Asteroids have rocked our world in the past
Raymond Bambery scans the skies for the next killer asteroid.
Raymond Bambery is principal investigator of the Near Earth Asteroid Tracking task, part of NASA?s Near Earth Object Program, which studies asteroids and comets through observations from Earth and from spacecraft probes. Bambery scans the night sky looking for asteroids that might someday collide with Earth.
Earth & Sky’s Jorge Salazar spoke with Dr. Bambery in 2005 about the likelihood of our human world encountering a killer asteroid.
Salazar: Thanks for speaking with me today, Dr. Bambery. To start with, how did NASA get involved with tracking asteroids that pose a possibility of harm to humans here on Earth?
Bambery: Basically, in the 1990s, NASA got interested in the fact that an asteroid could strike the Earth, and the amount of devastation that it could do by a study of geology, where they discovered deposits in Europe which signaled that this happened in the past. So, they started this program, where they ask whether telescopes could do this in a reasonable amount of time. And they came up with a ten-year program to search the skies every night with telescopes, and survey for ten years to see if any of these objects would strike the Earth or come reasonably close.
Ultimately NASA had two goals in mind. One is to determine the threat of these things, as well as the fact that as they pass close by, they offer unique opportunities for you to go out with a robotic spacecraft and explore these things in a very short period of time, in weeks or months, instead of spending years going out there to find these things.
So, basically, all we do is have a telescope – my particular telescope that I use is one in Maui, located in Hale Akula, and the other one is on the Palomar mountain in San Diego – we survey the heavens every night looking for asteroids. And basically, a simple procedure is to take three pictures of the sky, separated by about thirty minutes, and see these things move against the fixed background stars. We detect somewhere on the order of of 3000 asteroids a night, and somewhere in the neighborhood of five or six are interesting, in the sense that they signal that these have unusual motions and that we should investigate them further.
It takes about 2-3 months to really pin down that an asteroid is going to fly by the Earth, and it takes several years to pinpoint how close it will come in the future. And, as I said, the information is used for two reasons. One is to determine the threat hazard. And the other is to determine whether these are good candidates for planetary exploration.
Salazar: You mentioned detecting thousands of asteroids every night. How many asteroids are there near Earth, and how many are possibly dangerous?
Bambery: As I said, we detect about 3000 asteroids that might come by. And there are several people who have looked at these things, so it’s probably on the order of 6000-7000 asteroids that we look at a night. Most of them are in a belt between Mars and Jupiter, and they offer no threat to Earth. These few that do come in – there’s several thousand now – range in size from 2-3 kilometers down to 10-15 meters in size. And the closer they are to Earth, the smaller we can detect them, and the farther away they are, the larger they have to be.
So, from what we do every night, we plot these things and watch them over a period of time. Now there’s well over 100,000 asteroids that are very well known. Like I said, 99% of them are outside of the orbit of Mars and offer no threat. And the 1000-2000 that come near Earth really don’t offer any major impact on Earth. We do have several hundred that have the potential to be hazardous.
We define a hazardous asteroid as one that comes within five million miles of the Earth and is 1 km in size, because that is pretty devastating when it hits the Earth. Typically, we use different scales to determine the threat, and then those particular objects become of prime interest, and we follow them very closely for a period of time. And you may have heard that a few of these are projected to strike Earth in the next hundred years. But as you watch them, you realize that they’re not going to.
Nevertheless, we have not found any that are really going to hit Earth in the next hundred years. But since they come close to Earth over a period of time, the other gravitational bodies in the solar system such as Jupiter, Mars, Earth, Venus, can perturb their orbits, and they could potentially intersect that of the orbit of Earth at a very inopportune time. But nothing right now that we know of is due to impact the Earth in the next several thousand years, at least.
Salazar: How do you determine which asteroids to get concerned about?
