Near Earth Asteroid Rendezvous

Transcript of the Jan 31 Press Conference

NEAR End-of-Mission Press Conference
January 31, 2001
NASA Headquarters, Washington DC

Dr. Edward Weiler
Associate Administrator for Space Science, NASA Headquarters
Washington, DC

It's a real pleasure to be here today to tell you a total success story. NEAR has been a shining example of the success of the faster-better-cheaper program. It's a mission that was completed in 26 months from start to launch, for a total run out cost of $223 million.

This mission has achieved many firsts and I'll take a minute to read them:

The mission is now completed and has done the science it needed to do. The question then was what do we do with it? Although this mission wasn't designed to be a lander, you're going to hear about how were going to end the mission: by doing a controlled descent toward Eros.

Notice I didn't say landing - it's not a lander.

Some of you may remember some non-controlled descents we did in the early Apollo program, called the Ranger program, and when I was a kid I remembered these things coming in with their old-fashioned television-tube camera, and you saw the images come down in very high resolution. Sometimes only half the image would be returned because oops, it crashed.

This is a controlled descent, and we were just debating whether a linebacker from the Baltimore Ravens could actually catch this thing as it's coming down, because its only coming down at about 7 miles and hour. But it is hundreds of kilograms, so it would take a strong person. On the other hand, he might be able to throw it back into orbit. And if you're an "Erosian" watching this thing come down, it has a transverse velocity - that is, it's orbital velocity, roughly - is about the speed of a slowly walking individual. It's not the kind of orbital velocity you're used to because, remember, Eros has very little gravity. I believe its gravity field is .001 G.

And finally, if you're not thrilled by the science and the basic exploration we accomplished, and you ask why we spent $223 million on this program - which is about a dollar per American over the life of the mission - let me tell you why it might be important. This marks the first time we really began the in-depth reconnaissance of the so-called class of asteroids, near-Earth objects. These are objects that in the past have caused some bad days for species on the Earth, namely the dinosaurs.

We are not the agency that has the responsibility for protecting the Earth, but we do consider it a responsibility to learn as much as we can about these objects, and what you're going to hear today is the beginning of that process . . .

Dr. Andrew Cheng
NEAR Project Scientist
Johns Hopkins University Applied Physics Laboratory
Laurel, MD

For me, this mission has been the thrill of a lifetime. It's been 10 years of my life and worth every moment. And it's not just because we have met our scientific objectives - in fact, we exceeded them all. It's just being able to share with others the thrill of being able to see back to the very beginnings, to see where we all came from and look all the way back to the time when the solar system was forming and the planets were being built.

The reason I say that is that the asteroids are the remnants from the process of building a planet. So, we had some big questions when we went out to study a near-Earth asteroid. When we build a planet out of a cloud of debris, rocks, dust and ice that's orbiting the sun - it's a flattened cloud we call a disk - we have to assemble these chunks of solid matter into a planet.

That's done with collisions; we have to have little bits of rock collide with each other gently enough that they stick together and make a bigger object after the collisions. If we have too violent a collision, then these objects will break each other up into small pieces again, and we have to start over. So the kinds of collisions we have is important to understanding how a planet is built.

Also, we want to know what kind of materials are out there. Specifically, is Eros, our satellite, made of materials that date back to the time when the planets were being built?

So what's out there? What kind of collisions took place? And when we get out to Eros, can we see the effects of these collisions? Is Eros going to be so battered by collisions that it's not a coherent body at all, but what we call a rubble pile? [A rubble pile meaning that it had been shattered into lots of little pieces that are now held together only by their own gravity, or barely more than their own gravity.]

What kind of a regolith does Eros have on it? [Regolith is the fragmented surface layer on a body - it's what we call dirt on the Earth.] How much regolith is there and what is it like?

So these are some of the questions, and the good news is that we've made a lot of progress in understanding these basic questions. We know from X-ray/gamma-ray data from NEAR that Eros is fact a very primitive body made of material that dates back to the time when the planets were being formed. It's an undifferentiated body unlike the Earth and other planets, that when they first built up to planet size they heated to the point that they melted and they separated into a core, a mantle and crust. That process never happened for Eros or happened for a parent body of Eros. Because Eros, as we find, is in fact very similar to the material we find in the meteorites we call ordinary chondrites . . .

