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.