[meteorite-list] Thruster Problem Latest Hurdle for Hayabusa

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Nov 30 13:58:02 2005
Message-ID: <200511301729.jAUHTGn14963_at_zagami.jpl.nasa.gov>

http://www.spaceflightnow.com/news/n0511/29hayabusa/

Thruster problem latest hurdle for asteroid probe
BY STEPHEN CLARK
SPACEFLIGHT NOW
November 29, 2005

After weeks of confusion and setbacks, the Japanese Hayabusa probe
finally met with success last week in its mission to retrieve the
world's first assortment of asteroid specimens that will be bound for
Earth within the next few weeks -- if controllers can put an end to a
potentially ruinous problem with thrusters needed for the trip home.

The spacecraft carried out its descent to the surface of asteroid
Itokawa throughout Friday night and Saturday morning on its final chance
to try to collect the precious samples to achieve one of the primary
goals of the $100 million mission. An earlier attempt a week before had
reached Itokawa, but had failed to gather samples due to a last-second
abort ordered by an on-board sensor.

Hayabusa finally touched the surface at a velocity of around four inches
per second as scheduled in the early morning hours Saturday in Japan,
and the retrieval procedure went according to plan. Data giving the
first confirmation of the historic success reached the control room as
the craft began to ascend away from Itokawa after spending just one
second on the asteroid.

The design of the sampling system called for a 16-inch diameter funnel
to first make contact with the surface, followed by the firing of two
bullet-like projectiles made of tantalum metal into the asteroid at
several hundred miles per hour.

The high-speed impact of the pellets is meant to blow bits of dust and
rock through the collecting horn and into a chamber that will contain
the specimens throughout the journey back to Earth that will culminate
in a parachuted landing in June 2007 at the Woomera test site in central
Australia. Two bullets were fired to increase the amount of samples to
be transported back for study in laboratories, but the exact quantity
will not be known until the return capsule arrives at Earth.

Previous estimates of the total sum of material amassed over two
separate passes were around one gram, or about two one-thousandths of a
pound.

However, the return to Earth hinges on a remedy for thruster problems
that have plagued the mission since Saturday's sample retrieval. The
trouble showed up during the ascent back to a staging position near the
asteroid when the ground team noted a shaking and vibration in
telemetry, reports said.

The snag sent Hayabusa into a safe mode for the second time in a week,
and controllers have struggled to regain control of the spacecraft over
the past few days but so far to no avail.

Next on the schedule for the intrepid space mission is the departure
from the vicinity of Itokawa, which has lingered nearby for almost three
months. If possible, that milestone is set to occur within the next ten
days before an early December deadline, after which Hayabusa will be
unable to make it back to Earth because it will be out of position in
its orbit around the Sun.

It took just over nine hours for the 1,000-pound probe to make its way
from over 3,000 feet in altitude to the surface. During the descent,
officials opted not to release the last remaining target marker, and
chose instead to rely on guidance cues offered by an identical indicator
already deployed during the first sampling attempt on November 20.

At a distance of around 115 feet, the autonomous navigation system
aboard Hayabusa began receiving precise measurements from a laser range
finder. The device feeds range and closure rate data to the control
system by using four laser beams to help determine not only the height
but also information on the terrain below the probe.

When Hayabusa reached around 45 feet in altitude, it began to adjust its
orientation to match that of the slope of the surface of the asteroid
directly below. Communications with the craft was also switched to a
beacon mode as the high gain antenna was pointed away from Earth, which
was located 179 million miles away on the far side of the solar system.
This led to a reduction in the amount of telemetry reaching controllers
in real time, and teams had to rely on Doppler data for updates on the
status of the descent.

During the last few critical moments of the approach to Itokawa,
Hayabusa used a set of abort commands that limited the reasons to call
off the attempt. The November 20 sample collection effort ended in an
abort after a sensor detected potentially unsafe obstacles, although the
spacecraft continued to descend to the surface, where it sat for up to
thirty minutes before taking off again. That marked the first takeoff
from an asteroid in the history space exploration.

The new software package disregarded the sensor that caused the previous
abort, but stipulated that the maneuver be stopped if the laser
altimeter lost sight of the asteroid, the range finder ceased to
function, or the probe's attitude varied beyond set restrictions.

No issues arose as Hayabusa descended through the final feet to the
surface, and touchdown occurred at 7:07 a.m. Japan Standard Time on
Saturday morning (2207 GMT Friday). During the climb away from Itokawa,
telemetry reached controllers indicating the on-board computer sent the
signal to fire the bullets. This initial evidence of success came less
than a half-hour after the momentary landing.

Officials also pointed to signs that the sample funnel was somewhat
warped following its soft collision with the surface of Itokawa.

Other than the pressing thruster issue, Hayabusa has already countered
several challenges during its mission, including a delay in the arrival
at Itokawa after a solar flare damaged the capability to produce
electricity, the loss of two reaction wheels, and aborts of a rehearsal
and sampling pass earlier this month. These issues - though primarily
the reaction wheel failures - have caused the craft to consume more
chemical propellants than expected, leaving the future of the mission in
the balance of dwindling fuel reserves.

"Hayabusa may face more difficulties on its way back to Earth, but we
should renew our determination and do our utmost to complete this
challenging mission successfully," said President Keiji Tachikawa of the
Japan Aerospace Exploration Agency, or JAXA.

If returned, the samples will be the first material to be brought to
Earth from any celestial body other than the Moon. The specimens will be
analyzed for the precise chemical components of Itokawa to help
scientists better string together the evolution of the early solar
system, of which asteroids are believed to be ancient relics.

The material will also help bridge a connection between the asteroid and
certain types of meteorites that have fallen to Earth. Other scientists
will be anxious to see the results to determine what minerals and
elements on Itokawa would be commercially viable to mine if such
ventures become reality in the coming decades.
Received on Wed 30 Nov 2005 12:29:15 PM PST