earth Object Program Office, whose purpose is coordinating NASA–sponsored efforts to detect, track and characterize potentially hazardous asteroids and comets that could approach the Earth. Yeomans spoke with Earth and Sky’s Jorge Salazar about how scientists are anticipating the possibility of a large asteroid colliding with Earth.
Salazar: Thank you for speaking with me today. What are some of the asteroids that pose a threat to life here on Earth? There’s been quite a bit written about asteroid Apophis.
Yeomans: Right – you mentioned Apophis, which is an asteroid about 300 meters in size that will get very close to the Earth on April 13, 2029. In fact it will get beneath the geosynchronous satellites – the same satellites that are probably used to beam your radio signals to your listeners. Certainly television and Earth surveying satellites are at geosynchronous orbits, and this asteroid will get beneath them and become a third magnitude naked–eye object for a period of time. So that’s kind of exciting. But – it won’t hit the Earth.
If it had, or if it were predicted to hit the Earth, there are a number of ways that we could deal with it. The key is to find these things early. That is the whole goal of the NASA’s Near Earth Object program is to bring under contract observatories such as NEAT here at JPL, to find these objects early enough so that we can use the observations that they provide to determine their orbits, predict where they’ll be in the future.
For example, in this case we know that it’s going to make a close earth approach. And if we can find them all, and predict where they’ll be in the near future, we have an excellent opportunity to deflect them should we find one on an Earth–threatening trajectory. So, it’s just a question of applying a small change in the asteroid’s velocity now so that in 10–20–30 years it will miss the Earth. On the other hand, if we find one on an Earth–threatening trajectory, say that’s going to occur within a year, there’s not a whole lot that we can do about it, frankly, because it would require too much energy to deflect it in that short a time. But if we can find them 10, 20, 30, 40 years in advance, then it’s relatively simple – well not simple, but it can be done.
For example, with the Deep Impact spacecraft we just showed, rather dramatically, that we can autonomously collide a comet with a fairly heavy spacecraft, and so you can imagine that a spacecraft like that can be used to ram an asteroid that was on an Earth–threatening trajectory, and as long as it was done a few decades in advance of the predicted impact, it only takes a fraction of a centimeter per second to change that object’s velocity, and it can be done with a fairly modest–sized spacecraft.
On the other hand, if you wait and don’t discover that object for a year or so prior to impact itself, then you need vastly more energy to change its path so that it misses the Earth. And, if you discover it only a few months in advance, you can’t blow it up with some sort of a nuclear weapon, because now you’ve got a shotgun effect instead of a single bullet, which could be worse. You could use a nuclear weapon, I suppose, if you found the object early enough, because you could disperse the pieces of the object so that they would all miss the Earth.
However, there are many more green techniques that could be used to deflect an asteroid. You could send up a solar mirror, for example, that could concentrate sunlight on one side of the asteroid head that rotated underneath you, so that you would oblate the front side of the asteroid and introduce a rocket–like thrust in the opposite direction. And, over time, that would change the asteroid’s position enough so that it would miss the Earth. You could, if you wish, could mount a shuttle engine on the asteroid. You could thrust and change its trajectory enough that way. But again, you would need several years advance notice. And again, that is the whole goal of NASA’s Near Earth Object program, to find these objects soon enough that current technology could deal with them.
Salazar: Since Apophis isn’t going to hit the Earth in 2029, are there any other objects that scientists are concerned that might?
Yeomans: Well, actually, that object is of concern. Not in 2029, when it makes its close approach, but – once you do have a close approach like that, it makes computing the subsequent orbital position of that object more difficult. So, if that object passes through a 600 meter sized keyhole, in 2029, that is, a location in space that is only 600 meters wide, if it passes through that, it will indeed hit the Earth in 2036. Now the chances of it actually passing through this 600 meter sized keyhole in 2029 is extremely low, and we’ll know whether it will or won’t probably next year when we get additional radar data in May of 2006. And if we can’t rule it our then, there’s an additional radar opportunity in 2013 that will almost certainly rule out this possibility. In the unlikely possibility that we don’t rule it out in 2013, there’s still time to mount a mission to deal with it. This object illustrates the point rather well. It was discovered early, so we have lots of options. The first is to wait until 2013, when this whole thing will almost certainly go away.
Salazar: Could you tell us a little bit more about what scientists know about Apophis?
Yeomans: Well, as I’d noted, we have optical and radar observations of the object. So we have a fairly good idea of its size – it’s about 320 meters in size, a decent size but not a real large asteroid. There has been spectral observations made, and it seems to be similar to what we call an ordinary condroid meteorite, which means that it’s probably silicate rock for the most part. There isn’t yet a good radar shape model for it yet, though. We know its basic size, and likely structure, although we don’t know the structure for certain, and wouldn’t, unless we had a spacecraft in orbit around it.
Again, if next year’s radar data does not eliminate the threat in 2036, and again in 2013, if we can’t eliminate the threat with additional observations that would show that the object was not going to enter this 600 meter sized keyhole, then there’s still time that would first investigate the surface characteristics, the size, the mass, the chemical composition of the object. And then a subsequent mission could be used to deflect it, either with a kinetic impact, or perhaps with some other technique that actually rendezvoused with the object and pushed it.
