Andrew Cheng: One Giant Leap on an Asteroid

To the Presidential Commission on Moon, Mars and Beyond, Dr. Andrew Cheng, Project Scientist for the first robotic landing on an asteroid, presented his views about sending astronauts to an asteroid on the way towards lunar and martian exploration.

On Valentine’s Day, 2001, the NEAR-Shoemaker spacecraft successfully landed on the asteroid, Eros. Its remarkable journey–to soft-land on a peanut shaped asteroid about 176 million kilometers (109 million miles) from Earth, prompted Cheng to note: “On Monday, 12 February 2001, the NEAR spacecraft touched down on asteroid Eros, after transmitting 69 close-up images of the surface during its final descent. Watching that event was the most exciting experience of my life.”

The asteroids can be considered as an exploration destination. There’s great science because the asteroids tell us about the early solar system, tell us about how earth-like planets formed. There are great resources on asteroids because there’s material you find there that cannot be found anymore on the surfaces of planets like earth that have differentiated. And, finally, the role of humans in exploring asteroids is something that we should not ignore because humans can respond to unexpected discoveries.

Large, smooth basin. At bottom lie scattered boulders that appear like pebbles by comparison to the crater.
The rocks inside a crater on the Asteroid Eros. Numerous small impacts on the asteroid show brown boulders visible interior to the less exposed (white) lip of the crater. False-color for emphasis. Credit: NEAR Project, JHU APL, NASA

One of things we found when we studied an asteroid up close for the first time, this was Eros with the NEAR mission, we found that the asteroid did not look at all like we thought it would and that if we had astronauts there, I’m sure they would have responded, they would have looked at the features, thing like these ponded deposits that we saw, things like tectonic features, the bridges, the grooves, the liniment things that were not expected. The astronauts would have said, hey, we’ve got to get a better look at that. We better do something.

On a robotic mission, NEAR was a great mission, but we had to plan everything in advance. We had to plan the landing years in advance. We were not allowed to talk about it, but we had to plan it years in advance. Everything else in the mission was planned weeks or months in advance. But if we had astronauts there, they would have gone-they would have seen these features and said, hey, we need to look at it more.

And the other reason we need to study asteroids is that there is a great natural hazard associated with asteroid impact that every few hundred thousand years you can expect an impact that would have a destructive energy, something like global nuclear war. Simultaneous detonation of all the nuclear weapons on earth. We don’t know when the next one of these impacts will occur. We had NASA already trying to find, discover at least 90% of the asteroids large enough to do that kind of devastation by 2008. But the point I wanted to make is that even after we find them, there is more that we need to know. Suppose you find something and you think I don’t like the orbit it’s in, I want to change that orbit. How would you do that?

Well, we’re not really sure. The only thing I know is that if we’re going to try anything like that, blowing it up, deflecting it, whatever, that we don’t want to do something that would make the problem worse. That would be bad. Right now, we do not know enough about how these asteroids are put together, what kind of materials are in there, how strong they are, whether they’re single rocks or they’ve been fractured, how they were made. We don’t know enough about them to be able to say this is what you’ve got to do to solve the problem.

But that would not be the purpose of doing human exploration of asteroids. You would do that because you are interested in the asteroids, because you’re interested in maybe in the resources that could be found on asteroids, and you’d also be interested in proving out the manned exploration, manned expeditions and may want to go somewhere that, for example, you might want to spend a few months at. If you had to come back in a hurry, you could do it because it isn’t that far away from earth. That would be the purpose.

first view of annefrank
First images of Annefrank asteroid from Stardust
Credit: NASA/JPL, U. Wash

So when would it be best to undertake this human mission to an asteroid? I would say it was pre-Mars [on a timeline of exploration]. It’s a way of testing crew support-life support equipment and vehicles and so on. You know, that would need to survive years-long journeys through interplanetary space. I don’t think you’d want to send it near an asteroid. But it would stay there and use it to support astronaut exploration of asteroids.

I probably should also emphasize that you have resources on asteroids that you probably don’t have on either moon or mars and there might well someday be economically viable reasons for wanting to go out to an asteroid and use some of those things. So, there’s resource utilization experiments that I think you want to do on asteroids that would be independent. You wouldn’t be testing exactly the same thing that you would put on either the moon or mars, but there are things you want to do on asteroids and resource utilization.

The asteroid, depending on its size, would have very low gravity. A near-earth asteroids, most of them, we’re talking about objects a few miles across or smaller, those have very little gravity almost like being near the space station. You would be practically weightless nearby. You think of it more like operating an [extravehicular activity] EVA, let’s say, by the space station than on the moon.

Now on a larger asteroid, something the size of Eros, there is enough gravity you can imagine walking or hopping, you can imagine actually pitching some kind of structure or putting a flag into the surface. That kind of thing you could imagine doing on larger asteroids and also an asteroid as large as Eros you can go into orbit around. Very tiny asteroid, the one kilometer and smaller, they’re so small, you probably cannot even go into orbit around it. So, it is really more like station keeping. It’s more like extravehicular activities around the space station. So, it is different from what you would do on Mars. That’s surely true.

So, there is an exploration challenge there as well, which is not only to understand the record of the early solar system that is contained in the materials to be found in asteroids. Also, there is a problem of understanding the present day asteroids, their structures to know how to deal with one if we find that we’ve got to change it a little bit because it’s too dangerous. So, the final thing is that I’m interested in the asteroids as well. I think all of us should be because they’ve really played a central role in the development of life on earth.

The strange thing is that the earth and the moon are known to have undergone a tremendous early massive bombardment. These bombardments that are far bigger-the impacts that we’re talking about are so-called basic forming impacts, far bigger than we’ve talked about so far. Nothing like that has happened in the last billions of years on earth. But up to about 3.9 billion years ago, the moon and the earth were plastered with these huge, terrible impacts.

There’s no question that life could not have sustained itself before these impacts ended. But the strange thing is, truly strange thing is that as soon as that massive bombardment ended, basically life did start on earth. That when the earliest known evidence of life on earth dates essentially to the end of the massive early bombardment.

close-up of Eros as it rotates.
Video: NEAR Shoemaker flyover of Eros. Credit: Johns Hopkins Univ. AP

Now the question is, is that a coincidence?

We’re not sure. A lot of people think it’s not a coincidence.

So, what is the relationship between the bombardment of the earth and the moon by these asteroids and the development of life?

The one ended, another began.

Is it because these asteroids established the conditions favorable for life? We’d like to know.

Related Web Pages

Museum of the Galaxies
Stardust’s Success
Extraterrestrial Capture
Reading Rosetta
Coma for Halley’s Comet
Neat! Comet Crossing