From the abundance of the major elements as measured by the X-ray/gamma-ray instrument on NEAR, we find Eros is very similar to that material . . .

The other question we learned a lot about is the structure of Eros. We find that that is not a rubble pile . . .

What we see . . . is that the asteroid does have coherent structural features, typically in the form of long degraded ridges a hundred meters or so high, hundreds of meters wide, and several kilometers long. These are the kinds of coherent structures that indicate to us that the asteroid is in fact a consolidated body. It was very likely fragmented from a larger parent body that was broken up in the distant past. This parent body was undifferentiated . . .

The final surprise, when got close to the asteroid, we found there were aspects of the regolith on the surface we did not expect . . .

Dr. Mark Robinson
NEAR imaging Team member
Northwestern University
Evanston, IL

I'm going to step you through four levels of detail of what were seeing on the asteroid with the imaging experiment.

First we'll go through a movie that was taken from a 200-kilometer orbit, and step down to individual images about 100 kilometers, 20 kilometers, down to 6 kilometers from the surface . . .

You start out looking at the 180-degree end. Illumination will be up toward the southern hemisphere of the asteroid, so what will come into view is the depression Himeros, then Shoemaker Regio will come around, then over the end, before we see this beautiful crater, Psyche, and see the sun setting on the on the interior of the crater. Then we'll finish up looking on this heavily cratered end.

One full rotation of the asteroid, illumination is in the southern hemisphere.

When you run it slowly, first you can see the asteroid is pretty much heavily cratered on the 180- degree area. As the large depression Himeros - which was probably formed as an impact crater - in the lower left hand corner of the screen you can see a brightly illuminated cliff; that's a structural feature indicating there is some sort of internal strength to the asteroid, and that cliff is about 100 meters high. Now passing into view is Shoemaker Regio, a large impact crater filled with very thick debris, or regolith. Coming up on end you'll see the crater Selene, which has bright albedo features inside it, which is very unusual. Then, on the lower right coming into view is Psyche, which is the 5.3-kilometer impact crater. The terminator moves along the inside of the crater going into sunset, then on the right you can see the heavily crated surface indicating that its relatively old. The craters, as a general rule, are very rounded and degraded looking, indicating that the surface is very old, that it suffered a lot of micrometeorite bombardment on top of the larger craters that have worn them down.

Let's move into the next level of detail . . .

This is a really spectacular view - a mosaic - looking into the large 10-kilometer depression, Himeros. In the south end, looking toward the north, you can see the terminator in the upper left-hand portion of the picture. You can also see a little bright streak, which is actually that large cliff in the movie.

Some important things we've seen are that the surface is covered with tens of thousands of boulders. The boulders are irregularly distributed; sometimes they form in clumps and sometimes they're found singly. To get a sense of scale, you're looking at about 3 kilometers width across the bottom of Himeros. The somewhat sharp crater in the upper right is about 300 meters across, and the smaller crater on the bottom right is about 200 meters across.

What you can see is that this area of the asteroid isn't as heavily cratered as we saw in the movie. Most of the asteroid you can see, especially near the terminator, there are craters on top of craters on top of craters. The surface is saturated. But we also can see that some of the craters, especially the smaller ones 100 to 300 meters, are almost completely infilled with material. This is good evidence that there is a relatively thick regolith on the surface, regolith being the loose, broken up rock on the surface. Probably during impacts elsewhere on the asteroid the ground shakes and this material is able to move around and move into craters.

This image shows the next level of detail. You can see the scale bar in the lower right. The right crater right above it is about 300 meters in diameter. Once we start getting closer we see even more boulders; they keep appearing at all scales.

We also see craters in various stages of degradation. If you look on the right-hand side of the screen there is a crater that is so subdued it's almost difficult to identify it is a crater, but it is a subdued depression. At left-center is a crater that still has a rim, and at middle center is one that is somewhat in between of being degraded and relatively fresh.

But what's very striking is that we see in the center-left of these craters is a very flat surface in the middle of that degraded depression. We can see there are three boulders on the right-hand edge of it casting a shadow; these are what we're calling right now ponded deposits. They're very unusual; we didn't expect this and we don't see similar features elsewhere in the solar system.

A working hypothesis right now is these are possibly very fine portions of the regolith that have been shaken and moved into depressions during seismic events on the asteroid.

