February 8, 2000
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 lighting.
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