Bambery: Well, the fact was that people have done planetary studies, planetary dynamical calculations. We have mathematicians who model the behavior of objects as they circle the solar system, and then of course they have to understand how much gravitation the Earth has, Mars, Jupiter, and then see how these things change over a period of time. From that particular analysis, we hypothesize that there might be about 1500 objects that might fly by the Earth and be of some kind of threat over a 1000 year period, for example. So, we have that as an object – somewhere between 1000 and 1500 of these objects are candidates. But we embarked on a ten-year program in 1998 to actually do a complete census of these objects. And we’re now within two years of completing that, in 2008 we will finish that initial candidate, trying to find those objects that are 1 km or bigger, and plot those very accurately. And so it’s true, some of them get lost for a while, but eventually we pick them back up over a period of time, they come back into view, and one of the other telescopes will find them. They could be missing for months – but of course then they may be on the other side of the sun, so you can’t see them, obviously, and so that’s the reason for the ten-year program, to make sure that we actually have seen them over and over and over again to know that in fact we know their orbits very well.
Salazar: How do scientists go about studying asteroids?
Bambery: There’s two types of things that people actually do. First is the survey program – which is what I’m doing. And the survey program is just out there to detect them and monitor them and check them out in terms of what their future orbits are. The second type of thing that we do is to try to understand their composition and structure, because if we do it that way, then we might have an idea of where these things come from. We know that asteroids are not all the same composition – they’re all different. So, if they come within a certain distance to us, then we can start using high resolution spectrometers, which can give us the chemical composition. We use radar to detect their surface properties, and things of that nature. So, there’s basically two different thrusts that people are doing in order to try and characterize these things – what place do these things come from.
We do know in fact these are basically objects that are left over from the formation of the planets and the solar system over the last several billion years. And eventually, they collide with something, or they collide with something, or they fly by something close, and they’re thrown out of the solar system or they’re thrown into the sun.
We do have study teams, and these study teams consist not just of NASA scientists, but they’ve had the Department of Defense people in them, scientists interested in planetary dynamics who aren’t actually part of the Near Earth program, who sit down and think about ways that the danger could be mitigated. We have periodic conferences where that issue becomes the central spot. Right now, there’s a number of techniques, and I believe you’re going to be talking to Don Yeomans here in a little while, and he often is a chair at some of these meetings, and he’ll give you a better idea of what things they’ve proposed to mitigate the problems.
Salazar: Has there ever been a close call with an asteroid?
Bambery: In a sense of close calls, we have them quite often. Four or five times a month, an object comes within five million miles of the Earth, but they offer very little danger, at least in the past. We had one here that came by a year and a half ago that flew by at 22,000 miles past the Earth. It was very small – only 22 meters in size. But the interesting thing, at 22,00 miles above is where geosynchronous satellites are held, the communications satellites that provide us with cable TV and things like that. And so that area is where all these satellites are. And while there’s a lot of space there, this thing flew by but it didn’t hit anything. It didn’t even endanger Earth. But nevertheless, at 22,000 miles away, that’s very, very close in terms of what we’re looking for. But again, it was only a few meters in size, very small. But it could have had a devastating effect if it had hit a satellite. But it really didn’t come close to any that I know of.
Salazar: What happens when one of these large asteroids gets too close to Earth?
Bambery: When they fly by, and we spot them – we know that they’re going to fly close to Earth – we alert the community. There’s a place called the Minor Planet Center, in Cambridge, Massachusetts, which keeps track of all of this information. And so when these things come by, an alert is sent out to all of the people that told you, there’s two types of people that look at these kinds of things – one is the survey, like us, and there’s the people who are looking with the higher resolution instruments and want to study the composition. And they’re called, and we set up radar targeting and high resolution studies do be done at other observatories around the world, because then, if each one of these, if they look like they have different composition, they become a target for thinking about whether you want to send a robot out there to look at this thing and determine what the composition is, because that’s what interesting to planetary scientists.
