March 13, 2000

Selected Comments and Responses to Questions
Press Conference on NEAR Science Returns
March 13, 2000
31st Lunar and Planetary Science Conference
Houston, Texas

Dr. Andrew Cheng
JHU Applied Physics Laboratory
NEAR Mission Scientist

NEAR has been in orbit around Eros for just about a month. The spacecraft is fine, data is just pouring in, and that's about as close to heaven as a scientist can imagine . . .

Today we're going to focus on some exciting new developments. What we have seen over the past month does continue to support the developments that we announced in February: that there is a global fabric consisting of long systems of ridges, grooves and chains of craters that seem to run from end to end - basically the east-west direction on the asteroid - and are consistent with strata, or layers or systems of faults that are approximately aligned with the equatorial plane of the asteroid . . .

NEAR is in a 200-kilometer, approximately circular orbit around Eros. We're going to be there for the rest of the month, and in April we're going down to a 100-kilometer orbit, and in May we go down to a 50-kilometer orbit around Eros. The 50-kilometer orbit is going to be a polar orbit for the first time; until then we are in moderate inclination orbit that is mostly studying the equatorial and lower latitudes of Eros . . .

As we get down lower and lower, the resolution of all the instruments will improve, and there will be a lot more to come . . .

Dr. Clark R. Chapman
Southwest Research Institute
NEAR Imaging Team Member

You might remember from the flyby pictures of a year ago [December 1998] that there were two large features on Eros . . . Since then we've seen that Eros is very heavily cratered . . .

The two big impact craters are not as big in relation to the whole size of Eros as the craters that NEAR saw on Mathilde a few years ago [in June 1997], but then Eros has a very skinny banana shape, and relative to the smallest radius of Eros, those are pretty big craters. The shape of Eros is not dominated by these big craters . . .

In one region we see craters down to sizes smaller than 100 meters in diameter, about the size of a football field. Most of Eros is literally covered in these craters . . .

It's similar in density to the heavily cratered terrain on asteroid Ida - imaged by Galileo - but there are many more craters than we saw on Gaspra. All of these asteroids seem to have different personalities when it comes to their crater populations, and it's clear that Eros has had a very heavy cratering history during its lifetime. That's a little surprising because Eros is in an Earth-approaching orbit and it doesn't spend much time out in the asteroid belt. So presumably it came from the asteroid belt, and received most of its cratering when it was out in the main asteroid belt, where there are all those projectiles . . .

Dr. Peter C. Thomas
Cornell University
NEAR Imaging Team Member

Eros is a battered creature. Its saddle, which will surely be mentioned throughout the course of this mission, may be a very large old crater, but it certainly is exposing some interesting features . . .

Some of the products of the cratering on Eros do appear different from what we've seen in the other asteroids we've flown by and some of the small satellites that we use as analogs for asteroids . . .

We do see lots of blocks scattered all over the surface, sort of the size of football fields and smaller. They're greatly scattered, and there seems to be slightly more of them than on asteroid Ida, and perhaps more widely spread than on other objects . . .

Another aspect that is somewhat different from other asteroids is the apparent difference in brightness in materials within craters. You don't see ejecta blankets thrown out from the craters, but you do see, within craters, the evidence that materials either exposed from below the surface or as different particle sizes within the crater moving under gravity, give you different textures that show up as different brightnesses. This pattern of brightnesses within craters is very different from the other asteroids we've seen so far, and as we get closer we'll work out whether this is just different particle sizes or different compositions. But it's certainly enticing for when we get much closer . . .

Some other patterns of brightness are related to troughs, grooves and ridges. In the saddle, with the sun at a different orientation, you don't see the bright and dark pattern that you see in other pictures . . .

Dr. Maria T. Zuber
Massachusetts Institute of Technology
NEAR Laser Ranging Investigation Team Leader

The laser ranging device was turned on two weeks ago. It has a fixed ranging limit, but we needed to wait until we got into an orbit that would be compatible with obtaining ranges off the surface of Eros . . .

The data we're presenting today actually were collected at 4 to 6 times the distance the laser ranging device is actually designed to operate. So these results are preliminary but they show the performance of the instrument is excellent, and that bodes will for the future . . .

So far, we have collected 16,000 valid ranging shot . . . Because this is a high orbit and not optimal for laser ranging, we're essentially riding along most of the time with the camera and taking shots when we have the opportunity to be within the field of view of the asteroid . . .

The shape of the asteroid is important because it tells us something about the collision history of Eros and provides us with information, when combined with gravity data, about the inner constitution . . .

The mean radius is about 9.1 kilometers, and the maximum radius we are seeing is about 18.6 kilometers. It looks like the largest radius of the asteroid is within the southern hemisphere. The minimum radius we've been able to measure so far is about 5 kilometers, but we don't have a uniform distribution of points on the asteroid. Contrast this to what you'll see within the next month, because it's going to improve dramatically . . .

This is really just a teaser in a sense, to show that we're making great progress. This is the first instrument that has ever ranged successfully to an asteroid and we hope to have much more to show later this month . . .

Dr. Jacob I. Trombka
NASA Goddard Space Flight Center
NEAR X-ray/Gamma Ray Spectrometer Team Leader

We have less data than the other instruments at this point, but exciting data. The X-ray/gamma ray remote sensing system essentially utilizes the solar x-rays, the cosmic rays and natural radioactivity to obtain spectra characteristic to the chemical composition of the asteroid . . .

