ASTEROID RENDEZVOUS

February 14, 2000
		NASA project scientist Andrew Cheng discusses the agency's latest mission -- launching the first spacecraft to orbit an asteroid.

Online NewsHour Special Report: Space Exploration Aug. 26, 1999: Chandra Observatory, has relayed spectacular images of exploding stars to NASA July 20, 1999: The 30th anniversary of Neil Armstrong's first steps on the moon. Dec. 8, 1998: An Online Q and A: The International Space Station Nov. 20, 1998: Russians send the first component of the International Space Station into orbit. March 6, 1998: NASA scientist Alan Binder discusses the new discovery of water on the moon. Feb. 27, 1998: Is the universe evolving more rapidly now than it has in the past? Dec. 31, 1997: The Hubble Telescope's clear images reach Earth. Oct. 15, 1997: NASA begins its seven year mission to explore Saturn. Oct. 2, 1997: Forty years after Sputnik first circled the Earth, historians examine its impact. Sept. 30, 1997: An interview with Mir astronauts. Browse the NewsHour's coverage of science   NASA NASA's NEAR Web site	GWEN IFILL: Today a NASA spacecraft completed its journey to the asteroid named for the Greek god of love, Eros, becoming the first craft ever to orbit an asteroid. The NEAR spacecraft was launched in February 1996 from Cape Canaveral, Florida. SPOKESMAN: We've ignited the other three solid engines. All three of the first set are off. GWEN IFILL: Taking four years to reach Eros, 160 million miles away from the earth. Discovered in 1898, Eros is the second-largest near-earth asteroid, and has been studied longer than any other. If all goes as planned, NEAR will conduct a year-long scientific study of the asteroid's chemical and physical features, hoping to learn more about its evolutionary history. Joining us now for a discussion about the mission is project scientist Andrew Cheng. Welcome. Tell us exactly what is an asteroid first of all.
	What is an asteroid?
	ANDREW CHENG, Project Scientist, NEAR Mission: Okay. Well, we all learned as children that the solar system contains nine planets orbiting the sun. But in addition to those nine planets, there are a large number of what we call minor planets. These are small bodies that also orbit the sun. And, of those, they fall into basically two categories. There is the kind that when we look at them in the sky, we see a glowing tail of gas and dust. Those are the comets. And then there's the other kind that don't make this kind of tail. Those are the asteroids. There's probably hundreds of thousands of them, maybe even half a million of them that are bigger than one kilometer in diameter. So they're the most numerous members of our solar system. GWEN IFILL: What are meteors and meteorites in that constellation? ANDREW CHENG: Okay, the meteorites, some people would also include as members of our solar system. They are. They're just smaller. They're pieces of asteroids, even smaller. Going down even more than that is dust. We have hundreds of tons of dust from outer space falling into earth's atmosphere everyday. We live and breathe in that stuff --we just aren't aware of that. GWEN IFILL: The one that we are talking about is imaginatively named Eros. Describe it to us. We have a picture of it. Tell us what we're looking at that you released today, this picture. ANDREW CHENG: This picture is actually very important because it was the first picture taken after we got into orbit -- an hour after we got into orbit. First thing it means is that the spacecraft is working fine and we are in orbit. It also tells us that the camera is working fine. The way we have to take this picture is we have to tell the spacecraft in advance which way to turn and then when to take the picture. GWEN IFILL: And what are we looking at in that picture? Is it a crater? What is it? ANDREW CHENG: We have looking at a crater, probably the second-largest crater on Eros, it's on the eastern face. And, some days ago, for those of you who follow the Web site, I called it about the most perfect crater I ever hope to see or something like that. Now that I got a good look at it, it's clear. It's nothing of the sort. It's quite imperfect and all the more interesting for that. It turns out it's a complex of several craters and other interesting features. Okay. The large crater you see there is about three miles across. And we see inside, although you may not be able to make it out on the screen, there are boulders, some of them about a hundred or a couple hundred feet across. We see grooves. These are troughs in the surface. GWEN IFILL: So, how large is the entire asteroid? ANDREW CHENG: The entire asteroid is about 30 kilometers, which is 20 some miles long. We are in this picture from end to end of the picture looking at about one half of the asteroid.
	Watch for falling rocks
	GWEN IFILL: It heard it described somewhere as the size of a highway toll booth. So, when we think about asteroids in this country, which think maybe one might fall from the sky and hit the earth. What's the chance of that happening? ANDREW CHENG: Well, asteroids fall from the sky... pieces of asteroids, meteorites, they literally fall to earth every day. Now, smaller impactors hit the earth more often. The large impacts, a kilometer in size, let's say, those are the sizes that are big enough to cause global catastrophes when we hit. Those happen about once every million years or a few times every million years. GWEN IFILL: So this is not happening in our lifetimes. ANDREW CHENG: Well, not in our lifetime. The largest recent impact was in Tunguska, which is a remote area in Siberia in 1908, a 60 meter or smaller than a football field asteroid hit. And it destroyed several thousand square miles of that forest. GWEN IFILL: So, is it possible that a collision with the asteroid and the earth? Is that science fiction panic or is it something that's real? ANDREW CHENG: It's definitely real. It's definitely real.
