http://messenger.jhuapl.edu/news_room/details.php?id=120

MESSENGER Mission News
February 9, 2009

MESSENGER Continues Hunt for Ever-Elusive Vulcanoids

MESSENGER reaches its orbital perihelion today and passes within 0.31
astronomical units (AU) of the Sun (one AU is nearly 150 million
kilometers or 93 million miles). The mission's imaging team is taking
advantage of the probe's proximity to the fiery sphere to continue
their
search for vulcanoids - small, rocky asteroids that have been
postulated
to circle the Sun in stable orbits inside the orbit of Mercury.

Vulcanoids are named after Vulcan, a planet once proposed to explain
unusual motions in Mercury's orbit. Scientists have long suspected
that
these small, faint "space rocks" exist. There is a gravitationally
stable region between the orbit of Mercury and the Sun, which means
that
any objects that originally formed there could have remained for
billions of years and might still be there today. All other such
regions
in the solar system are occupied by some type of debris (e.g., Trojan
asteroids at stable points along the orbits of Jupiter and Neptune and
Kuiper Belt objects near and beyond the orbit of Pluto).

The so-called vulcanoid region between the orbit of Mercury and the
Sun
is the main gravitationally stable region that is not known to be
occupied. The region is, however, the most difficult to observe. Any
vulcanoids would be difficult to detect from Earth because of the
strong
glare of the Sun. Previous vulcanoid searches have revealed no bodies
larger than 60 kilometers in diameter. But MESSENGER's travels in
near-Mercury space enable a search for vulcanoids from a vantage never
before attempted, says MESSENGER Science Team Member Clark Chapman,
who
is spearheading the team's search along with his associate, William
Merline.

"With MESSENGER, we can search for vulcanoids as small as 15
kilometers
across," said Chapman, a senior scientist at the Southwest Research
Institute in Boulder, Colorado. Between February 7 and 11, the
wide-angle camera of MESSENGER's Mercury Dual Imaging System will have
snapped 256 images in the areas east and west of the Sun. Because of
the
danger of the Sun's glare, the camera will have to peek just past the
probe's sunshade to capture images.

"We are making the same observations on each day," MESSENGER team
member
Nancy Chabot explained. "This cadence will allow us to reject cosmic
rays and to distinguish, by its motion, the class of each object
imaged"
(e.g., vulcanoid vs. near- or inner-Earth asteroid).

The team carried out a similar imaging campaign over a nine-day period
in June 2008, capturing 240 images of the outer portions of the would-
be
vulcanoid belt. "This sequence was designed to refine our observing
techniques, assess limiting magnitudes, verify detectability of known
objects, and make an initial search,' Chapman explained.

"Vulcanoids, should they be found, may provide scientists with
insights
into the conditions prevalent in the early solar system," Chapman
said.
"In particular, if they exist or once existed, they would represent an
additional population of impactors that would have cratered no other
planet but Mercury, implying that the geological processes on Mercury
have happened more recently than we would calculate if we assumed
thatMercury's craters formed at rates equivalent to cratering on the
Moon
and Mars."

If vulcanoids are found not to exist, then we could be more confident
that most of Mercury's volcanic plains formed billions of years ago,
as
on the Moon, according to Chapman. The absence of vulcanoids would
also
focus scientists' thinking on why vulcanoids never formed or, if they
did form, why they are no longer there.

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MESSENGER Thermal Engineer Maintains Cool in Extreme Environs

The Mercury-bound MESSENGER spacecraft will be assaulted by
temperatures
as high as 700°F as it orbits the planet closest to the Sun, and the
only thing that will stand between its room-temperature science
instruments and the blistering heat is a handmade ceramic-cloth quilt
just one-quarter of an inch thick. Carl Jack Ercol, the man largely
responsible for making sure that MESSENGER will be able to stand up to
such harsh heat once imagined he'd make his living in a darker, much
cooler environment: the coal mining industry. Read more about Ercol at
http://messenger.jhuapl.edu/who_we_are/member_focus.html.
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MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and
Ranging) is a NASA-sponsored scientific investigation of the planet
Mercury and the first space mission designed to orbit the planet
closest
to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and
after flybys of Earth, Venus, and Mercury will start a yearlong study
of
its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie
Institution of Washington, leads the mission as principal
investigator.
The Johns Hopkins University Applied Physics Laboratory built and
operates the MESSENGER spacecraft and manages this Discovery -class
mission for NASA.
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