The Fiery Return of NASA's Space Dust Cargo

http://www.nasa.gov/centers/ames/res.../stardust.html

John Bluck
NASA Ames Research Center, Moffett Field, Calif.
Phone: 650/604-5026
E-mail:

The Fiery Return of NASA's Space Dust Cargo
November 29, 2005

In search of clues about the origin of life on Earth and other secrets,
a crew flying on a NASA DC-8 aircraft will study the small, speeding
Stardust capsule returning from space early in 2006.

Two years earlier, in January 2004, the Stardust spacecraft flew within
147 miles (236 kilometers) of the comet Wild 2 (VILT-TWO) and survived
the high-speed impact of millions of dust particles and small rocks up
to nearly two-tenths of an inch (one half centimeter) across. With its
tennis-racket-shaped collector extended, Stardust captured thousands of
comet particles.

The returning Stardust capsule will strike Earth's atmosphere during
the
early morning darkness in mid-January 2006, at eight miles (12.8
kilometers) per second ? more than 10 times faster than a speeding
bullet. That is fast enough to go from San Francisco to Los Angeles in
only one minute. The DC-8 crew will face the daunting task of tracking
and observing the 101-pound (45.7 kilogram) conical object as it
hurtles
through the atmosphere and slows before the spacecraft finally
parachutes down in a Utah desert.

Scientists aboard the DC-8 also will assess how well the Stardust
capsule's heat shield protects its precious cargo of comet dust and
interstellar grains.

Though scientists will study this captured space dust for years to
come,
the separate team of researchers aboard the DC-8 will gather data only
during the brief re-entry of the space capsule into Earth's atmosphere.

The mini, Apollo-like capsule will shoot down through the air at the
highest spacecraft re-entry speed into Earth's atmosphere ever,
generating extremely high temperatures. The capsule's special
carbon-based heat shield, developed at NASA Ames Research Center in
California's Silicon Valley, will protect the priceless cargo of comet
dust and interstellar grains. During this blistering re-entry, the DC-8
crew will take surface-temperature and shock-radiation measurements of
the heat shield as part of it burns away. Shock radiation is light
emitted from extremely hot air. Scientists will study this light to
learn how hot the capsule gets and what chemical reactions are taking
place. These chemical reactions will result from of the violent breakup
of air molecules that collide with vapor in front of the speeding
capsule.

At the same time as the DC-8 crew is flying its mission, amateur
astronomers, willing to endure the cold of the bitter winter, may
contribute to the study by simply photographing the incoming capsule,
noting their global positions and later providing that information to
mission scientists.

One of the goals of the researchers aboard the DC-8 is to measure the
capsule's re-entry brightness. Scientists expect it to peak at
approximately the brilliance of Venus for roughly 90 seconds. The
capsule will be brightest 37 miles (60 kilometers) high over the town
of
Carlin, Nev., as the spacecraft approaches. This will occur in the
early
morning cold and darkness on Sunday, Jan. 15, 2006, shortly before the
spacecraft parachutes to a landing at 3 a.m. MST. The landing zone is a
restricted area ? the Utah Test and Training Range, located southwest
of
Salt Lake City.

"As the observer sees the approaching capsule, it will appear as a
point
of light," said Peter Jenniskens, principal investigator of the
Stardust
Sample Return Capsule Re-entry Observing Campaign. Jenniskens is a
meteor astronomer at the SETI Institute, Mountain View, Calif. "After
it
passes the observer, the back of the capsule will be less bright, and
it
will quickly fade. Each observer will have a different experience,"
Jenniskens added.

The special carbon-based heat shield material designed to protect the
Stardust capsule is a candidate for potential inclusion on NASA's next
planned spaceship, the Crew Exploration Vehicle (CEV), NASA engineers
say. This prospective future use is one reason they plan to study the
Stardust capsule as it slams into Earth's atmosphere, and the shield
rapidly heats due to friction with the air.

"Our main interest is the performance of the heat shield and the
chemistry that takes place in it as it vaporizes and erodes during the
descent and re-entry," said Dave Jordan, a NASA Ames engineer and
project manager for the capsule observation mission.

The spacecraft will penetrate Earth's atmosphere as if it were a normal
meteor, according to Jenniskens.

"The capsule will be an artificial meteor that we can study for clues
about how life's molecules may have first formed on Earth," Jenniskens
said. "The carbon from the heat shield will react in the shockwave,
making new molecules that would have seeded Earth at the time of the
origin of life. The carbon in comet dust could have done the same,"
Jenniskens ventured.

After scientists examine the dust carried within the capsule, they may
soon learn what carbon compounds are found in comet dust. The
spacecraft
flew through comet Wild-2's dust cloud and captured some of it in a
very
light substance, called 'aerogel.'

"It's a little
bit like collecting BBs by shooting them into Styrofoam," said Scott
Sandford, an astrophysicist at NASA Ames and a Stardust mission
co-investigator. "Some of the grains are likely to have exotic isotopic
ratios that will give us an indication that we're looking at materials
that aren't as old as the solar system, but are, in fact, older than
the
solar system," Sandford asserted.

Another mission objective was to expose the spacecraft to the
interstellar dust stream for 150 days to grab interstellar particles.
After collecting the particles, the aerogel collector retracted into
the
capsule. Stardust will be the first mission to capture and return a
substantial sample from outside Earth's moon system.

