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.

Andrew Cheng

NEAR Project Scientist