http://msnbc.msn.com/id/8131678/
Japan shoots for a piece of an asteroid
Hayabusa sample-return mission nears critical stage
By James Oberg
NBC News space analyst
Special to MSNBC
June 7, 2005
A celestial smash-and-grab space mission that could become the
greatest triumph in the history of the Japanese space program is
entering its most challenging stage in deep space.
Together with NASA's comet-crashing Deep Impact mission and the European
Rosetta spacecraft, Japan's Hayabusa probe is helping inaugurate a new
era of interplanetary exploration: the physical examination of the
surfaces of small bodies such as comets and asteroids.
The Hayabusa probe is slowly closing in on a distant asteroid named
Itokawa. Within a few months, after surveying the asteroid thoroughly
from a safe distance, Hayabusa will swoop down to its surface and grab
samples of the dirt for return to Earth, like a spacefaring bird of
prey. In fact, the spacecraft's name comes from the Japanese word for
peregrine falcon.
Already closer to the asteroid than the distance between Earth and its
moon, Hayabusa is approaching at a relative speed of 225 mph (100 meters
per second) and is firing its gentle but exceedingly persistent engine
to further slow its speed.
The probe uses an ion-drive system pioneered by NASA's Deep Space 1
comet scout, but with a distinctive design innovation. It is the first
probe to use microwaves to ionize the xenon fuel. Through electrostatic
deflection - like charges repel each other - the four engines can
produce a slow-but-steady gain or loss in total speed amounting to as
much as 27 mph (12 meters per second) per day.
That's about the same velocity gain that a rising rocket booster racks
up in half a second. But traditional thrusters operate only sporadically
for a few seconds or a few minutes at a time. In contrast, Hayabusa has
been thrusting for two years, and now it has essentially reached its
distant goal.
The distance is within 400,000 kilometers [250,000 miles], project
manager Junichiro Kawaguchi of Japan's Institute of Space and
Astronautical Science reported in an e-mail exchange with msnbc.com.
Although Hayabusa took detailed pictures of Earth
during a high-speed flyby last year, it has yet to turn its camera onto
its target. The asteroid is small, Kawaguchi explained, and also the
attitude constraint [for firing its braking engines] restricts the
camera to be oriented to the object.
The probe and nearby asteroid will pass behind the sun next month,
restricting radio contact. Images should start coming in once the radio
interference clears up, Kawaguchi said. In mid-August, it should be
seen at the magnitude of 5 or so, he explained, initially only as a dim
speck of light that will grow gradually brighter. (As seen from the
spacecraft's point of view, a magnitude-5 object would be slightly
brighter than the dimmest light sources that can be seen from Earth with
the naked eye.)
Delayed arrival
When Hayabusa was launched two years ago, mission planners expected the
probe to be already at the asteroid by this time - but the schedule was
delayed because of a crippling event that occurred during the voyage.
It is due to the heavy solar cell damage caused by the historical
largest solar flare that occurred in 2003, Kawaguchi explained. The
consequent reduction in electrical power from the craft's solar cells
caused Hayabusa's electrical propulsion system to lose thrust.
The probe is now expected to arrive at its home station stand-off
position early in September.
The delay has been worrisome, however, because there's only one safe
route home with the samples, and that requires the probe's departure
from the asteroid in mid-November. For the kind of trajectory that the
craft's ion-powered engines can achieve, it's a matter of leaving on
time, or becoming a permanent asteroid resident.
Consequently, all the scouting and remote-sensing operations that had
originally been planned to take three or four months must now be
accomplished in half the time. This advance reconnaissance is essential
if the actual landings - and several are planned - are to have any
reasonable chance of success.
The target
Hayabusa is aimed at a small piece of space debris once known as
Asteroid 1998 SF36 - signifying that it was discovered only a few
years ago and was too small for anybody to bother to name. But once it
had been selected as a sampling target, the International Astronomical
Union acceded to Japan's request to name it after Hideo Itokawa, the
father of Japanese rocketry.
The potato-shaped asteroid measures about a quarter-mile (500 meters)
wide, with a gravitational pull hardly more than a millionth of Earth's.
Itokawa's gravity is so faint that the probe won't even bother to orbit
the asteroid. Instead, it will hover about 12 miles (20 kilometers)
away, surveying the surface both from the full sunlit side and then
later from above the boundary between day and night.
Itokawa's shape and density
