On February 9, 2000, NEAR crosses a milestone – images
of Eros from then on are at a higher resolution than has
ever been achieved before. The best previous images of
Eros were obtained during NEAR’s flyby of Eros in
December, 1998. At that time we obtained our highest
resolution image from a range of 3827 km with resolution
of 363 m per pixel. The NEAR imager is a digital camera
that uses a charge-coupled device (CCD) to capture an
image. The image is divided into little blocks like a
mosaic, and each block is called a pixel (which stands
for picture element). A resolution of 363 m per pixel
means that one pixel in that best image spanned a
distance of 363 m on the surface of Eros. Roughly
speaking, a feature is said to be resolved if it is at
least two pixels across, or in this case, 726 m across.

However, as photographers know well, there is more to
imaging than resolution, and lighting conditions are
also critical. An excellent illustration of that can be
seen in the image-of-the-day for 7 February 2000, which
already reveals previously unsuspected features on Eros,
even though the range to Eros was 7700 km and the
resolution did not yet equal the best achieved in
December 1998. If we compare the images from 06:48:05
and from 11:09:45, for example, we are looking at almost
the same face of Eros in both cases. The same large
crater appears in the center of both frames (it has a
beautifully defined rim – I think I shall never see a
more perfect crater). Another large crater is seen
easily in the 06:48:05 frame near the end of Eros to the
upper right (this is the 180 degree longitude end – more
on coordinate systems another time). The point is, the
same crater near the end is almost invisible in the
11:09:45 image, and the principal difference is

Of course, we cannot move our source of light, which is
the sun, so we must make observations over a long time
as the sun moves across the sky. However, even over the
course of an asteroid day, not all parts of the asteroid
see the sun, so we must observe long enough for the
seasons to change at Eros (for more on seasons, see the
January 24 update). The only way to do this is by
orbiting around Eros, and that is one of the principal
reasons why NEAR must achieve orbit to accomplish its
primary science objectives. There are other reasons
relating to the composition measurement by x-ray and
gamma ray spectrometry, but that is a topic for another
time. For now, our most important milestone is orbit
insertion on February 14, now less than a week away.

Andy Cheng

NEAR Project Scientist