NEAR Science Status: Dancing the Tango 17 October 2000

NEAR is now eight months into its year-long rendezvous with Eros. We have
seen Eros from as low as 35 km orbit for about ten days back in July, but
then returned to higher altitude. For the last five weeks, we have been
mapping Eros from 100 km orbit, but we are now preparing for our closest
descent yet, a 6 km flyover scheduled for October 25, 2000. Why has NEAR
Shoemaker performed this elaborate tango with Eros? One answer is that we
chose one of the most attractive partners on the dance floor, and we have to
pay the price if we wish to dance very close – it costs extra fuel and
requires frequent maneuvers. But another reason for dancing both up close
and farther out is that we scientists want it that way.

The design of the mission has been the result of a complicated interplay
between science and engineering requirements. To start with, NEAR Shoemaker
was designed as a simple spacecraft, with fixed antennas, fixed solar
panels, and fixed instruments. This simplicity makes for a more reliable and
robust spacecraft, but it also places operational constraints on the
mission. The spacecraft must always keep its solar panels pointed at the sun
– it cannot survive even for an hour without solar power, not only because
the electronics would stop operating, but also because instruments and
subsystems would be damaged by cold and the fuel would quickly freeze. The
instruments must be pointed at the asteroid in order to acquire data. The
main antenna must be pointed at Earth to send data back to Earth at high
rate, although the spacecraft status can be monitored and the spacecraft can
be tracked at lower data rates using other antennas, even when the main
antenna is not pointed at Earth. If we had designed a more complex
spacecraft, we could have lifted many or all of the operational constraints,
but it would have cost more. And indeed these restrictions complicate the
day-to-day operations of the spacecraft, but it turns out that the
ever-vigilant enforcement of simple rules is a task better suited for
computers than for humans, and this task is largely automated for NEAR.
Paradoxically, the use of a simple spacecraft leads to an overall
simplification of mission operations, despite operational restrictions,
because there are fewer options to be studied.

In any case, the spacecraft design constrains NEAR Shoemaker to fly in an
orbit plane that is within about 30 degrees of perpendicular to the line
from Eros to the Sun. The spacecraft can then keep its solar panels pointed
at or close enough to the Sun at all times, while for 16 hours a day it
keeps the instruments pointed at Eros for data taking, and for 8 hours a day
it points the main antenna at Earth for data transmission. This tight
constraint on the orbit plane at Eros, plus the constraint that the orbit is
flown at a particular time, already fairly well settle three of the six
parameters required to specify an orbit completely. In some sense the orbit
is now almost halfway designed, although in real life our engineers
determine these parameters to 10 decimal places. Those of us who don’t
actually have to fly the spacecraft can afford to take a more relaxed
attitude. The remaining three orbit parameters deal with how low and how
high the orbit goes, and precisely where it dips low.

That is where science comes in, although even at this point operational
constraints are still critical. Some of our science operations are best
performed at higher altitudes, while others require that the spacecraft be
at low altitude. For instance, our imaging team desires to map the whole
asteroid under a variety of lighting conditions and from a series of orbit
radii, specifically 200 km, 100km, and 50 km. In addition, the team requires
both monochrome (black-and-white) and color imaging, and the ideal lighting
conditions for the one are not ideal for the other. For monochrome images,
we prefer the sun to be low in the sky so that shadows accentuate the
structures, whereas for color images we prefer the sun to be higher to
reduce the shadowing. In addition, there are seasons on the asteroid. Until
late June of this year, portions of the southern hemisphere never came into
sunlight at all. The reverse is now true in the southern hemisphere summer,
when the north polar region is always in the dark. Beyond all these
requirements, the x-ray and gamma ray teams need to have the spacecraft in
low orbits of 50 km or less as long as possible to achieve the highest
possible signal-to-noise ratio. Also the laser rangefinder team obtains the
highest resolution and measurement accuracy in the low orbits, and the study
of the asteroid’s interior structure, through determination of its gravity
and magnetic fields, achieves the highest sensitivity in the low orbits.

So there are many science tasks that require low orbits, but there are also
science tasks that require high orbits, and in both cases, the spacecraft is
required to fly over all parts of the asteroid at the altitudes in question.
In addition, particular solar illumination geometries are often required,
such as for color and spectral observations as well as the x-ray
measurements. Hence the choices of how low to fly, and where, and when, are
complicated, and we really needed to spend a full year at Eros. Moreover,
there are engineering requirements which derive from orbit stability. The
spacecraft cannot be put into an orbit that would be so unstable that we
could not predict with sufficient accuracy where it would be a week later,
or so unstable that, if for any reason we could not contact the spacecraft
or correct its orbit for a week, it would crash or escape from the asteroid.
Furthermore, we avoid orbits that would require excessive fuel expenditures
or corrective maneuvers more often than once a week. Finally, we cannot send
the spacecraft into an orbit that would carry it into the shadow of the
asteroid, where the Sun would be eclipsed and the spacecraft would die.

Generally speaking, the orbits we need to worry about are the low orbits. As
we discussed earlier (April 18, 2000), the irregularity of an object’s shape
produces greater and greater distortions of its gravity field, the closer
one approaches to the object. At large distances from any object, its
gravity field becomes monopolar and spherical, so higher orbits tend to be
better behaved in terms of being more like ordinary elliptical orbits. There
is a caveat, which is that the gravity from the object must remain the
biggest force field around; if we get too far from the object, then we also
have to worry about other forces like solar gravity and radiation pressure,
and orbits become complicated again. For Eros, too high in this sense means
above about 1000 km. Hence the 200 km orbits are fairly stable and ordinary
– that is, not too close and not too far. The 50 km orbits, on the other
hand, are close enough to be strongly perturbed by the irregular shape of
Eros. The most serious disturbance is that the orbit plane is continually
torqued around (that is, it precesses), so it would quickly violate the
operational constraint we started with unless we perform maneuvers to
correct the orbit. In other words, we need to fire the rocket engines to
keep the orbit plane within the allowed angles to the line from Eros to the
Sun. It turns out that the precession rate depends on the orbital
inclination to the Eros equator as well as the orbital radius.

The upshot is, there are only certain times of year when NEAR Shoemaker can
fly in 50 km orbits or lower, without using too much fuel or putting the
spacecraft at too much risk. Even so, we have no choice but to get up close
to Eros to make the measurements we need. This is why we are dancing a tango
with Eros, sometimes close, and sometimes far. Like the real tango, our
dance with Eros has been exciting, full of mystery, and much hard work – and
more is still to come. Our closest view of the surface to date is eight days
away.