Salazar: Could you detail a bit how asteroids are studied?
Yeomans: Usually when we have observations of objects, they’re telescopic, visual observations, that is the object is located in the sky with respect to background stars that happen to be in the neighborhood. So you determine the right ascension and declination, which, you know, is similar to longitude and latitude here on Earth. So, you have an angular position of where the object is in the sky. And the third component is the distance between you, the observer, and the object. That is provided by radar. You can actually use one of two planetary radars – one in Goldstone, in the Mojave desert in California, and the other in Arecebo in Puerto Rico. And what is done with these giant radio antennas is to send a pulsed beam of radiation to the asteroid, it bounces off the asteroid and comes back and is received. And by noting how long it takes for that signal to leave your antenna, bounce off the asteroid, and come back, and be received by your antenna, by measuring that time, and you know the speed of light, that gives you the distance from your antenna to that asteroid. And that gives you the third component. You already have the other two on the plane of sky from your angular observations done with telescopes. And then you have the distance, provided by the radar. So, that’s an extremely powerful data type. And so radar data can give us a much better understanding of this object’s orbit than would be otherwise available only from the more traditional optical observations of the object.
Salazar: In the event that an large asteroid is found to be headed for Earth, what can be done about it to avoid disaster?
Yeomans: Well, there hasn’t been a great deal of work done on asteroid mitigation techniques. Some papers have been done though. But basically, if you find the object soon enough – and again, that’s NASA’s goal – then you have a number of technologies available to you. The easiest is to simply run into it, like Deep Impact, and slow it down, or speed it up, depending on which direction you’re headed from, and then monitor its motion. And if the first impact didn’t do it, then you’d do another one and monitor its motion. And so, if you had plenty of time, then the kinetic impact, or impacters would probably be the easiest and the cheapest.
If you wanted to actually rendezvous with the object, that would take a longer time, because you’d have to match positions and velocities with the asteroid. And if you wish to use this solar concentrator that we talked about, that would require a rendezvous mission. If you wish to land on the surface and use a shuttle engine to deflect the object’s motion, that would require a rendezvous as well. So, these techniques that require a rendezvous require a lot more time, and are quite a bit more expensive than the kinetic impacter which actually just runs into it. Or, if you actually wanted to just use a flyby and some sort of a nuclear device that would shatter the object – you’d actually have to bury the device into the surface before you shattered it. If you have the time, the kinetic impacter is the cheapest and easiest.
Rendezvous with a solar concentrator, or rendezvous and just simply using a spacecraft tug to push it out of the way a little bit – that’s a concept that Rusty Shweikerk and the B612 folks put out some time ago in Scientific American, that would work. There are other techniques which have been suggested. You can use a mass driver, sitting on the surface of the asteroid, that electrostatically threw rocks and material off the surface and, the equal and opposite thrust would move the asteroid over time. The problems with all of these techniques that actually sit on the surface of an asteroid is that you can only do it at certain times, because the asteroid’s rotating, and you want all of the thrust to be in the same direction. So, you’d have to pulse–thrust – every time the object rotated once, you would thrust, wait for it to rotate to the same position, and thrust again. So, using a technique that requires landing probably doesn’t make much sense. If you have a standoff solar concentrator, the object can rotate underneath you, and you would oblate the surface that is closest to the mirror and introduce a thrust that is in the same direction all of the time. That would be a cute technique. But again, that requires a rendezvous.
So, if you have the time, I think the kinetic impacter is the best way to go. If you don’t have enough time to deal with it in that way, then you’d have to try and shatter it with some sort of an explosive device so that the pieces would have time to disperse and miss the Earth. You don’t want to have a shotgun effect. That would require that your blast be at least a year out. The easiest are blast techniques that would try and shatter a small asteroid, or kinetic impacters which would try and nudge it one way or the other, so that in 10 or 20 years, when it was predicted to hit the Earth, it would simply be out of the way. If you have a large asteroid that was on an Earth impact trajectory – first of all we would discover it decades in advance, so that we would have plenty of time to deal with it. It’s the small ones that are vastly more numerous than the large ones, that are most likely to sneak up on us. One of the issues that NASA is wrestling with now is, should they extend the Near Earth Object Discovery program, like NEAT, and others, LINEAR, LINEOS, should those programs be expanded to try and find out some of the smaller objects as well as the larger ones. I mean we have discovered lots of small ones, but there’s a vast number of them that we haven’t. And so that’s the current issue for NASA headquarters to wrestle with.
You’ve got this peculiar duality, that near Earth objects, and asteroids in particular, are the closest objects to the Earth – I mean even closer than the moon, and represent a danger. And at the same time, they’re also the easiest objects to land on – some of them are easier than the moon itself. So, while some of them are a threat, others may well be a boon to future space exploration. We may be able to hop on some of these objects and mine them, and use the materials that are there to build future space habitats. So, we’ve got both friends and foes in our near Earth objects.
