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NEAR Shoemaker Science From Surface of Eros

Near flyover art by Kees Veenenbos
by Andrew Cheng
NEAR Project Scientist
Laurel - Feb. 20, 2001
On Monday, 12 February 2001, the NEAR spacecraft touched down on asteroid Eros, after transmitting 69 close-up images of the surface during its final descent. Watching that event was the most exciting experience of my life. I was asked immediately afterwards how I felt, and I mumbled something about being tired and happy, but I missed the point.

I realized afterward what I should have said: it was like watching Michael Jordan on the basketball court, when the game is on the line and he is in the groove. One miracle after another unfolds, and we are left stunned and speechless. When we learned that the spacecraft had not only landed on the surface, but was still operational, we hardly knew what to think.

Over the past week, we have started to come to our senses again and to appreciate how fortunate we are. The final weeks of low altitude operations revealed bizarre and surprising aspects of surface structures on Eros, including one type of feature we noticed for the first time in the very last image taken by the spacecraft (the incomplete image taken from a height of 120 meters, 2001 Feb 12F ).

As we discussed previously, there are markedly fewer small, fresh craters on Eros than we would expect from our experience at the Moon, and an amazing profusion of boulders, likewise more than we expected. We do not know just what is happening on the surface of Eros to cover and/or obliterate craters while making and/or uncovering boulders.

We have seen many examples of mass motion on Eros - loose material sliding downhill - and that is no doubt part of the story, but maybe not all of it. We also believe that at least some of the bouldery debris found on Eros is comprised of ejecta from impacts on Eros; some of these ejecta do not escape but fall back to the surface.

Some of the strange features we are beginning to think about can be seen in the low altitude images obtained during the past few weeks. The new type of feature seen in the last image returned ( 2001 Feb 12F ) can be found, for example, at the bottom of the image (just above the vertical streaks indicating loss of signal), to the left of center.

It appears to be a collapse feature, formed when support is removed from below the surface, and it is about the size of one's hand. Other strange sights are clusters of boulders (e.g., the upper right of 2001 Feb 12E ) - are these cases of disintegration in place? - and extremely flat, sharply delineated areas in the bottoms of some craters (e.g., the two left panels of 2001 Jan 31 ).

The mere existence of sharp boundaries, called "contacts", is surprising in itself, especially if the entire surface of the asteroid is thought to have been blanketed by debris from impacts. These boundaries can be incredibly sharp on Eros, as evidenced by the last frame, 2001 Feb 12F (compare the upper right and lower left of the image).

The images tell us a tale whose outcome we don't yet know, but there is more: the story of Eros's composition is likewise still emerging. Our orbital data from the x-ray spectrometer showed that the abundances of key elements on Eros are very similar to those in the undifferentiated meteorites called ordinary chondrites, but there was a discrepancy.

The abundance of the volatile element sulfur is less than we would expect from an ordinary chondrite. However, the x-ray spectra tell us only about the uppermost hundred microns of the surface, and we do not know if the sulfur depletion occurs only in a thin surface layer or throughout the bulk of the asteroid.

Fortunately, the spacecraft is now in a position to help answer the question (on the surface, that is). The gamma ray spectrometer measures the composition to a depth of about ten centimeters, and it is much more sensitive on the surface than it was in orbit. We are now in the process of trying to obtain our best yet gamma ray spectrum of Eros.

We will try to determine the abundances of the volatile element potassium and the major element iron from this spectrum, to look harder at the match between the compositions of Eros and the ordinary chondrites, and to look for evidence for bulk depletion of volatiles. The latter would suggest that Eros has undergone significant heating (a geologist would call it "metamorphism").

It is sad for me to say, but the gamma ray measurement will be the last from NEAR - one more miracle is what we ask of this little spacecraft. Its job is almost done, but ours is just beginning.

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NEAR Shoemaker Reconfigured For Surface Operations On Eros
Laurel - Feb. 15, 2001
NEAR Shoemaker has been configured by mission controllers to collect and record data from the surface of Eros. Commands to prevent the rest of the spacecraft from sending data to its onboard recorder have also been sent, with the only reliable telemetry link via the spacecraft's low-gain antenna.

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