Another thing, is that on July 4, 2005, we sent a spacecraft that flew by and impacted a comet that’s not too far out away. So, we were watching on the ground as that took place, but, of course, the best pictures came from the spacecraft that flew by at the time that this big, huge copper ball was sent into the comet and was exploded.
Salazar: What were scientists trying to accomplish there, with the Deep Impact mission you mentioned?
Bambery: The primary purpose of the mission was to impact it with something of a known mass, and just see what would happen. This is a very large object, tens of kilometers range. So, this thing that was only 300 pounds or so that struck it wasn’t going to bust it apart, but the idea was that – there’s basically two things that we talk about when we talk about near earth objects, one is asteroids and the second is comets. Asteroids tend to be rocky or very hard objects, while comets are composed of mostly water with a lot of intermixed rock inside of it. And the question was, if you impacted this big huge ice ball with an object, would you shatter it into millions of pieces, because ice, if you drop it on the ground, it will shatter, right? Whereas if you drop a rock, it will just bounce. So the interesting thing about that is that it impacted and created a huge explosion of water and ice that flew out of it, and it was very bright. It turned out that it had a very rigid body, meaning that the rock and the ice make a nice, solid matrix that doesn’t just shatter very easily. So, that was a very interesting piece of information.
Salazar: How likely is it that a massive asteroid will hit Earth?
Bambery: If you look at the history of it, it’s only been in the last few years that we’ve spent looking at it that we’ve realized that the Earth has been impacted a large number of times in the past. But, the question was, well, how often is it? Just because you spot 40 or 50 of these big impact craters on the North American continent, did these things all occur at one time, or did they occur over a period of time like a million years, did they occur every tens of thousands of years? A large study has gone on to try to figure out whether or not this thing happens very often. The answer is that we do know that the Earth is impacted by these things. And we know that the impacts are great and have had devastating results on here on the Earth. But those have been in the far past, not in the recent past. The last know crater, I think, that we had is in Arizona, and that was about 20,000 years ago. In 1907, one flew by and blew down a bunch of trees in Siberia, and we really haven’t had another incident since that time. So, it’s been about 100 years since anything really made close to an impact here on Earth. Again, if it was 1 km in size, it would make a huge problem for the area that it was hit by. And if you could imagine that one struck in the middle of the ocean, then considering the fact that the Earth is 80% ocean, that a huge tidal wave, or tsunami would endanger the shores of most of the continents of the world. That’s the threat that we’re more worried about. We do know what a tsunami, which was only 30 feet tall, did in Malaysia and southeast Asia. And we know what Katrina has done to New Orleans. So, those are the threats that we’re trying to understand. We deal with hurricanes every year, but something like this, which could raise a tsunami up to a mile high, that’s a significant impact. Now, what you would do in that case is up to other people. We don’t really deal with what-if scenarios like that. But, if something comes around like that you can imagine the large number of study groups that would try to figure out what we could do.
Salazar: How much lead time would scientists have in the event that an asteroid were speeding on a collision course with Earth?
Bambery: That’s the reason for the ten-year program that we have. If something were to strike in the next two weeks – we already saw what would happen with Katrina and Rita – the fact is that with just the knowledge several days in advance, for us, a lead time of a few days is very little help. That’s the reason for the survey program. We want to know years in advance – as much as a hundred years, so that we can think about two different things. We can try to push the thing out of the way so that it doesn’t strike us. And a hundred years is plenty of time to develop technology to make an asteroid not strike Earth. But if you have a seven-year lead time, that’s pretty hard to stop one of those, to prevent that. And if you only had two weeks, well that’s pretty devastating. It’s very hard to figure out what to do. That’s the reason for the ongoing program, to make sure what’s really out there, there’s nothing going to be a surprise to us. And truly, the only thing that we have to worry about is if it’s unknown. But I think for the most part, this program (NEO Survey) has done just about everything it’s been asked of it to do, and while we’re still discovering new ones all the time, each time that we’ve discovered them and followed them, we’ve found that they’re pretty benign out there.
