Clark R. Chapman is a planetary scientist at the Southwest Research Institute at Boulder, Colorado. He studies small objects in our solar system—asteroids, comets, moons, and the planet Mercury. He was among the first researchers to draw attention to the danger of asteroids and comets striking Earth, and he has spent the past two decades evaluating that risk and comparing it with other dangers in daily life.
Why is it important for us to know what asteroids are composed of?
If in the future we want to mine asteroids, there is a big difference between a hunk of metal—which would be very difficult to process and do anything with—and an asteroid that has a lot of water bound up in its rocks that could be harvested and used, for example, by astronauts traveling to Mars. That is the practical side. From the scientific side, the minerals and compositions of asteroids provide clues to the earliest epics of the solar system. These objects are representative of the small bodies from which all the planets were made.
You can't find that out by studying planets?
C: The planets have undergone enormous transformations, due to the gravity, due to the heat, all the chemical reactions. In our own planet, we can see what has gone on in the last millions of years but very little about what happened billions of years ago. Asteroids and comets have transformed less.
Any other reason to care about little objects?
C: You could have one of the smaller asteroids come crashing into our planet, and that would be the end of civilization. Little can be important.
Have we found all the near-Earth asteroids that could potentially collide with our planet?
C: We have found all the near-Earth asteroids larger than five kilometers in diameter, and about two-thirds of those larger than one kilometer in diameter. In 1998 Congress and NASA authorized a search to find 90 percent of them within a decade. As we go down in size, they are harder to find, and there are more and more of them. We have found almost none of the asteroids that are the size of a house.
Could something the size of a house do harm?
How much longer until we find the rest of the asteroids that are one kilometer or larger?
C: It depends on the size of the house. My wife and I just visited the Biltmore estate in Asheville, North Carolina, the largest house in North America—250 rooms. An object that size would penetrate the atmosphere and make an enormous crater and do considerable damage. If our house were to hit, there is a marginal chance it would make it through the Earth's atmosphere. A smaller house would explode in the upper atmosphere and make a very bright flash. Some remnant meteorites might make it through.
Some colleagues of mine believe that we'll come close to finding 90 percent by the end of 2008, and other colleagues think we won't do it until the 2020s unless we build bigger telescopes. I don't know. It's tricky. Many asteroids are between us and the sun, where it is difficult to search. So while we are reducing the threat by finding that this asteroid and that asteroid are in orbits that aren't going to hit us, we'll never find them all.
How does finding them reduce the threat?
C: The way I think about it is that there is a natural threat—things are out there flying around, and some of them are going to eventually hit us. Others will crash into the sun, others will be flung out of the solar system, others will hit Venus, and so on. Suppose we find them all and calculate their orbits accurately and discover that within the next 100 years, none of them are going to hit Earth? That doesn't change the natural statistics for the far-distant future or what it has been in the past, but it seems to me that if you know you've found them all and you know they aren't going to hit, you are perfectly safe. If we find 90 percent of them and know that none will strike, we are down to being 10 percent in harm's way.
If we find one that is going to hit, what can we do?
C: The process of studying it will alert public officials, NASA, the Department of Defense, international agencies, and the countries that might be in danger. Then you follow the normal civil defense measures or extrapolations of them, such as evacuating people from ground zero, like they've tried to do with marginal effectiveness before hurricanes. You might store up food and medical equipment. The unique thing with the impact hazard, compared to other hazards, is that there is the technological possibility of moving the asteroid out of the way, if we have enough warning time and if we do it right.
In 2003 controllers at Johns Hopkins University landed the NEAR spacecraft on the asteroid Eros, proving we could do it and giving us a huge leg up on a mission that would push an asteroid out of its orbit by landing on it and setting off a rocket engine. You were involved in that mission. Were you surprised they did it?
C: The danger to that landing on Eros was the possibility that NASA would actually forbid it. They tried to. Dan Goldin, NASA's administrator then, was grimacing in the control room. I was on TV with Miles O'Brien, telling CNN about this thing, and then I walked into the next room as the landing was happening, and we were looking at pictures as they were coming in. Dan Goldin was outside in the hall. Even at that point he wanted nobody to say anything about landing on Eros.
Was he afraid that if it failed it would make NASA look bad?
You've made calculating the risk of impact something of a science. What are the odds as you see them?
C: I think so. Fear of failure had gripped NASA. Goldin was particularly fearful of failure because of the failures that had happened on his watch with the "faster, better, cheaper" philosophy, which focused too much on cheaper. There were definitely orders given not to refer to that landing as a landing. I've seen a letter that a NASA official sent telling them not to do it. The odds of it working were extremely good. The landing speed was equivalent to being dropped from a height of five feet. Even though NEAR wasn't built with landing pads, it was built strongly enough to withstand the accelerations it was subjected to and the shaking of the launch. There was little doubt that it would work. There was a 5 or 10 percent chance of failure—not because it would crash and not land but because something else would go awry with the electronics. But the chances of success were excellent.
You are much more likely to die from an asteroid impact that creates global climate disaster than you are to win your state lottery. But you are even far more likely to be killed in an automobile accident. Things that are extremely dangerous like smoking cigarettes and driving automobiles and famine and war have much higher risks than asteroid impacts. On the other hand, people worry about lots of other things that have very low risks, like shark attacks. Where I live, people worry about mountain lion attacks. They are much more likely to die from an asteroid impact.
Why do people get more worked up about mountain lion attacks than automobile accidents?
C: Human beings are not objective computers. They have feelings and attitudes and experiences. Research has shown that certain kinds of risks are more frightening because you are not in control. In an airplane, you are not in control. Being in an airplane is very safe. You undergo more risk by far driving to and from the airport.
How should we rethink our ideas of risk?
C: We should, as a society, understand all of these risks and think about them as carefully as we can and then decide to apportion our funds in a more balanced way than we currently do. I don't know where the asteroid-impact hazard would come out in that equation. What worries me is that it is all very haphazard. There seems to be no long-term planning in our society. It is very hard to get people to plan decades or centuries ahead.
Does that mean we will be poorly equipped to deal with asteroid impacts?
C: Yes. On the other hand, there are members of Congress of both parties who are interested in this topic and keep it alive. Last year, congressional committees passed measures that take small steps toward taking an impact hazard seriously. So it is not as though it is completely ignored.