Watching the Stardust re-entry

The capsule will approach the landing zone from a westerly direction.
The best opportunities for viewing the re-entry will be along Highway
80
between Carlin, Nev., and Elko, Nev., and further east to the Utah
border, where the capsule's front side can be observed before it passes
over the observer on the ground. The peak brightness will decrease
further from Carlin, lessening to about the brightness of Venus (+0
magnitude) when seen from Boise, Idaho, and Salt Lake City. Viewing
will
not be as good at sites east of Carlin where the craft will be seen
from
behind.

For certain viewing locations just north of the trajectory line, the
capsule will appear to pass by the moon (above it, or below it,
depending on the viewer's location). By choosing their positions
carefully, some observers will be able to see the capsule pass in front
of the moon. As seen by the naked eye, the capsule will disappear in
the
glare of moonlight, but by looking through telescopes, observers may
see
a tiny dot, perhaps trailed by a dark wake of dissipating heat shield
material and hot air. The trail may form a thin line behind the
capsule,
especially near the point of peak brightness where ablation (erosion of
the heat shield and dissipation of the heat that results from the
friction of the heat shield with the atmosphere) is most intense.

"If somebody could see that line, that would be fantastic, because it
would tell us how much carbon is being lost by the heat shield at that
moment," observed Jenniskens. "It would be better yet if several
observers at different locations were to videotape the entry of the
capsule appearing in front of the moon because then we could trace the
ablation of the carbon along the capsule's trajectory, especially at
locations between Carlin and Elko," he added. Videographers should fix
the focus of their cameras at 'infinity,' because auto focus may be
unreliable for nighttime recording, according to mission technicians.

The best way to see the capsule pass in front of the moon would be with
a large telescope at high magnification, according to Jenniskens. Due
to
the long viewing distances, the tiny capsule will appear as a dark dot,
only 1-2 arcseconds across, but darker if the capsule is clearly
visible. One arcsecond is 1/3600th of a degree.

Moving at many times the speed of sound, the capsule will take only two
to three video frames to appear to pass by the moon.

When the Stardust capsule does not appear to be near the moon,
observers
should look for 'chemi-luminance,' a faint glow in the wake of the
capsule. This glow may be created by chemical reactions between the hot
air in the capsule's wake and ozone in the air.

According to scientists, observers using the naked eye will likely see
the capsule as a very bright pinpoint of pink-white light. This color
is
the signature of excited atoms and molecules in the shock wave formed
as
the capsule strikes the atmosphere, according to George Raiche of NASA
Ames.

Light emission is caused by violent breakup of the air molecules that
collide with the speeding capsule. For the most part, these molecules
are oxygen and nitrogen atoms and ionized nitrogen molecules,
Jenniskens
noted. Ablation products could cause violet light resulting from
chemical reactions between carbon and air.

"What the re-entry of the capsule will tell us is how those carbon
compounds might be chemically changed when comet dust enters Earth's
atmosphere. Life's molecules need nitrogen and oxygen combined so that
they can become useful ingredients for living things," Jenniskens said.

In effect, the capsule will be an artificial meteor that we can study
for clues about how life's molecules may have first formed on Earth,
according to Jenniskens. "This will be the first time we will have
scheduled observation of a manmade object entering the planet's
atmosphere at speeds comparable to natural fireballs," he noted.

While most of this chemistry happening at high speed is very quick,
complex and elusive, it is possible to recognize the most brightly
radiating compounds in this fiery process and to look for clues about
what conditions the molecules must endure.

Finally, the conical spacecraft will drop straight down over the
restricted zone in Utah, floating down by parachute.

Once the capsule has landed in Utah, researchers will collect debris
from the surface of the shield and study how much of the heat shield
was
lost during re-entry. "This is called an ablative heat shield," said
Michael J. Wright of NASA Ames, another scientist working on the
project. "By vaporizing some of the material from its surface, the heat
shield vapor carries some of the heat from friction away from the
capsule, keeping the payload cool," Wright explained.

After they recover the capsule and its precious cargo, scientists will
transport the space dust to a laboratory at NASA Johnson Space Center,
Houston, for analysis.

"The sample(s) brought back from comet Wild-2 will tell us is what
carbon compounds are in cometary dust," Jenniskens observed.

"There'll be a small team of us at Johnson Space Center who will assess
what we actually got back from the comet so we can verify we did get a
useful sample," Sandford said. "A small portion of the samples will
then
be used to make a preliminary study of the returned material. After the
preliminary examination is complete, all the samples will be made
available to the general scientific community for more detailed study.
My guess is people will be asking for and working on these samples for
decades to come."

Besides NASA Ames and the SETI Institute, several other institutions
are
partners in the NASA DC-8 airborne study: the University of Alaska at
Fairbanks; the University of Utah at Logan; Sandia National
Laboratories; Los Alamos National Laboratories; the Aerospace
Corporation; the U.S. Air Force Academy; Kobe University, Japan; and
Stuttgart University, Germany. The University of North Dakota operates
the DC-8 aircraft for NASA.

Amateur astronomers who wish to submit photographic, video or other
data
to Jenniskens will find directions and more information about the
airborne campaign at:

http://reentry.arc.nasa.gov

The Stardust spacecraft was launched on Feb. 7, 1999, from Cape
Canaveral Air Station, Fla., aboard a Delta II rocket. NASA's Jet
Propulsion Laboratory, Pasadena, Calif., manages Stardust, a part of
NASA's Discovery Program of low-cost, highly focused science missions.
For more information about Stardust and a background audio interview
with Sandford, visit:

http://stardust.jpl.nasa.gov

http://www.nasa.gov/centers/ames/mul...ust/sdust.html