[meteorite-list] Asteroid Hopper is Lost in Space

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Nov 14 12:11:11 2005
Message-ID: <200511141709.jAEH9i624060_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn8311-robot-asteroidexplorer-is-lost-in-space.html

Robot asteroid-explorer is lost in space
Maggie McKee
New Scientist
14 November 2005

A small hopping robot meant to explore the asteroid Itokawa was lost in
space after being released from Japan's Hayabusa spacecraft on Saturday.
But mission officials say the main probe will still try to land and
collect samples of the space rock at least once before beginning its
return flight to Earth in December 2005.

The 10-centimetre-tall robot, called Minerva (Micro/Nano Experimental
Robot Vehicle for Asteroid), was designed to hop around the
600-metre-long rock, snapping close-up images with three cameras and
taking temperature measurements of the surface.

It was originally to be released on 4 November while Hayabusa was
approaching the asteroid in a test of its "autopilot" landing system.
But this "dress rehearsal" was cut short because mission
controllers could not accurately guide the spacecraft using its fuel
thrusters - a contingency solution devised following the failure of two
of the craft's three stabilising reaction wheels.

Now a similar problem on a rehearsal descent on Saturday has prevented
the robot from ever reaching its target.
          
Breakdown in communication

In the early phases of the spacecraft's descent, mission officials had
used height readings from an onboard laser altimeter and the craft's
speed to estimate when Hayabusa would be at Minerva's release point, 70
metres above the asteroid.

But the slope of the asteroid's surface had apparently caused the
altimeter to misjudge those first estimates of the craft's altitude. So
mission officials were shocked when later readings revealed the
spacecraft was actually much closer to the asteroid than anticipated.

"As soon as we realised we were below 100 metres, we decided Minerva
should be separated," project manager Jun'ichiro Kawaguchi told New
Scientist.

Unfortunately, that point came when the main data link with the
spacecraft was being switched from an antenna in Japan to one in
Australia. During this 40-minute antenna change, information about the
spacecraft's vertical motion was unavailable to ground controllers.

So when they sent the release command, they did not realise the
spacecraft had just fired its thrusters to maintain a minimum height
above the asteroid - an adjustment it performed every 10 minutes or so
during the descent.

The command took about 16 minutes to travel from the Earth to the
spacecraft, and when it arrived, the thruster firing had raised
Hayabusa's altitude to about 200 metres. So Minerva was released while
its mother ship was moving away from the asteroid at about 15
centimetres per second - faster than the space rock's 13 cm per second
escape velocity - and the robot simply drifted into space.
          
More rehearsals

"This is [our] responsibility," says Kawaguchi, who says officials
simply did not account for all possible mission scenarios during the
antenna changeover. "Our readiness was not so complete."

Nonetheless, he says the mother ship's practice descent was a success.
It approached the asteroid to within 55 m and its laser range-finder -
which had never before been tested - successfully helped the craft
maintain its orientation with respect to the asteroid's surface.

Still, he admits that mission controllers do not fully understand how to
deal with the spacecraft's motion after the periodic thruster firings.
So rather than attempt two sample collection landings as planned on 19
and 25 November, officials may decide to do another rehearsal descent on
19 November.

"We may end up cancelling the second sampling," says Kawaguchi. During
the sample-collecting rendezvous, the spacecraft will attempt to briefly
touch down on an expanse of fine dust in the middle of the asteroid
called MUSES-Sea, fire a pellet into the surface and scoop up the
resulting debris.

After gathering its sample, it will leave Itokawa in December and then
drop the first-ever asteroid samples back to Earth, over Australia, in
July 2007.
Received on Mon 14 Nov 2005 12:09:44 PM PST