[meteorite-list] Hayabusa Begins Daring Close Encouter With Asteroid

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Sep 14 19:28:43 2005
Message-ID: <200509142327.j8ENRXO26299_at_zagami.jpl.nasa.gov>

http://www.spaceflightnow.com/news/n0509/14hayabusa/

Probe begins daring close encounter with asteroid
BY STEPHEN CLARK
SPACEFLIGHT NOW
September 12, 2005

A $100 million Japanese space explorer parked in the vicinity of an
enigmatic asteroid this week, allowing scientists to get a first glimpse
of the mid-sized rock that will become the source of the first samples
of such an object to ever be returned to Earth.

After methodically tweaking its course - first by electrical ion
propulsion, then by conventional chemical thrusters - toward its target
over the past few months, the Hayabusa probe finally arrived at its
station keeping position some 12 miles from the asteroid early Monday.

Officials timed the arrival by when the 1,000-pound craft's closure rate
relative to the object reached zero, indicating the probe was now
essentially anchored in the "gate position" located about 12 miles from
the space rock. That moment occurred as the spacecraft commanded its
maneuvering jets to fire one last time at about 0117 GMT Monday, or in
the late-morning hours in Japan.

Hayabusa's ion drive propulsion system took a the leading role for the
rendezvous up until August 28, when control switched to the
liquid-fueled chemical thrusters. That milestone left the four ion
engines with a cumulative burn time of almost 26,000 hours, and the
system will be re-started once the probe embarks on the last leg of its
journey back to Earth.

The delicate space ballet took place almost 200 million miles from
controllers on Earth, who left responsibility for the rendezvous to an
on-board navigation system that is designed to operate without ground
intervention.

The goal of the mission is to study asteroid 1998 SF36 - later named
Itokawa in honor of an early Japanese pioneer in rocketry. The object
was discovered in September 1998 by a joint team consisting of
scientists from the U.S. Air Force, NASA, and the Massachusetts
Institute of Technology.

Details about Itokawa have largely remained elusive in the seven years
since its discovery, but scientists now have a much clearer picture of
the potato-shaped asteroid estimated to measure 2,000 feet by around 900
feet. Its orbit stretches from inside Earth's out to a distance of 157
million miles from the Sun, making it a member of the Apollo class of
near-Earth asteroids that pose potential impact threats to our planet.

Astronomers studied Itokawa via several ground-based telescopes during
its last Earth fly-by in 2001, and they found evidence that the asteroid
was brighter and more reflective than first expected. Scientists had a
difficult time predicting what Hayabusa would find as it closed in on
the object, and many questions were left unanswered.

However, many of those issues can now be thoroughly remedied with the
new higher resolution images Hayabusa's optical asteroid multi-band
imaging camera has captured. The pictures show a contrast of rocky and
hilly terrain with smooth regions, but detailed analysis and sample
retrieval will be conducted before scientists can announce the results
of their detailed study.

A first look at the new images appears to show a loose layer of dust and
dirt-like material covering the smooth surfaces of parts of the
asteroid, which is a surprise to some project officials.

"According to a certain theory, small objects do not have regolith,"
said Hayabusa project manager Jun Kawaguchi. "But this asteroid seems to
have smooth portions that appear (to have) some regolith."

Work with Hayabusa's science payload has already begun, with the
near-infrared and X-ray spectrometers now gathering measurements. The
craft's laser altimeter is also conducting observations to learn more
details about Itokawa's terrain.

The pair of spectrometers will labor to determine the elemental and
mineral composition of Itokawa to help astronomers in their quest to
link asteroids and comets to meteorites that have fallen to Earth and
been recovered.

"The analysis has just started and no conclusive results (are) given
until the measurements are detailed and analyzed," Kawaguchi told
Spaceflight Now.

"When we talk about the shape just from impression and not from a
scientific point of view, the shape apparently looks like an object
(with many) smaller objects united. And also apparently, a small
(object) and some other objects are accreted with a larger one. These
are not interpreted at all and definitely have to wait for detailed study."

Hayabusa will have to occasionally refine its position relative to the
asteroid because the object's gravity is too weak to keep the craft in a
stable stationary position.

Controllers plan to order Hayabusa to close further toward Itokawa at
the end of September, when more precise science activities are
scheduled. Once in the "home position" under five miles from the
asteroid, remote sensing observations will continue as the object
completes one rotation about every 12 hours.

A goal of the remote sensing phase will also serve to choose up to three
sites that are scientifically interesting and safe enough to bring the
fragile spaceship to the surface for sample retrieval passes beginning
in November. During each approach, a 16-inch funnel aboard Hayabusa will
make contact with the asteroid, followed by the firing of a small metal
pellet into the rocky crust at hundreds of feet per second.

Rubble and dust ejected from the high-speed impact will make their way
through the funnel and into a chamber designed to carry the samples on
the return trip to Earth and through the fiery re-entry into the
atmosphere. Officials expect up to one gram - or two one-thousandths of
a pound - of material will be gathered.

It is expected that the ground team will eventually choose two sites for
sample runs, leaving a third as a rehearsal to test the operation and
feasibility of the plan.

Also during the first pass, Hayabusa will deploy a tiny 1.3-pound rover
known as MINERVA, which will leap, hop, and jump across the surface in
the extreme low-gravity environment. MINERVA carries three stereo
cameras for pictures, and a suite of six thermometers to measure
temperatures.

Hayabusa - formerly known as MUSES-C - was launched on May 9, 2003,
aboard an M-5 rocket from Japan's Kagoshima launch site. The spacecraft
flew past Earth a year later to receive a gravity boost that aimed
Hayabusa toward this year's encounter with Itokawa.

Time spent in the vicinity of asteroid Itokawa was reduced by a 2003
solar flare that slightly damaged the ability of Hayabusa's two solar
panels to produce electricity, which is vital to the operation of the
craft's primary ion propulsion system. The degradation caused the four
ion drive engines to produce less total thrust with less operating time,
delaying arrival from mid-summer until now.

Plans call for Hayabusa to depart Itokawa in early December to begin its
trek back to Earth, culminating with a homecoming in June of 2007 by way
of parachuted landing at Woomera in the Australian outback.
Received on Wed 14 Sep 2005 07:27:32 PM PDT