[meteorite-list] Comet Chasers Reel in Dust, Chart Soft Landing on Tiny Rock

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Jul 18 13:37:56 2005
Message-ID: <200507181632.j6IGWkw14314_at_zagami.jpl.nasa.gov>

http://www.sfgate.com/cgi-bin/article.cgi?file=/c/a/2005/07/18/MNG09DPGQ21.DTL

Comet chasers reel in dust, chart soft landing on tiny rock
  
Researchers seek remnants from solar system's birth

David Perlman
San Francisco Chronicle
July 18, 2005

When Deep Impact blasted a crater in a distant comet early this month,
the tricky feat of navigation from Earth to a moving target across 83
million miles of space astonished the world -- but a European spacecraft
already flying toward another comet has an even more demanding mission.

This robot vehicle, named Rosetta, is on a 10-year voyage looping across
a billion miles of space in an attempt to land on its tiny target so
softly that the spacecraft can photograph the comet's unknown surface
and drill into it to find out what lies underneath.

A third remarkable cometary mission, meanwhile, is nearly over. A
pioneering NASA spacecraft named Stardust is headed back to Earth,
carrying a priceless cargo of microscopic particles it has collected
from the glowing coma surrounding comet Wild 2 and interstellar dust
from the region between Earth and the comet. Stardust's mission control
engineers will try to land its capsule filled with cometary samples by
parachute on a flat stretch of Utah desert early next year.

Rosetta's unprecedented goal is to orbit the distant comet called
67P/Churyumov-Gerasimenko; to fly with it at 18,000 mph along its path
around the sun for an entire year, the solar heat slowly turning the
comet from a cold and almost invisible object near Jupiter into a
blazing fireball closer to the sun; and finally to release a 220-pound
lander onto the comet's surface.

And that's the infinitely tricky part, for the comet's nucleus is less
than 3 miles in diameter and moving closer and closer toward the sun.
With so little gravity in which to work, Mission Control engineers at
the European Space Agency in Darmstadt, Germany, will have to ease the
three-legged Rosetta lander slowly down from its mother ship, inch by
inch, until it anchors itself by firing three small harpoons into the
comet's surface to keep from flying off.

Just to reach the comet Rosetta needed three swings around the Earth and
another around Mars to gain enough speed for the long voyage.

And if Rosetta gets there safely, the job will be really tough: the
lander must drill just 25 centimeters -- 10 inches -- into the comet's
crust for samples that will be baked in a miniature oven to vaporize
them for chemical analysis. The lander also will take images of the
surface, measure the comet's radiation environment and relay all the
data to the orbiter flying above, which must take its own complex
instrument readings on its environment.

This will be "very different and more difficult" than Deep Impact's
cratering crash, said Paolo Ferri, Rosetta's operations manager, in an
e-mail message. "The problem is so complex, especially in an unknown
environment, that at the moment we can only give it some statistical
solutions."

The final decisions about how and where to land will take months of
analysis while Rosetta is flying in orbit around the comet gathering data.

"It is difficult to characterize unknown dangers," Ferri said.
"Certainly at this time the most concerning aspect is that we don't know
how difficult it will be to find a safe landing solution -- but we are
confident that we'll find one!"

The European scientists of the Rosetta mission have been following the
Deep Impact mission closely, and Americans are participating in many
aspects of Rosetta, too, as part of what Gerhardt Schwehm, Rosetta's
project scientist, calls "the worldwide community of cometary scientists."

In fact, Schwehm noted in an e-mail message, Michael A'Hearn, Deep
Impact's chief scientist, is also a co-investigator for Rosetta's
imaging system and its ultraviolet spectrometer. Another American,
astrophysicist Alan Stern of the Southwest Research Institute in
Boulder, Colo., heads the international team for the spectrometer.

"Our joint goal," Schwehm said, "is to understand the physics of comets
and their evolution, and all our missions provide pieces of the puzzle."

Physicists, astronomers, geologists and even biologists are all seeking
to understand the detailed nature and composition of comets because of
the clues they hold to the origin of the solar system's planets, and
perhaps to the origin of life itself on Earth.

Most of these starry wanderers were formed some 4.6 billion years ago
from the same clouds of interstellar dust and gas that slowly condensed
and heated into the incandescent object we call our sun. The planets
were forming by accretion from the same materials, too -- but unlike the
giant sun or the planets, the comets remained like icy pebble remnants,
frozen on the very edges of the nascent solar system.