This is a really fantastic view of the surface taken last October, when the spacecraft swooped down to within approximately 6 kilometers of the surface. What we see is when we get down closer and closer - about 5 times higher resolution than previous images - is again that more boulders and more indications that craters are subdued and infilled at all scales. This is really a magnificent example of that in this image is that in the right-center of the two overlapping craters - there are very subdued, degraded and filled in. When we do the measurements - we can do this by shadow measurements or with the laser altimeter - of these bowl-shaped craters we see the very fresh ones have a depth about one-fifth the diameter of the crater. When we look at these heavily degraded craters the depth is much shallower, so it's further evidence they've been worn down or filled in.

If you look to the side, in the lower left, there is another one of these ponded deposits. And to the lower left of the pond there is a boulder that has an apron around it. This is not uncommon; we've seen this in several places around the asteroid associated with these ponded deposits. It's very possible this is material from the boulder as it's being degraded by micrometeorite bombardment, or it had loose material on it when it landed, and the material has sloughed off the sides.

So it's been really interesting to look at the surface to see these landforms associated with a thick regolith . . .

Dr. Jessica Sunshine
Senior Staff Scientist
Science Applications International Corporation
Chantilly, VA

Not being a member of the NEAR team, it's been a pleasure watching the results come in from NEAR over the last year. They've asked me to stand back and look at what we've learned . . .

Probably the principal reason were looking at asteroids is because asteroids, meteorites and comets are windows into the early solar system - a record of which on Earth has really been obscured by subsequent processing. We want to understand what's going on in the early solar system because these fundamental materials and processes are what ultimately formed our planet and what we evolved from.

NEAR has met its primary science goal, which is to try to understand how we can relate these meteorite samples we see here to asteroid bodies. NEAR was designed to do that and it had the right complement of instruments to do that: the X-ray/gamma-ray spectrometer, the imager and the near-infrared spectrometer, and we're able to see a primitive composition on the surface of Eros that is similar to these types of meteorites. We see evidence of regolith processing in the imagery, and we also see evidence in the spectra that the regolith processing is affecting how we observe the color of asteroids, and how they look when we look at them with telescopes. That has been a problem in asteroid science that we had these meteorites, but we didn't have asteroids that matched them. It turns out the regolith is a problem here in that it changes the color of what were looking at.

Ultimately one of the principal conclusions of the NEAR mission is that we know how to relate these things; that was in large part what the mission was designed to do. But NEAR, like any other mission, had a lot of unexpected results . . .

The first is that if you look globally, even compared to other asteroids, Eros is very bland in terms of its color. That was surprising. I think our expectation coming in was that we would see relatively fresh impact craters that show differences in the amount of regolith that cover them, or ejecta that had differences in their color, and we'd see more evidence from this that we've seen in other asteroids because we now had our first close-up look, and it turned out that wasn't the case, and that was very surprising.

In fact, when we go to even smaller scales, we had a whole new series of issues that Mark Robinson discussed, and I would like to comment on that . . .

When NEAR first got images back they were in black and white, and the first thing you noticed is the distinct brightness differences that are quite pretty and quite stark. There is almost a brightness factor of two between those bright and dark areas.

And again, we were very excited waiting for the color and spectral data because we thought we'd see some compositional differences associated with this, and if not a compositional difference, then certainly a difference in its weathering process . . .

This is a color image and it's quite subtle. It's an infrared color composite, and even in the infrared, the difference between these dark regions and these bright regions are very subtle. The bright regions in this color are slightly bluer than the background, which is red. This is significantly less than we would have expected to see, and to be quite honest, I don't think we really understand why this is the case.

So what were seeing there is downslope movement of regolith off the very steep slope, exposing relatively fresh material, but not fresh enough to see what the original composition looked like, at least optically. I think were going to continue to try to struggle with understanding why that's occurring, as well as the morphologic and geologic processes involved in regolith, over the next few months. This is going to become very important as we move toward the future and Eros becomes our frame of reference for the study of small bodies throughout the solar system . . .

Dr. Robert Farquhar
NEAR Mission Director
Johns Hopkins University Applied Physics Laboratory
Laurel, MD

We've been having a lot of fun over the last few years and we've fulfilled all the primary science goals of the mission. But all good things must come to an end, and the mission is scheduled to end in [February]. So we're trying to find some way that we can end the mission on a high note, and we've come up with a way to do some bonus science, and also do some things with the spacecraft that have never been done before . . .

