MESSENGER Mission News                                   September 30, 2009

MESSENGER Gains Critical Gravity Assist for Mercury Orbital Observations


MESSENGER successfully flew by Mercury yesterday, gaining a critical gravity
assist that will enable it to enter orbit about Mercury in 2011 and
capturing images of five percent of the planet never before seen. With more
than 90 percent of the planet's surface already imaged, MESSENGER's science
team had drafted an ambitious observation campaign designed to tease out
additional details from features uncovered during the first two flybys. But
an unexpected signal loss prior to closest approach hampered those plans.

At approximately 5:55 p.m., the spacecraft passed by Mercury at an altitude
of 142 miles and at a relative velocity of more than 12,000 miles per hour
according to Doppler residual measurements logged just prior to the closet
approach point. As the spacecraft approached the planet, MESSENGER's Wide
Angle Camera captured this striking view, which shows portions of Mercury's
surface that had remained unseen by spacecraft even after the three flybys
by Mariner 10 in 1974 and 1975 and MESSENGER's two earlier flybys in 2008.

"This third and final flyby was MESSENGER's last opportunity to use the
gravity of Mercury to meet the demands of the cruise trajectory without
using the probe's limited supply of on-board propellant," says MESSENGER
Mission Systems Engineer Eric Finnegan of the Johns Hopkins University
Applied Physics Laboratory in Laurel, Md.

A portion of the complicated encounter was executed in eclipse, when the
spacecraft is in Mercury's shadow and the spacecraft - absent solar power -
was to operate on its internal batteries for 18 minutes. Ten minutes after
entering eclipse and four minutes prior to the closet approach point, the
carrier signal from the spacecraft was lost, earlier than expected.

According to Finnegan, the spacecraft autonomously transitioned to a safe
operating mode, which pauses the execution of the command load and "safes
the instruments," while maintaining knowledge of its operational state
andpreserving all data on the solid-state recorder.

"We believe this mode transition was initiated by the on-board fault
management system due to an unexpected configuration of the power system
during eclipse," Finnegan says. MESSENGER was returned to operational mode
at 12:30 a.m. with all systems reporting nominal operations. All on-board
stored data were returned to the ground by early morning and are being
analyzed to confirm the full sequence of events.

"Although the events did not transpire as planned, the primary purpose of
the flyby, the gravity assist, appears to be completely successful,"
Finnegan adds. "Furthermore,all approach observing sequenceshave been
captured, filling in additional area of previously unexplored terrain and
further exploring the exosphere of Mercury."

"MESSENGER's mission operations and engineering teams deserve high
commendation for their professional and efficient approach to last night's
spacecraft safe-mode transition," says MESSENGER Principal Investigator Sean
Solomon, of the Carnegie Institution of Washington. "They quickly diagnosed
the initial problem, restored the spacecraft to its normal operating mode,
and developed plans to recover as much of our post-encounter science
observations as possible. Most importantly, we are on course to Mercury
orbit insertion less than 18 months from now, so we know that we will be
returning to Mercury and will be able to observe the innermost planet in
exquisite detail."

Additional information and features from this encounter will be available
online at http://messenger.jhuapl.edu/mer_flyby3.html. Be sure to check back
frequently to see the latest released images and science results!