MESSENGER Mission Passes Five-Year Mark

http://messenger.jhuapl.edu/news_roo...ils.php?id=127

MESSENGER Mission News
August 3, 2009

MESSENGER Mission Passes Five-Year Mark

It's been five years since MESSENGER was launched atop a Delta II
rocket
on August 3, 2004, and they have been busy years. It has been a long
journey, says MESSENGER Mission Operations Manager Andy Calloway, "not
just in distance travelled - just over 3.5 billion miles so far - but
also in terms of significant milestones and accomplishments."

MESSENGER has executed five planetary flybys - one of Earth on August
2,
2005; two of Venus, on October 24, 2006, and June 5, 2007; and two of
Mercury, on January 14, 2008, and October 6, 2008. "These were not
merely gravity assists, but also major science data collection
endeavors
that required months of detailed planning," says Calloway of the Johns
Hopkins University Applied Physics Laboratory (APL).

The probe has completed four major deep-space maneuvers (DSMs) and 12
trajectory-correction maneuvers, and mission controllers have been
able
to forgo six additional planned course corrections by using
MESSENGER's
solar panels creatively, harnessing solar radiation pressure to adjust
the spacecraft's trajectory.

"Because of the implementation of solar sailing, the MESSENGER team
has
not used propellant to correct the cruise trajectory of the spacecraft
since December 19, 2007, in advance of the first Mercury flyby," says
MESSENGER Mission Systems Engineer Eric Finnegan, of APL. He credits a
solar sailing team of engineers - Ken Williams in navigation, Jim
McAdams in trajectory design, and Dan O'Shaughnessy in guidance and
control - "for increasing mission performance while lowering mission
risk by making this technique operational."

Over the years, MESSENGER's circuitous journey has presented
opportunities within challenges. "We've been through periodic extended
communication outages as the spacecraft travels on the far side of the
Sun from Earth during superior solar conjunctions and frequent close
passages by the Sun during perihelion crossings," Calloway says. "But
MESSENGER has traveled as close to the Sun as only three tenths of the
Earth-Sun distance during these crossings, providing valuable
information and experience in preparation for orbital operations,
which
begin following the critical Mercury orbit insertion maneuver in
2011."

In addition to nine major instrument software loads during these five
years (with two more planned this month), the third main processor
flight software update of the mission was successfully completed on
July
14, 2009.

"This new software significantly increases the spacecraft's
capabilities
for the upcoming orbital phase," Calloway says. "These processor loads
require months of testing, and then once the software is loaded to the
spacecraft, the processor has to be rebooted for it to take effect.
This
rebooting results in a transition to a Sun-safe rotisserie mode in
which
the spacecraft transmits a beacon signal as it rotates slowly every
3.5
hours. Operators in the control center monitored the signal as it
briefly swept through the ground station's field of view one rotation
after the reboot, and seven hours later they sent a command with
precise
timing to halt the rotation and begin restoring the spacecraft to its
nominal operational mode."

More than 90% of Mercury's surface has now been imaged after
MESSENGER's
flybys, including territory never seen before by spacecraft, and
planning is in full swing for the orbital phase of the mission. Up
next
is a third pass by Mercury on September 29, 2009, a fifth DSM on
November 24, 2009, and then on March 18, 2011, MESSENGER will become
the
first spacecraft to enter into orbit around the innermost planet.

"When MESSENGER launched five years ago, even our first encounter with
Mercury seemed far off," recounts MESSENGER Principal Investigator
Sean
Solomon, of the Carnegie Institution of Washington. "Our first two
Mercury flybys last year produced an explosion of new and exciting
observations, and our cruise through the inner Solar System as of this
important anniversary is more than 75% complete. The entire MESSENGER
team is eagerly awaiting the first observations of Mercury from
orbit."
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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.