The project I worked on this summer was part of the Near-Earth Asteroid Rendezvous Mission, or NEAR for short. I worked for Mark Robinson and Ann Harch, and with Jessica Edmonds. Our job was to simulate which pictures the multispectral imager, the MSI, would take when the NEAR spacecraft was in its 100km orbit around asteroid 433 Eros.

Three weeks ago, when Jessica gave her talk, she told you the specifics of what we did in order to simulate this. So I'm going to talk more about NEAR's instruments, the significance of this mission, and the difficulties that are involved in taking pictures of Eros.

Or, "Who cares about a dumb asteroid," and "why did this take you a whole summer?"



Right now, everything we know about asteroids comes from telescopic observations and four accidental fly-bys of asteroids. So we know a fair amount about the color and albedo of asteroids, and this allows us to make inferences about the composition of the asteroid, but we have no way of knowing how accurate our guesses are.

The NEAR mission is going to gather conclusive data on the mineralogy and chemical composition of Eros. It has six instruments with which to do this.



The Multi-Spectral Imager is what Jessica and I were simulating. It will analyze the morphology and mineralogy of Eros at a high resolution -- about sixteen meters by ten meters (in the 100 kilometer orbit).

The Near-Infrared Spectrograph also gathers mineralogical data. Its resolution is lower than the MSI -- about 300 meters by 300 meters, at best.

The NEAR Laser Rangefinder has by far the highest resolution -- in can be as low as two meters by two meters, and on average is three and a half meters by three and a half meters. It gathers data about the topology of Eros.



The X-Ray and Gamma Ray Specrometers both gather data on the chemical composition. The X-Ray Spectrometer, with a resolution of two kilometers by two kilometers, analyzes the chemical composition of the surface of Eros to a depth of one centimeter. The Gamma Ray Specrometer, whose resolution is about a quarter of the asteroid, goes to a depth of about ten or twenty centimeters below the surface of Eros.

By comparing the data gathered by these two instruments, we ought to be able to determine whether Eros's composition is homogenous.

And finally, there's the Magnetometer, which measures Eros' magnetic field.

This data is going to tell us a lot about what Eros is made of. We can use this information to calibrate the telescopic data we already have, and we can also apply it to two other problems:


The process that started the birth of our solar system was a supernova, which occured very near to a nebulae.



The shock from the supernova fragmented the nebulae, and it broke into a bunch of pieces. One of these pieces became what's knows as the solar nebulae.



The solar nebulae was a slowly rotating cloud of gas. Its dense center attracted more gas from the rest of the nebulae, and dust from the supernova. It became even denser and hotter as a result, and eventually became so hot and massive that it started to emit heat and light. This was the birth of our sun.



Meanwhile, the rest of the solar nebulae had flattened out into a spinning disk.

The gas and dust in the disk coalesced into tiny clumps known as planetesimals. These planetesimals stuck to each other and turned into planets and asteroids.

There's a mathematical formula which predicts, with high accuracy, what the radius of each planet's orbit around the sun is. However, it also predicts that there should be a planet where the asteroid belt now is. Because of this, scientists used to think that there had been a planet there and it has somehow been destroyed. However, scientists now believe that Jupiter's huge gravity simply prevented the planetesimals in that orbit from ever forming into a planet, and they formed into a lot of smaller clumps -- the asteroids.



At this point, the solar system had essentialy the same form that it does now.

Since then, the planets and sun have undergone a lot of changes -- fusion in the sun, and chemical reactions on all the planets.

Asteroids, on the other hand, haven't changed much at all. Some of them, especially the C-type (carbonaceaous) and S-type (stony) asteroids, are made of the same stuff as the solar nebulae was. Eros is an S-type asteroid, and knowing more about its composition should help us to understand the details of the evolution of our solar system better.



NEAR will hopefully also tell us about the origin of meteorites.

About 85% of meteorites are ordinary chondrites, which are made of iron and silicates such as olivene and pyroxene. Since S-type asteroids are the most common asteroids, they're thought to be the parent bodies for ordinary chondrites because of the numerical corelation.

Right now, we don't have enough information to compare the compositions of the two. The data from NEAR ought to conclusively tell us whether or not Eros is an ordinary chondrites, and it will allow us to calibrate the data we have on other asteroids to determine if they're parent bodies for ordinary chondrites.



Now for the second question -- why did this project take us so long?

Well, it turns out that taking pictures, and assembling a sequence of images that covers the whole asteroid, is really difficult, for a bunch of reasons.

First of all, Eros has a really funky shape. Different people have described it as a sock, a potato, a peanut with a tumor, and a boomerang. What this means for us is that when the NEAR spacecraft is in orbit, the distantance between it and the surface, as well as the angle of the surface below the spacecraft, is constantly changing. This makes it rather difficult to get a good picture.



Another problem comes from Eros's rotation -- its axis of rotation is in same plane as orbit its around the sun. This means that, for five months, its northern hemisphere is in permanent daylight and its southern hempishere is in permanent night. For another five months, it has alternating day and night, and then for another five months, its northern hemisphere is in permanent night. We can only take pictures of regions that are illuminated by the sun, and since we go into orbit while the southern hemisphere is in permanent night, we only get data on the top half of Eros for a while.

One of the constraints we're working under is that, because all of NEAR's power comes from its solar panels, it always has to orbit around Eros's terminator (the line between night and day). This gives us very little control over what part of Eros we're looking at.



The quality of the data we gather depends on two things: the incidence angle, which is the angle between a normal to the surface and a line from the sun to the surface and the emission angle, which is the angle between a normal to the surface and a line from the spacecraft to the surface. The sum of these two is known as the phase angle. In order for the data we get to be useful, the emission angle should be low -- preferable less than 30 degrees. We can change the emission angle by changing where the spacecraft point, but this will also change the incidence angle. We would like to keep the incidence angle fairly low too, but since we orbit the terminator, it's not easy to get both of these to be less than thirty degrees.



These are the problems we encounter when we're just taking one picture. When we want to take a whole bunch of pictures, we have a whole new set of constraints and problems. For one, we can take a maximum of 600 pictures a day. Also, there are limitations of the time we're in the 100 kilometer orbit, and what we can do while we're there. We spend about 10 days in the 100 kilometer orbit, and we need to spend 8 hours every day sending data back. While this is happening, we can't take pictures because the spacecraft is no longer pointing at Eros. We also might not be able to take pictures the three days that the NEAR laser rangefinder team has control over spacecraft. The combination of these constraints means that there are some lattitudes that we just can't take pictures of.

A final problem we encountered in doing these simulations was uncertainly. We only know the shape, mass, and moment of inertia of the asteroid to within about ten percent. If any of these were off by even five percent, a lot of our simulations would be completely useless. So we had to run simulations for a couple of different cases.



So, to summarize:

We care about this dumb asteroid because it can tell us a lot about the evolution of our solar system and the origin of meteorites. Specifically, it can help us determine an accurate shape map which will tell us Eros' density. It will also show us if there are small-scale heterogenieties on Eros' surface, and a crater count can tell us Eros' relative age. All of this will help us learn what Eros is made of and tell us about the beginnig of our solar system.

Finally, it took a whole summer to do because there are a huge number of things we had to take into consideration -- it's just a really difficult problem.

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