	Why study asteroids?
	GWEN IFILL: Why is this mission... why is it important for us to go and study asteroids? ANDREW CHENG: Well, we have a couple of reasons. One of them is the one that you already mentioned, that asteroids, one of these days we will discover possibly an asteroid that poses an immediate threat to earth. At that point you might want to consider pushing it aside so it doesn't hit the earth. And in order to be able to do that, we have to know the physical nature of that asteroid, whether it's a single solid rock or a loose pile of rubble. That's the kind of thing we'll find out about Eros for example. GWEN IFILL: I wanted to correct myself. I said Eros is the size of a highway toll booth -- it's actually the spacecraft. I'm getting my metaphors mixed up. Describe the spacecraft to us because it's one of the faster, better, cheaper models that NASA's so in love with. ANDREW CHENG: That's right. This whole mission cost every man, woman, child in the country less than $1. We could spend $7 to watch a movie. We're spending $1 for the NEAR mission. GWEN IFILL: A year ago it was a near miss. ANDREW CHENG: A year ago it was a near miss. That's true. I felt an awful lot better this time. GWEN IFILL: So it's solar power. ANDREW CHENG: That's right. Solar panels are six foot long by four foot wide. The whole spacecraft is about six foot tall. GWEN IFILL: What is it equipped to do, just send pictures back? ANDREW CHENG: It's making all kinds of measurements. In fact, one of the most important things it will be doing is trying to figure out if the asteroid Eros is made up of material that's been around basically since before even the earth formed, from the very beginning of the solar system. And so we're taking measurements that will tell us whether this material has undergone the main process of planet formation. So, that's the process in which the planet inside gets hot enough to melt. Different liquids separate out. So that it's very similar to what happens in your salad. Your oil and your water don't mix together. They separate out. The same thing happens when you have a planet. Inside it's warm enough to melt the rock. And then the different constituents of the rock separate out. When that happens-- and we see pieces from the inside of such an object or the outside, they have different compositions. We will try to look for that. GWEN IFILL: Eros is potato shaped. Somehow this spacecraft has to orbit Eros in the opposite direction that it's turning. It seems kind of tricky. ANDREW CHENG: Very tricky. Eros has very little gravity. It something like 1,000 or 2,000 times less gravity than the earth. And that means when we're in orbit, we're only going something like five miles an hour not thousands of miles an hour like an orbit around a planet. We have to control the orbit of the spacecraft very, very precisely. If we're wrong by even a few miles per hour, we're in completely the wrong orbit or in danger of running into the asteroid or something. GWEN IFILL: We didn't know what an asteroid looked like until recently. When the fly-by happened, when it overshot its mission a year ago, we did get to get some pictures. What did we learn from that? ANDREW CHENG: Okay. At that time we got a preliminary value for the mass. We also learned something about what the shape of the asteroid was. It turned out to be a little smaller than people had thought before. GWEN IFILL: So, before I get to that question, let me edit myself here. What exactly happens in a year? Will they see this spacecraft ever land on the planet? ANDREW CHENG: Well, that has not been decided yet. We're going to actually focus for the first several months on just opening all the Christmas presents we've been given and just see what kinds of secrets the asteroid is going to tell us. Toward the middle of the year we'll be deciding what to do at the end of the mission. And landing is one possibility, yes. GWEN IFILL: So there is actually a possibility that that wouldn't be gathering up, it doesn't have a scoop mechanism or anything to send anything back to earth but it would just be a little bit more up close and personal. Would that be the whole point? ANDREW CHENG: That's the idea. Every time you get closer to a target, you learn more about it. Just in the same way that we just saw with the image we saw earlier, what looked like a very perfectly round and ordinary crater turns out to be very complicated and very interesting.
	The asteroid of love
	GWEN IFILL: Okay. I have to ask. It's Valentine's Day. Here we have a sighting of an asteroid named Eros. This is not a coincidence, is it? ANDREW CHENG: No, it's not entirely a coincidence... GWEN IFILL: How do you plan something like that? ANDREW CHENG: Okay. It was partly a coincidence. Without causing too much extra use of fuel, we could have arrived a week or two earlier or later. But given that choice, of course, we would arrive on Valentine's Day. GWEN IFILL: Why not? It gives you a little bit of extra boost. ANDREW CHENG: That's right. It proves we're all romantics at heart. GWEN IFILL: Well, Andrew Cheng, thank you very much and Happy Valentine's Day to you. ANDREW CHENG: Thank you.