At this time, the intensity of that emission in the normal operational mode of the sun is such that we wouldn't see any significant signal. But on March 2 there was a major solar flare at 8:30 Universal Time and we were also lucky enough to be looking at the asteroid at the time . . .

We were able to see signals from magnesium, aluminum, silicon, calcium and iron. Statistically they're rather poor and because of the inclination angles it's very difficult to obtain abundances of significance, but it does indicate the great promise of this experiment . . .

Our major mapping period will be in the lower orbit, around 50 kilometers. At that time we should be able to get the maximum of the 2-kilometer resolution in terms of chemical composition on the surface of the asteroid. The gamma ray results right now are mainly background. The other part of it is that it has become a very significant burst detector for the International Planetary Network, so that is working very well . . .

Responses to Questions:

Q: What do the number of craters tell you about the age of the asteroid?

Dr. Chapman: There are countless craters. Even from the most recent images, we see craters down to smaller sizes, and I'm sure we'll see smaller ones yet, crater diameters smaller than roughly 1 kilometer down to many tens of meters. There is about as many craters as you can fit on the surface, so the crater density is as heavy as on any cratered surface that we've seen - not more - but like the most heavily cratered parts of the moon, for craters that size. So that means we can't tell how many craters were ever formed because successive craters destroy the previous craters . . .

In looking at the shapes of these craters a lot of them are degraded. They're softened in profile, they don't have very sharp topography, and that's also very characteristic of degradation by smaller impacts that we see in the smaller craters that we see in the lunar maria. So it looks like this thing has just been peppered by projectiles . . .

How old is it? We need to not only count the craters, but ask, how often was the object hit? For asteroids in the main asteroid belt, like Ida and Gaspra, we can calculate from the known asteroid distribution how often they're hit, so we have approximate ages for those asteroids. But Eros is in a transient orbit - it's currently detached from the asteroid belt, but it hasn't always been. So it's for us to work backward on this . . .

Dr. Cheng: Most of the craters we see on Eros were formed when Eros was in the main belt, but we don't know at that time if it was in its present configuration or whether it was still part of a parent body . . .

Q: What's the actual albedo or contrast of the light material within the craters?

Dr. Thomas: I hesitate to use the word 'albedo' because all these images that you can see were taken from one way, when the sun is coming in at a phase angle of 90 degrees. So one of the things you are sensitive to is roughnesses that are much smaller than the pixel size of the camera . . .

For some things, such as the markings in the saddle, it's clear that as you look at them in different orientations, the contrasts change quite a bit. The absolute amount of contrast is on the order of 20 percent or less - at one particular viewing angle. If you translated that into you looking down, with the sun coming straight down - and at the moment, we haven't had the pleasure of being in that geometry and we really won't with the camera - and seeing exactly what the possible brightness, hence, maybe compositional differences are. For these we'll infer those by the higher-resolution MSI [multispectral imager] color data and spectral data when we get there . . .

How do these contrasts change when you do change the lighting - is it really due to texture or something else that is brighter? It strikes me that given lots of these brighter things are on the upper parts of crater walls, that they may relate to how different-size materials are coming down the walls, rather than that you're slicing different layers. But there may be component of both of those involved . . .

I think that's an example of why you like orbital missions around these objects. You go whipping by and you sort of get one view, but we've seen enough different views already to realize that some things are brighter no matter how you look at them, and some change their relative brightness depending on the location of the spacecraft and the sun . . .

Q: Will the spectrometers have enough resolution to determine whether these features have compositional variations?

Dr. Chapman: The near-infrared spectrometer will have that resolution, and the other instrument will have that resolution only for the very biggest of the features . . .

Dr. Trombka: The other thing that's interesting about the X-ray/gamma ray instrument is that the x-ray essentially looks about 100 microns deep into the surface, where as the gamma ray goes down to about 10 to 20 centimeters. A difference that's going to be very important in terms of interpreting the infrared is how much the surface distribution is like the subsurface materials, and that's another area of investigation . . .

Q: One of the techniques coming out of the Mars Global Surveyor is the ability to discern what's going on inside by combining gravity data and topography. Do you anticipate being able to do that here, especially being able to address the notion of porosity inside Eros?

Dr. Zuber: You bet, as soon as we can get the two data sets together. We do have plans to put together global topography and gravity models to look at the internal density distribution. We've never dealt with a potato-shaped planet before, so one can expect there will be some challenges, but combined with the mass information we do feel like we can do something about the internal structure . . .

The altimetry and gravity data sets are so incredibly precise that if there are any small changes in the mass distribution or the shape due to settling, we hope to be able to see that . . .

Q: Is there any portion of the asteroid that you haven't been able to get an illuminated image of, and do you have a sense of whether the cratering pattern is uniform all the way around?

Dr. Chapman: We have not seen at high resolution most of the southern hemisphere. We saw it from a great distance during the flyby [in December 1998] and though most of it is in shadow now, it won't be in few months. By the end of the mission we'll have excellent data from around the asteroid . . .

However, in the illuminated side that we do see, there are spatial variations in the number of craters. In the cavity of the saddle, for example, there are not so many small craters, and although most of the asteroid is saturated with small craters, there are other places that seem to lack these small craters and we'll be investigating why as time goes on . . .

If, from more careful analysis, we see that the regions that lack the small craters are adjacent to other features, then you can say that those other features happened later, and covered up the preexisting smaller craters. So it might be possible to get a relative, time-ordered sequence of when features formed . . .

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