http://messenger.jhuapl.edu/news_room/details.php?id=119

MESSENGER Mission News
December 23, 2008

MESSENGER Approaches Three Billion Miles, Enters Fourth Solar
Conjunction

On December 26, the MESSENGER spacecraft will have traveled three
billion miles since its launch, marking somewhat more than 60 percent
of
the probe's journey toward its destination to be inserted into orbit
about Mercury.

"That MESSENGER's odometer reading has reached another major milestone
reminds us of the long and complex route that our spacecraft must
follow," offers Principal Investigator Sean Solomon, of the Carnegie
Institution of Washington. "The year now ending has seen the first two
spacecraft flybys of the innermost planet in more than three decades,
encounters that have yielded a rich lode of new observations. The
journey is far from over, but MESSENGER has a skilled team to guide it
the rest of the way."

Mercury orbits deep within the Sun's gravity well. So, even though the
planet can be as close as 82 million kilometers (51 million miles)
from
Earth, getting the probe into orbit around Mercury depends on an
innovative trajectory using the gravity of Earth, Venus, and Mercury
itself to slow and shape the probe's descent into the inner solar
system. On its 4.9 billion-mile journey to becoming the first
spacecraft
to orbit the planet Mercury, MESSENGER has flown by Earth once, Venus
twice, and Mercury twice. Still to come is one more flyby of Mercury
in
late September 2009.

Today the spacecraft entered its fourth superior solar conjunction of
the mission, placing it on the opposite side of the Sun from Earth.
(To
see where MESSENGER is now, visit
http://messenger.jhuapl.edu/whereis/index.php.) The Sun-Earth-probe
angle will be between 2° and 3° until January 6, 2009, so during the
next two weeks there will be no communication with the spacecraft.

To support the conjunction period, the MESSENGER team performed
several
activities to prepare the spacecraft and keep it safe, explains
MESSENGER Mission Operations Manager Andy Calloway. Examples include
extension of the onboard command detection timer and inclusion of
attitude alternations to avoid an autonomous propulsive burn to unload
spacecraft angular momentum. In addition, all instruments have been
turned off except for the Gamma-Ray Spectrometer sensor, which will
remain in a maintenance mode to control closely the temperature of its
cryogenic cooler.

"The team will gather for a spacecraft health assessment on the first
contact after the conjunction, and then the payload will be powered on
again, timers will be restored, and nominal operations will resume,"
says Calloway, of the Johns Hopkins University Applied Physics
Laboratory in Laurel, Md. "The next superior solar conjunction lasts
only five days - from June 6 to June 10 - and will therefore not
require
such extensive preparations." The next long conjunction spans about
two
weeks beginning on November 2, 2009.

The MESSENGER spacecraft is a little more than two years from reaching
its final destination, but the mission Science Team has been
collecting
data and sharing it with the larger scientific community. Those plans
and results are available online at
http://messenger.jhuapl.edu/soc/index.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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