Of course, when you try to do something like this there is always a little risk involved. But since we're trying to get bonus science and the primary goals have been satisfied, in my view, the only risk is not taking one at this point in the mission.

So we've come up with what we call a controlled descent to the surface, and there are two goals connected with this operation. The primary goal is to get higher resolution images of the surface, 5 to 10 times better than we have gotten to date. On the way down we're planning to get about two images every minute and send them back in almost real time.

The secondary goal is to try to impact on the surface in a relatively soft way - a soft landing - about somewhere between 1 to 3 meters per second, which is about 2 to 7 miles per hour. Now 7 miles per hour may not sound very fast, but I have personal experience with this kind of velocity. When I was in the paratroopers and we were jumping with World War II parachutes you would hit the ground at about 7 miles per hour if after swinging back and forth, so I know it's a fairly hard landing.

In any case, this is what we're trying to do on Feb. 12. This plot shows a view of the orbit around Eros on Feb. 12; it's actually the one we're in now and we're going to stay in this orbit until then. This is a view from the sun, and Eros' south pole is just about pointing at the sun right now. Eros is moving in a clockwise direction as it spins on its axis, and our orbit is going counterclockwise. We are going to de-orbit with the first engine burn about 4 ½ hours before we would impact on the surface.

As it comes down there is a kink in the orbit close to the surface; it takes about 4 hours to get there and not too much is happening, we're just drifting down. At that point, about 5 kilometers above the surface, is where we do our final controlled descent.

This is a plot of the altitude starting about 5 kilometers, altitude versus time. That first burn, which I call brake number one, lasts about 3 minutes and is followed a bit later by another engine burn, brake two, which takes about 5 minutes, then another one that lasts 6 minutes, then a final one at about 4 minutes.

You can see we're doing a whole series of these engine burns and we've never done this before on the mission. So this is rather complicated, but I have full confidence that we have very experienced teams both at the Applied Physics Laboratory and the Jet Propulsion Laboratory that will be in charge of planning and implementing this . . .

The engines will fire and we come out of orbit; the sun is facing the right side of the solar panels and we're getting power. The thrusters fire as it gets closer; we do a roll to position ourselves properly for the final engine firing . . .

All the way down we stay on the high-gain antenna; it points toward Earth, and about 20 degrees off of that is the sun, so were getting full solar power on the panels on the way down, and the imager is pointing down . . .

When the spacecraft actually hits it could roll, or we could go into what we call the ostrich mode [tipped over on its top]. We don't want to do that because it's hard to communicate with it. We are going to try to communicate with the low-gain antenna, but the chances of contacting it are probably less than one percent . . .

There have been a lot of firsts on this mission . . . and the one I'm proudest of is that we were the first planetary orbiter of a small body, which happened on Valentine's Day 2000. We've joined a rather select group of spacecraft starting with Sputnik I and ending with Galileo going around Jupiter . . .

Mark Robinson on the Landing Site

The landing site is right on the outer edge of Himeros . . . It's exciting geologically because we're on the edge of this large depression - which is probably a very large impact crater - and we'll be getting images as we come down of the inside of Himeros as well as of the heavily cratered terrain on the outside . . .

In the past week we've gotten this swath of low-resolution imaging . . . and we can see the surface is littered in boulders. But what's particularly exciting about in here is the possibility of looking in very close detail at this regolith that we've seen, which inside Himeros is very streaky. You can see in the images there have been landslides and dark material like trails coming down in there, and to get 10 to 20 times higher resolution of what we already have will really be a useful tool for us in interpreting the exact nature of the regolith, particularly how it's distributed and how the depth varies in Himeros and in the typical heavily cratered side.

So we got very lucky that this is the area we're going to land in, because we have the possibility to look at two very distinct terrains . . .

QUESTIONS

Q: Could Eros be a source of any meteorites found on Earth?

Cheng: Eros could be a source of meteorites; actually, almost any asteroid could be, because we found news ways that fragments of asteroids can be transported to Earth. We are not able to trace yet a definite connection between Eros and any individual meteorites in our collection. We don't have any compositional detail in the measurements yet to make that connection. But we can say that Eros is very much like a broad class of meteorites we call the ordinary chondrites.

