January 24, 2000
NEAR has just completed an intensive week of preparations for Eros encounter day (which is, of course, Valentine's Day). We went through a full rehearsal of the spacecraft and instrument activities leading up to the orbit insertion maneuver, which is the rocket engine firing that will finally put NEAR into orbit around Eros. All of these activities have to be scripted and loaded on the spacecraft in advance, and last week, we tested the command script on the spacecraft - happily, all went well. What makes the encounter day all the more 'interesting', however, is that critical science observations are scheduled just before the orbit insertion maneuver.NEAR has been targeted to pass directly between the Sun and Eros about 11 hours before the maneuver, so that a spectral map of Eros can be taken at "zero phase angle". We call this operation the "Low Phase Flyby". The phase angle is the angle made by the sun, Eros (at the vertex), and NEAR - when this angle is zero, NEAR is exactly between the Sun and Eros. In this geometry, there are no shadows on the surface as seen by NEAR. This geometry yields the best infrared spectrometer measurements of the brightness of Eros in various infrared "colors", or wavelengths of light - more on spectrometry in two weeks. The infrared spectrometer can't detect areas of Eros that are in shadow, so we want to minimize the shadow area.
Currently, it's summer time at the north pole of Eros, and the sun never sets there (just as in the land of the midnight sun on Earth above the Arctic Circle). When NEAR flies through zero phase angle on Valentine's Day, it will obtain spectral maps of the northern hemisphere of Eros. This is the only time during the entire Eros rendezvous that NEAR will be able to obtain zero phase observations over the northern hemisphere. Once NEAR is in orbit around Eros, after the orbit insertion, the phase angle will remain near 90 degrees (because that is better for imaging science - more on that next week).
Of course, since it's summer in the northern hemisphere of Eros, it's winter in the south, and the sun never rises there. That means NEAR can't take images or obtain infrared spectra of the southern regions, where the sun does not shine, until the seasons change later in the year. Of course, we do intend to map all of Eros, so NEAR is scheduled to make another zero phase observation of the southern hemisphere of Eros next October.
So, the only times that we will obtain observations at small phase angles will be during the encounter day itself, looking at the northern portion of Eros, and on one other day in October looking at the southern part of Eros. For science reasons alone, these are critical observations - they are expected to yield the highest quality infrared spectra that will distinguish the mineral composition of Eros. But we must also get into orbit successfully only a matter of hours later. Since the entire sequence, low phase flyby plus orbit insertion maneuver, must be loaded in advance and executed autonomously by the spacecraft, last week's test included both sets of activitites. Here's hoping that encounter day goes just as smoothly.
