SPACE.com: Why are asteroid impacts a hobby for you?

David Morrison: Several of us who have been interested for 10 years or so in the question of whether Earth is at hazard from asteroids and what's the right way to handle this issue have, for the most part, not received any direct funding from NASA to do it. It is not our primary job.

SPACE.com: In the search for potentially threatening asteroids, what are we doing right?

DM: We are efficiently finding the NEOs 1 kilometer [0.62 miles] or larger in size, which is a range that includes anything that is a global threat; that is, that could produce a global environmental catastrophe. We're more than halfway there. In fact, we have reduced the risk from an unexpected asteroid strike by about a factor of two.

We reduced this risk without actually having to move anything.

We do not expect any of the remaining [undiscovered] objects to be on a collision course with Earth. It would be bad luck if they were. On the other hand, if one is on a collision course, we want to know it. I think probably by 2008, when we have 90 percent of the larger NEOs catalogued, we will have concluded that none of them is a risk.

But the possibility that we're unlucky, that an impact might create a truly global catastrophe and kill hundreds of millions of people, motivates us to carry out the search and be concerned about this issue even though it's a low probability risk.

We deal with such things in ordinary life. For instance, when you buy fire insurance on your house, you actually don't expect your house to burn down. Most people go through their entire lives without having a major fire in their house. But you still buy the insurance.

SPACE.com: We always hear that eventually, Earth will be hit again, statistically speaking. When?

DM: A large impact is not something we expect to happen in our lifetime, in our childrens' lifetime, or even our grandchildrens' lifetime. It would be very bad luck if it did happen. But it could happen at any time.

Ultimately, the reason we can deal with this scientifically is that it's not a statistical random chance. Somebody doesn't throw the die every year and decide if we're going to be hit that year. If there's an object out there that's going to hit us, say, in the next thousand years, it is already on a collision course. So it can be found, its course can be determined. Asteroids don't change orbits capriciously, as is often depicted by Hollywood.

SPACE.com: Scientists and journalist constantly spout statistics about the asteroid threat. But in some sense the statistics are meaningless and may fuel some apathy among the general public.

DM: That's right. The issue is not one of refining the statistics. It's not whether it's a 1-in-a-million or a 1-in-2-million chance that it will happen this year. It's an absolute thing. Will it or won't it happen?

SPACE.com: What are we doing wrong in the NEO search?

DM: We have not yet seriously considered what the next step should be. In 2008, when we have found 90 percent of these larger NEOs, do we just keep going to get 95 percent or 99 percent? [The final few will be the toughest to track down, experts say.] Do we try to segue into larger telescopes so we can find fainter [and thus smaller] objects, those a few hundred meters in size that could produce a tsunami and wipe out the coast around an ocean basin?

We have been so focused on the immediate, higher-priority problem that there hasn't been much thought given yet to the next level.

One group that has considered the next level is in the United Kingdom. The UK NEO Task Force recommended two years ago that we set another goal, that we raise the bar and focus on [smaller and thus dimmer] 300-meter [roughly equal to three football fields] objects, which requires a new generation of search telescopes.

In the United States, we haven't done anything either to build such telescopes or even to plan for it.

SPACE.com: Is this just because the plate is full?

DM: It's partly because the plate is full. It depends on whether you think of the asteroid search funds as a fixed sum of money. If we are level-funded at $3.5 million a year, which is what NASA is now investing, then that pretty well all goes to the current search.

On the other hand, the National Research Council has recommended that the U.S. build a Large-aperture Synoptic Survey Telescope (LSST), a new instrument that could in fact do the survey down to 300 meters, by itself, as well as a lot of other good astronomy. Astronomers at the National Science Foundation are looking at that.

SPACE.com: If I promised you a billion dollars a year, where would you put it?

DM: I would not know how to spend a billion dollars a year on asteroids.

SPACE.com: But we keep hearing that it will be very expensive to find smaller asteroids.

DM: The smaller you go, the more expensive it is. This LSST has been estimated at about $180 million to build plus roughly $20 million per year to operate. So if you found that kind of money you could build such a telescope.

SPACE.com: So if I gave you a billion, you wouldn't put it all into NEO research?

DM: That's right. I wouldn't. Because I don't think it could be justified.

But there is another perspective that has to be seriously thought about. We always say that if we found an NEO on a collision course, we have the technology -- in principle -- to deflect it. But of course we've never actually done it. We've never done any experiments.

The alternate perspective says we should develop and test such deflection technology, that we should take an innocent asteroid that's not on a collision course and try sending a spacecraft to deflect it.

SPACE.com: A billion might come in handy for that.

DM: Yeah, it would [laughs].

The space program doesn’t normally operate this way, but you could set out a challenge, some sort of international prize for the first group that changes the velocity of an asteroid by 2 centimeters per second [0.04 mph]. They could do it with an explosion, nuclear or non-nuclear, by putting a solar sail on it, or whatever.

[A minor velocity change would induce a change in trajectory as an asteroid interacted gravitationally with the Sun, planets and other objects, putting it on an entirely new course.]

SPACE.com: Some vocal members of the NEO community are going to read this and say, "Here's the NASA voice again saying we're on track, we just have to worry about the big ones. But it's the small sucker punches we need to worry about, and we need to worry about them now." What do you say to those people?

DM: I'm not sure what point they're trying to make, because they speak as though with a small one we're likely to have less warning. I don't understand that argument.

Right now we catch any 1-kilometer or larger object that comes within a big volume of space -- within about 100 million kilometers [62 million miles] of Earth. To carry out a complete survey of 300-meter objects, you need to look at the same volume of space but detect fainter objects. The survey procedures and warning times are about the same.

SPACE.com: But it's easier for a small asteroid to escape detection, and there are more of them. So the chances are greater we'll get surprised by one, at least until they have been catalogued.

DM: That's right. But in that case you'll really be surprised. The first you'll know of it is when you see the sky light up as it enters the atmosphere.

If we haven't started building new telescopes within the next year or two, they won't be ready to take over in 2008. So now is the time to go on to the next step. But I think the search philosophy remains the same. Whether it's big ones or little ones, you carry out a survey, make a catalogue, calculate orbits, and predict future encounters with the Earth.

We can predict the impact of a 100-meter or 50-meter object just as far in advance as a kilometer object, once we find it.

SPACE.com: Aside from this conversation, you're wearing your astrobiology hat today. In your mind, is there any connection between asteroids and astrobiology?

DM: Yes, certainly. Astrobiology is more than just a search for life. Astrobiology is an effort to understand life as a planetary or astronomical phenomenon. We look at the long-term interaction of life, the environment, and the planet.

In that context, asteroid impacts are very important. We don't know how important, but it's at least possible that on Earth, impacts have been a major driver in evolution, because by producing mass extinctions, you essentially open up a huge number of ecological niches. After a mass extinction the rate of speciation is huge, a very quick radiation of new species. Impacts and their environmental effects, both past and future, are one of the elements of astrobiology.

To me, realizing that the end-Cretaceous extinction [dinosaurs, et al.] is due to an impact is illustrative of how fragile the biosphere is. That's a tiny impact compared to the planet as a whole. It's not enough to change orbit, or rotation, or magnetic fields or anything, yet it produced an ecological catastrophe, redirecting the course of biological evolution on our planet.

It is trite but true: Without the end-Cretaceous impact, humans would not be here.