[meteorite-list] Hopes Fade for Hayabusa

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Dec 13 11:43:21 2005
Message-ID: <200512131641.jBDGfUH04062_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn8451--hopes-fade-for-troubled-japanese-asteroid-probe.html

Hopes fade for troubled Japanese asteroid probe
Maggie McKee
New Scientist
12 December 2005

Hope that Japan's Hayabusa spacecraft will return to Earth is fading as
mission controllers remain unable to regain complete control of its
orientation.

The spacecraft was designed to bring the first-ever asteroid samples
back to Earth for analysis. But recent data suggest that, during a
landing attempt on 26 November, it did not fire metal pellets into the
600-metre-long asteroid Itokawa to draw up material for collection.

Now mission controllers have little hope the spacecraft will be able to
get back to Earth - even without its quarry - because of continuing
problems with its fuel thrusters.

"The situation is not optimistic," Hayabusa's project manager Jun'ichiro
Kawaguchi told New Scientist. The spacecraft was supposed to begin its
return journey by mid-December to take advantage of an ideal alignment
between the Earth and the asteroid. The next such alignment will not
occur for another three years.

Fuel thrusters

The fuel thrusters have been used to point the spacecraft because two of
its three stabilising reaction wheels failed in July and October 2005.
But after the landing attempt on 26 November, one thruster on the
spacecraft's upper panel sprang a leak, forcing the craft into an
emergency shutdown of all its non-essential systems.

As mission controllers worked to return the craft to normal operations,
other thrusters failed - possibly because their valves got locked shut
or because their hydrazine fuel froze.

As a result, ground controllers were not able to accurately orient the
spacecraft to ensure its solar panels faced the Sun. Around 1 December,
the resulting loss of battery power forced most of the onboard
instruments to shut down or restart incompletely.

As an emergency measure to restore power, ground controllers programmed
the craft to vent the xenon gas from its ion engines. These engines -
which use electric fields to accelerate a beam of ions - were designed
to propel the craft on its 2 billion-kilometre round trip to Itokawa.

Tricky feat

Firing the xenon thrusters successfully pointed the craft's solar panels
towards the Sun on 5 December - when the least manoeuvring was needed.
But it was not an easy feat. Kawaguchi says: "It is not a strong
thruster and is very easily subject to disturbance."

According to a recent mission status report, there is enough xenon fuel
both to point the spacecraft and return it to Earth. But the ion engines
are currently off and it is not clear how healthy the engines are. A
plan to reignite them on 14 December will likely be postponed, says
Kawaguchi.

"We are in a very tough spot," he told New Scientist. "We are waiting
for what we can do next."

The mission has been plagued by problems, including the loss of a robot
that was due to hop around and explore the asteroid. Mission members say
using commands from both human ground controllers and the spacecraft's
autonomous landing protocols sent the robot into space when it was too
far away from the asteroid.

Poor planning

Poor planning of spacecraft commands also appears to have been
responsible for the failure to collect samples during the spacecraft's
two landings on 20 and 26 November.

During the first landing, Hayabusa bounced twice on the asteroid then
sat on its surface for 30 minutes. But it did not fire a pellet -
apparently because two of its autonomous landing systems sent it
conflicting commands. One detected a potentially damaging boulder on the
asteroid's surface and commanded the spacecraft to rise away from the
asteroid, while another found that the spacecraft was not in the correct
orientation for an emergency ascent.

A similar problem may have occurred on 26 November, when the spacecraft
touched the asteroid for just one second. Telemetry sent back just after
the landing showed a command to fire the pellets had been made. But
other data sent back on 6 December suggest the pellets' control
mechanism had been accidentally "disarmed", preventing the pellets from
firing.
Received on Tue 13 Dec 2005 11:41:30 AM PST