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发帖数: 11530 | 1 A close-up with a comet
by Staff Writers
Chicago IL (SPX) Nov 12, 2014
On Nov. 12 a robotic lander called Philae is scheduled to touch down on a
comet, following a journey of 10 years and more than 3.7 billion miles. The
mission aims to provide insights into how the solar system evolved, and
whether comet impacts helped form Earth's oceans and brought chemical
precursors of life. (Image courtesy of European Space Agency).
Even as Tom Economou approached retirement age in 1994, he began planning an
instrument for the European Space Agency's Rosetta mission to a comet. He
still remembers the reaction of Riccardo Levi-Setti, then director of the
University of Chicago's Enrico Fermi Institute, after the latter reviewed
the mission timeline, which culminated with a comet rendezvous in 2014.
"Riccardo looked at me and said, 'Let's see, 1994, 2014. Where do you think
Tom will be in 2014?'
"Well, of course I expect to be here," replied Economou, who at age 77 is
still a Fermi Institute senior scientist. Economou's work will come to
fruition on Nov. 12, when a robotic lander called Philae, having detached
from the main Rosetta craft, is scheduled to touch down on the head of the
snowman-shaped comet 67 P/Churyumov-Gerasimenko. Among Philae's instruments
is the alpha particle X-ray spectrometer that Economou successfully proposed.
After a journey of 10 years and more than 3.7 billion miles, Rosetta and
Philae are poised to deliver new insights into how our solar system evolved,
and whether comet impacts helped form Earth's oceans and brought chemical
precursors of life.
The APXS detects alpha particles and X-rays. Of the Philae's 10 instruments,
it is the only one that can provide information about the elemental
composition of the comet's surface.
Other UChicago scientists following the rendezvous with interest are Thomas
Stephan, a senior scientist in geophysical sciences, and Nicolas Dauphas,
professor in geophysical sciences.
Stephan is a member of the team analyzing data collected by Rosetta's dust
analyzer. While working at the Westfalische Wilhelms-University of Munster
in Germany, Stephan helped build and prepare Rosetta's COSIMA (Cometary
Secondary Ion Mass Analyzer). In Germany and later at UChicago, he also
analyzed dust samples returned from comet Wild 2 by the Stardust spacecraft.
Although not directly involved in the mission, Dauphas operates UChicago's
Origins Laboratory, which examines questions pertaining to the formation of
these solar system objects.
"It's interesting to see what comets are made of because they could have
played a role in seeding life on Earth by bringing in some specific organic
molecules that were in the primitive soup of the early oceans," Dauphas says.
Tricky landing
Economou will be watching Philae's landing on the morning of Nov. 12 from
the lander control center in Cologne, Germany. There he will join colleagues
from Johannes Gutenberg University Mainz, who lead the experiment. Landing
Philae on the irregularly shaped comet, which measures less than 2.5 miles
in diameter at its widest extent, will be complicated and risky.
"Landing is tricky, because they have to release the lander at the right
speed in the right direction and from some distance," Economou says. ""If it
's a little bit off, it's possible that you'll miss the comet."
That's what happened when the Japanese space agency attempted to deliver a
mini-lander to the surface of asteroid Itokawa in 2005. Despite the loss of
the mini-lander, the Hayabusa mother ship did manage to successfully land on
Itowaka, collect a small quantity of dust particles from the asteroid's
surface and successfully return them to Earth.
Rosetta already has collected riveting images and valuable data from the
comet, ensuring a successful mission regardless of the lander's fate.
"I'm very happy about this mission by the European Space Agency," says
Dauphas, a native of France. "It's the most ambitious planetary mission I've
seen come from Europe in a long time."
Rosetta was originally scheduled to launch in 2003 for a rendezvous with
Comet 46 P/Wirtanen, but problems with the probe's Ariane rocket caused a
delay. By the time Rosetta successfully launched on March 2, 2004, Economou
had almost forgotten about it. Since submitting his proposal for the APXS,
he had become busy with four other robotic interplanetary space missions.
Economou built the alpha proton X-ray spectrometer that performed the first
chemical analysis of martin rocks aboard the Mars Pathfinder rover in 1997.
The two Mars Exploration Rovers that landed on Mars in January 2004 carried
alpha particle X-ray spectrometers that were similar to Pathfinder's
chemical sensor. The Spirit rover operated for six years. The Opportunity
rover has traveled 26 miles "and it's still going strong," Economou says.
And the Cassini mission to Saturn carries the high rate detector that
Economou helped develop along with the late John Simpson, the Arthur Holly
Compton Distinguished Service Professor Emeritus in Physics; and the late
Anthony Tuzzolino, a senior scientist in the Fermi Institute. The high rate
detector measures the dust particles that orbit Saturn and its moons,
including Enceladus.
One spacecraft, two comets
Economou also participated in the Stardust mission, which flew within 150
miles of comet Wild 2 in January 2004 with the Simpson group's dust flux
monitor instrument. The instrument collected data on the microscopic dust
particles streaming from the comet. After that mission, the spacecraft was
redirected to another comet encounter during theStardust-NExT mission in
2011.
Philae's APXS is a descendant of the alpha backscattering experiment
developed by the late Anthony Turkevich, the James Franck Distinguished
Service Professor in Chemistry, for pre-Apollo lunar studies in the 1960s.
Economou collaborated with Turkevich for 38 years.
The APXS will measure the number of atoms of light elements on the comet's
surface. It is well suited for analyzing the comet's light elements, such as
carbon, oxygen, and nitrogen, Economou says. "That's what a comet is. It's
half ice and half dust."
Some of the lander's other instruments will make measurements to help
determine whether comets provided any of the water to Earth's oceans during
a period of heavy bombardment early in the planet's history.
Dauphas is especially interested in the noble gas content of comets. These
highly stable gases, which include helium, neon, and argon, could offer
clues about the formation of Earth's atmosphere.
"The composition of the atmosphere is puzzling. We cannot relate it easily
to any of the building blocks of the Earth," Dauphas says. "When we look at
meteorites, there is nothing that really matches the composition of Earth's
atmosphere. Comets are one category of objects that we have never measured."
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