A lot of the slides are necessary to this talk; in places where it doesn't make sense without the picture, a link has been inserted to a small picture of the slide.


This summer I worked on the Near Earth Asteroid Rendezvous mission, planning the Multispectral Imager sequence for the satellite's 100 kilometer orbit of Eros. That basically means I figured out where to point the camera and when to take the pictures.


The NEAR spacecraft was built at the Applied Physics Laboratory at Johns Hopkins University. It was launched on February 17, 1996, and will rendezvous with the asteroid 433 Eros on February 14, 2000. On board there are six major instruments, but my work only used one of these, the Multi-Spectral Imager, or MSI.


The MSI determines basic morphology and topology of Eros, as well as color imaging (which is useful for determining composition).

I was working with Emily Peters most of the time, and our job was to construct picture taking schemes for the MSI. We worked under Mark Robinson, and we were mostly directed in this work by Ann Harch at Cornell University.


It's really difficult to take pictures! Eros's shape makes it hard to get a zero degree emission angle, and it's mass is oddly distributed, so it's hard to orbit normally. The NEAR spacecraft has to orbit around the terminator in order to keep its solar panels pointed towards the sun.

We want to view the asteroid with the camera at as close to perpindicular to the surface as possible (that's called zero degrees emission angle), and we also want to see everything in stereo, or from two angles, if that's possible.

One way to achieve this is to use longitude scans instead of lattitude scans.


There are two basic ways to take picutres:

We can orbit the asteroid with the camera pointing directly below the spacecraft the whole time. This is called a lattitude scan. This way, in six hours, we would see a section of the asteroid about this size, where the light gray part in the above picture is the visible asteroid.

Or, we can slew the camera up and down as we orbit. This is known as a longitude scan. In six hours, we would see this about twice as much of the asteroid -- you can see this is quite a bit more than we saw with the lattitude scans. (link)


Nominally NEAR is in a 100 kilometer orbit for about eleven and a half days. This is a timeline of NEAR's time in the 100 kilometer orbit. The numbers above the bars represent the day and hour of NEAR's orbit. For example, 101/18 would be the hundred and and first day of the year, hour eighteen. The dark horizontal bars represent downlink time (time when we can't take pictures because NEAR is pointed away from the asteroid in order to downlink to Earth). The dotted horizontal bars represent times that the NEAR Laser Rangefinder, another instrument on board, needs to take pictures, and the MSI can't. Finally, the blue horizontal lines are times that NEAR can take picures. The numbers across the bottom represent the latitude NEAR covers for each part of its orbit. You can see that usually we pass by each latitude more than once.

However, the vertical purple bar shows the latitudes that we can never image, given these constraints (and assuming the camera is only pointed at nadir). (link)

We also can only send back a certain amount of data per downlink, so that means we can only take a certain number of pictures.

Now, it's also possible that the mass of the asteroid could be greater than we originally anticipated, or less. If it is greater, the 100 kilometer orbit will be longer: fifteen days instead of eleven and a half. If the mass is less, the 100 kilometer orbital times will also be different.


Orbit is the program that Emily and I used to model all of this. It was written by Brian Carcich at Cornell University. It's a really cool program! To use it, we run a determined number of frames and save them. The menu in the lower left allows you to alter the timing of the frames, the pointing of the spacecraft, the duration of picture taking, and the time between frames.

Once we have these frames, we can run a process that makes the plate model. "Plate models" are images you make in which all the frames you took are put together in one model, to create a three dimensional image of what NEAR would see in a given time period with given camera angles. Most of my slides are plate models.


Our goal was to make a sequence for the time the spacecraft is in its 100 kilometer orbit, in order to cover as much of the asteroid as possible, at the best emission angles possible.

To do this work, we divided picture taking times into five-and-a-half hour periods, because that is approximately the time it takes for NEAR to orbit Eros.


We also divided Eros up alphabetically in order so that we could verbalize the sections covered by each set of frames.

For example, the section labeled S on this map is a region on the very bottom of the asteroid that we never, ever get pictures of.


There are three main steps to make a sequence:

First, we find the minimum number of orbits necessary to cover the whole asteroid.

Then, we see what happens when you make a plate model using all but ONE of the 5.5 hour time periods.

Finally, for the five-and-a-half hour periods that did not change the plate model significantly when they were removed, we can change the pointing of the camera to get spots we couldn't normally see, or just to improve emission angle.

First, I'll show you the process and sequence for the high mass case (Emily did the low mass case).


This is a plate model of all the time we were allowed to take picures in the high mass case, with the camera pointed directly below the spacecraft, or nadir, the whole time. Blue is 0 degrees emission angle (that's what we want - it means we are looking directly perpendicular to the surface). Red represents 90 degrees emission angle, and green is a gradation beween blue and red. We use this particular model as a control when we make alterations, in order to see if we improved the model. (link)


This plate model was made using only nine five-and-a-half hour time periods (to see what happens using the minimum number of orbital periods). (link)


For the high mass case, I found that removing one five-and-a-half hour time period was usually insignificant, so I went ahead and altered the pointing for one orbit at a time. (link)


This is the final sequence I came up with for the high mass case, with three five-and-a-half hour time periods pointed off nadir, and the rest pointed nadir. (link)


This first model is the final sequence we came up with for the nominal mass case, with three five-and-a-half hour periods pointed off nadir, and the rest pointed at nadir. Compare that to the second model, with no off-nadir pointing, and you'll see how much of a difference three orbits can make. (link)


And even more drastically, you can compare the final model to a model made using the minimum number of orbital periods. (link)


In summary: Emily and I were able to build sequences for the 100 kilometer orbit of NEAR around Eros. We made sequences for the nominal mass case, as well as for high and low mass cases. The sequences reasonably covered most of the asteroid. Most of the time we were able to point NEAR such that the MSI viewed the surface at nearly zero degrees emission angle.

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