[meteorite-list] Rosetta Will Encounter Asteroid Lutetia on July 10

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 8 Jul 2010 11:49:43 -0700 (PDT)
Message-ID: <201007081849.o68Inhhb026095_at_zagami.jpl.nasa.gov>

http://www.spaceflightnow.com/news/n1007/08rosetta/

Rosetta will encounter unseen asteroid Saturday
BY STEPHEN CLARK
SPACEFLIGHT NOW
July 8, 2010

The comet-bound Rosetta spacecraft will use its powerful instruments to
see and sniff asteroid Lutetia Saturday, taking advantage of a
fortuitous flyby of the perplexing object, which could be a chunk of
primordial rock from the ancient solar system.

Scientists don't know what to expect from Lutetia, which will be the
largest asteroid ever visited by a spacecraft. Lutetia is inside the
main asteroid belt between Mars and Jupiter.

The size of Lutetia is even up for debate. The best estimate is the
round-shaped asteroid is about 60 miles across, but other data points to
an elongated shape with a peak diameter of 83 miles.

Scientists also don't know the chemical make-up of Lutetia. The best
guess is Lutetia is a C-type asteroid, meaning it has stayed relatively
untouched through most of the violent 4.6-billion-year history of the
solar system.

C-type asteroids are dark and rich in carbon and organic molecules.
Scientists believe they are leftover relics from the formation of the
solar system.

"If it does turn out to be a C-type, which we all hope, then we have a
large object which is rather pristine showing us what the solar system
was like shortly after the planets formed," said Rita Schulz, Rosetta's
project scientist at the European Space Agency.

But some measurements from telescopes on Earth and in space suggest
Lutetia could harbor metals, a signature of an M-type asteroid. Schulz
said metallic M-type asteroids formed from rock from the interior of a
larger body after massive collisions fractured the parent object.

"It can't be, at the same time, a C-type and an M-type asteroid because
they are so different that it is not possible," Schulz said. "This is a
riddle that we can solve only by visiting this object because the
indications from all the observations we have right now are not
conclusive enough that anyone would dare to say this is for sure a
C-type asteroid."

Rosetta should answer all of these questions Saturday.

ESA reports Rosetta is right on course for the flyby, and engineers
don't expect to fire the craft's thrusters to correct the trajectory.

Built to study a comet, Rosetta will only have about two hours to get
the best views of Lutetia as the probe speeds along at a relative
velocity of approximately 33,500 mph.

Rosetta will approach within 1,965 miles of the asteroid at 1544:56 GMT
(11:44 a.m. EDT) Saturday. But the asteroid is 280 million miles from
Earth, meaning it will take radio signals more than 25 minutes to travel
from Rosetta back to Earth.

The flyby will occur at 1610:17 GMT (12:10 p.m. EDT) as observed from
Earth.

Saturday's encounter is the second asteroid flyby of Rosetta's
decade-long sojourn through the solar system. The robotic probe flew
past the much smaller asteroid Steins in 2008.

Rosetta launched in 2004 on a $1.2 billion mission to explore comet
Churyumov-Gerasimenko. The spacecraft will enter orbit around the
periodic comet in the summer of 2014 and stay with Churyumov-Gerasimenko
until the end of 2015. The probe also carries a small lander named
Philae to drop on the comet's surface.

Mission managers assembled a list of possible asteroid flyby
opportunities after Rosetta's launch to give researchers bonus science
on the way to the craft's ultimate destination.

"Steins and Lutetia was regarded as the best possible combination and
was selected," said Gerhard Schwehm, Rosetta's project manager. "Lutetia
is actually the prime asteroid target, and therefore Rosetta will
achieve a major milestone on Saturday. The asteroid chapter can be
closed and we will concentrate on the comet."

If everything goes as planned, Rosetta's visible camera will return
images of Lutetia to Earth later Saturday night. Schulz plans to present
the pictures as early as 2100 GMT (5 p.m. EDT).

"We will have pictures showing craters, valleys and all kinds of
features on the surface," Schulz said.

Rosetta's mineral-mapping spectrometers will determine the surface
composition of Lutetia, and the probe's gas analyzers will try to detect
an exosphere, or ultra-thin atmosphere, around the asteroid.

Other sensors will measure the magnetic field around Lutetia and the
asteroid's interaction with the solar wind.

"We hope to find some gases, or maybe ice and water, and we will look at
the density of the asteroid," Schulz said in an interview Tuesday.

Rosetta's findings should close the book on Lutetia's chemical make-up
by observing the asteroid's ratio of metallic and carbon compounds.

"After the flyby, we will know for sure what class of asteroid this is,"
Schulz said. "This is an accomplishment because asteroid scientists have
now started calling it an X-type, meaning we have no clue what it is."

Lutetia was discovered in 1852 and named for an ancient Roman city on
the site of present-day Paris.

If Lutetia is confirmed as a carbon-rich object, it would only be the
second C-type asteroid ever visited by a spacecraft.

NASA's NEAR Shoemaker asteroid probe flew by asteroid Mathilde in June
1997, the closest look scientists have of such a pristine object.

"Lutetia has always been our main asteroid target because we believe
this will provide us with the most precious information about how the
planets have formed, and what the status of the material was like during
the period of planet formation," Schulz said.

Studying asteroids is one way researchers learn about the early history
of the solar system, when small objects commonly collided with each
other to gradually build large planets and moons.

"What's left in the asteroid belt could not accrete into a bigger planet
and had to stay small," Schulz said. "The material in the asteroid belt
has not been altered as much as the planets have. These are interesting
bodies, for sure. Because they are very diverse, we have to look at them
all to understand better how the asteroid belt evolved."
Received on Thu 08 Jul 2010 02:49:43 PM PDT