Scientists theorize, therefore, the comets must still contain the same
water and rock and carbon compounds that created the planets so long ago.

It is a bit ironic that after the material that created comets also had
coalesced into planets, the Earth itself is thought to have been
bombarded for nearly a billion years by ferocious streams of comets.
That activity heated the young planet, churned up its barren crust and
-- many scientists believe -- provided enough water to create all the
Earth's oceans and quite possibly delivered the carbon-containing
compounds that were necessary for life to emerge some 3.8 billion years
ago.

Missions like Stardust and Rosetta are designed to identify the contents
of comets even more precisely than the data that Deep Impact's "flyby"
spacecraft has provided to mission scientists working at NASA's Jet
Propulsion Laboratory and at their home university campuses.

When the Deep Impact spacecraft's separate "impactor" vehicle steered
itself directly into the orbital path of comet Tempel 1 in a triumph of
space navigation at 10:52 p.m. PDT on July 3, it hit the comet's
onrushing surface at a relative speed of 23,000 miles an hour and
blasted a crater that scientists still have not been able to measure.

The blast blew out vaporized rock and water particles in a flash that
glowed brilliantly as the sun energized them, "but it's pretty clear
that this event did not produce a gusher (of water)," said astronomer
Gary Melnick in a release from the Harvard-Smithsonian Center for
Astrophysics after gathering ground-based observations last week from
telescopes in Hawaii and from NASA's orbiting Submillimeter Wave
Astronomy Satellite.

The relatively sparse output of water from the crater has been puzzling
astronomers, and so are their first analyses of instrument readings from
both the last moments of the impactor and the subsequent look backward
as the flyby craft moved away from the scarred comet's nucleus.

"The major surprise was the opacity of the plume the impactor created
and the light it gave off," said A'Hearn, the mission's chief scientist
from the University of Pennsylvania. "It suggests that the dust
excavated from the comet's surface was extremely fine, more like talcum
powder than beach sand."

Whether the surface of comet Wild 2, the Stardust mission's target, will
prove to be like Deep Impact's Tempel 1 won't be known until Stardust
completes its mission on Jan. 15, when it is scheduled to land by
parachute within the desert confines of an Air Force weapons test range
70 miles west of Salt Lake City.

After traveling since February 1999, the spacecraft used a swing around
the Earth like a stone from a slingshot to speed it on its way. It
reached its target just a year ago and endured a storm of rock and dust
particles as it passed through the comet's coma. Its unique collector is
a silicon-based, latticelike foam called an aerogel, which is nearly 99
percent empty space and the lightest solid ever known. The aerogel
trapped microscopic particles of interstellar dust as it flew to the
comet, and some of the cometary debris was even larger -- the size of
sand grains.

Stardust's encounter with Wild 2 also has yielded surprising insights
into the possible structure of all comets, traditionally once thought to
be little more than loosely compacted rubble -- "dirty snowballs" or
"snowy dirtballs," as scientists have called them.

The images Stardust sent back to Earth were striking: Wild 2's nucleus
is marked by rocky cliffs, pinnacles, deep impact craters and other
evidence of a solid rocky object. Particles from that surface, blown off
by the sun's energy and trapped in the returning spacecraft's collector,
should give scientists the first chance earthlings have had to determine
the composition of the solid matter that formed -- just as Earth did --
billions of years ago in the new solar system.

------------------------------------------------------------------------


ROSETTA

Rosetta was launched last year by the European Space Agency. -- Mission
Timeline

March 2, 2004: Launch

March 2005: First Earth-gravity assist

February 2007: Mars-gravity assist

November 2007: Second Earth-gravity assist

November 2009: Third Earth-gravity assist

May 2014: Comet rendezvous

November 2014: Comet landing.


STARDUST

Launched in 1999, NASA's Stardust spacecraft is on its way back to Earth.

-- Mission Timeline

Feb. 7, 1999: Launch

February-May 2000: First interstellar dust collection

Jan. 15, 2001: Earth-gravity assist

August-December 2002: Second interstellar dust collection

Jan. 2, 2004: Comet Wild 2 encounter

Jan. 15, 2006: Return to Earth
Received on Mon 18 Jul 2005 12:32:44 PM PDT