Q: Clarify the idea of why the inside of these large craters seem so smooth - is it the slumping of the regolith?

Robinson: Probably that's true. And the evidence for that is we can see in the walls of these craters several features; one of the most convincing that this downslope movement, is you can actually see inside Shoemaker Regio, little fields where there have been slides coming down the sides of the steep areas, and if you look inside Psyche, the big 5.3-kilometer crater, is that there are impact craters that are small - say 100 or 200 meters - you can just barely see . . . They've been filled in and there is barely an imprint of it now . . .

And just the fact that a very smooth and not impacted at the bottom - in some cases you can see lobes of material - very strong evidence this is fine-grained material moving down step slopes of crater wall. The slopes are on the order of 25 degrees . . .

Cheng: Up to 50 degrees, actually.

Q: About how much fuel will you have left on impact after touchdown, and what are the risks of that fuel getting loose and causing some dynamic reaction? And, at what altitude, due to the time lag in signal, will your last input instructions to the craft be possible?

Farquhar: I'll answer the second question first. We get our last input about an hour and half or so before actually impacting, and it's before we start any of the braking engine burns. We can't do anything about that series of braking burns; there's not enough time, because round-trip light time is about 35 minutes.

On the first question, we don't know precisely how much fuel we have left because there is an uncertainty of about 4 or 5 kilograms . . .so we wont have very much left . . .

I don't think it's going to hit that hard, but if it did, the only thing we really have to worry about is if the hydrazine somehow got mixed up with the oxidizer that we had for the bipropellant engine . . . there would be some kind of reaction. But we'd have to rupture more than one tank for that to happen, and there's very little oxidizer left, maybe 3 kilograms. We're not expecting any violent explosion.

Q: Is there one big question that NEAR has brought up, and what would be planned for addressing that in the future, in terms of visiting another asteroid?

Cheng: There are still two unresolved big questions. One is that we measured a pattern of abundances of five elements; four of them fit very well with the so-called low-iron ordinary chondrites. One of the elements that doesn't fit it is sulfur; there is very much less sulfur on Eros than you would expect if in fact Eros were just like the ordinary chondrites. And we're still not quite sure what that means. Either Eros is really not like these ordinary chondrites at all; it's still undifferentiated, but it may be partially melted. And there is a very rare class of meteorites that does appear to be in between. We're still debating whether Eros could be one of these, what are called primitive achondrites.

The other big question is in fact the nature of the regolith in very small scales at Eros. When we get up close what we see are very few craters of the size of this room or smaller, but a very large number of boulders of those sizes - and how the surface got that way is still very much a mystery. What do we do about it? One of the things were trying to do is to get up and take as many close-up pictures we can, get as close as we can. In the future we would probably like to land a scientific package on the surface of an asteroid and maybe bring back samples . . .

Sunshine: It's important to remember that Eros is a near-Earth asteroid, and that what exactly is going on and what might be different in the main belt and the space environment, are issues for potential future exploration.

Q: What happens if the first burns go all right or the last or second to last does perform adequately?

Farquhar: Then we'll hit a little bit harder. Actually, we've calculated that and if none of the final braking burns go properly, we would hit at about 9 meters per second - that's about 20 miles per hour, roughly - and that should probably do us in. If some of the other burns right at the end don't work, it might hit at about 5 meters per second . . .

Q: If it lands exactly as you planned and you're able to communicate with it, how long to do you expect the spacecraft to transmit pictures from the surface?

Farquhar: It will not be transmitting any pictures from the surface. We aren't able to get any telemetry - we have to be on the high-gain antenna for that - so the best we can hope for is to get some kind of beacon from the surface that would verify that it was still alive. It could stay alive for a couple of months, really, because we're in sunlight almost all the time if it lands properly.

Q: Since the camera is designed to take pictures from a distance, when you come in very close for the pictures, is it going to look like you're . . . too close to your targets in order to get a fine, crisp image?

Robinson: It's a very narrow camera, about 2- or 3-degree field of view . . .

Cheng: The camera remains in focus to a range of about 500 meters above the surface . . .

Q: What's the mass of the spacecraft when it actually does its landing? Given the mass of the spacecraft and the one-thousandth of a G, is there any good comparison and could somebody conceivably catch it?

Farquhar: It's about 500 kilograms at the present time, having burned a lot of fuel, about 300 kilograms worth. But I wouldn't want to try to catch this thing because it's still moving fairly fast.

Q: Is there a major, overall question about the origin of the solar system that you're hoping to get answered from this particular mission?

Robinson: One of the key things we'd like to find out from the NEAR mission about Eros is what class of meteorites can we connect what class of asteroids to, and it looks like we're making very good progress connecting Eros to ordinary chondrite meteorites. But you have to remember that there are hundreds or thousands of S-type asteroids and they fall into classes and subclasses themselves. And the reason we want to connect the meteorites to the asteroids is because that gives us a real good look into the formation of the solar system.

We have all these wonderful meteorites that we've learned a lot about the formation of the solar system from, but we'd really like to know their geologic context. Where did they come from within the solar system? A way of thinking of that is if someone went around Earth and collected a couple-hundred different samples, and took them to Mars and gave them to a Martian geologist who knew nothing about Earth. If you didn't tell them where the rocks came from it would be very difficult to put together a coherent story about the Earth. You could learn a lot about the Earth, whereas if I told them exactly where they came from and put them in their context, they become much more valuable and much more useful.

So, NEAR is really a good start toward understanding the early solar system, but you have to remember we've only looked at one asteroid in this sort of detail . . .

Q: What kind of detail can you expect to see in these final photos?

Robinson: If we get extremely lucky and say we get some imaging from 500 meters above the surface, I think the resolution will be on the order of 10 centimeters to 5 centimeters . . . and that would be about 10 times higher than the highest resolution we have so far. Remember that most of the imaging so far was from between 35 and 200 kilometers. We've made couple of excursions very low to the surface, and those data are incredibly valuable . . .

Cheng: In those final images we'll see objects as small as a few inches across, at best.

Q: Why Eros over another near-Earth asteroid?

Farquhar: It's because it's a very large near-Earth asteroid; technically it's the second largest. It's also one that is accessible, so it was possible for us to get a fairly large spacecraft there. There were some asteroids that might have been of interest also, but very difficult to get to.

Q: Who had the idea initially to attempt this landing, and how did that proposal come about?

Farquhar: I have to plead guilty to this. We started looking at this even before launch, but we weren't serious about it to begin with, but we thought his might be a nice way to end the mission and get some close-up images. Then as we looked more closely at it we figured we could use this series of braking engine burns to actually soft land, so this would be the way to get some flight experience for future missions. That's the main idea there.

There is a good scientific basis - to get the high-resolution images, that's the main goal - and also an engineering test.

Q: Gene Shoemaker, all through is life, waged a fairly consistent campaign to do more to try to look for these near-Earth objects, potential Earth-crossers. Is enough being done right now by NASA, the U.S. government and, for that matter, the rest of the world to identify the types of asteroid that could make us have a very bad day?

Weiler: We have a commitment to Congress over the next 10 years to map 90 percent of all near-Earth objects that could cross Earth's path and eventually cause a bad day . . . We're ahead of schedule on that. By 2008 we should have cataloged 90 percent of all asteroids that are 1 kilometer or larger that have a chance of crossing the Earth's path . . . so we're on our way to that goal. That is being done with ground-based telescopes.

But that's just part of the program; NEAR was another major part. As I said in the introduction, this is the first time we've really gotten up close and personal with one of these objects that could have been the type that eliminated the dinosaurs 65 million years ago. There are a lot of theories about what these things are made of. Some people talked about rubble heaps, and this one is not a rubble heap, it's fairly solid object. This is the beginning of understanding these objects, and this will be useful information to some generation perhaps, sometime in the future that may have to deal with this . . .

In addition to NEAR we have missions in the queue to go to comets. Deep Impact . . . will actually blow a hole in a comet and analyze the gas and dust that comes out, to understand some of the most primordial material we've ever seen, the pristine material that comes out of the comet.

So in addition to the asteroids, were looking at comets because comets are also potentially a threat someday in the future to Earth. So it's not just the few millions of dollars we're spending on the ground-based search for these objects; NASA is spending hundreds of millions of dollars on these missions. So we're doing as much as we can do with the budget we have.

Near Earth Asteroid Rendezvous