[meteorite-list] Nothing but Helium: Correction Maneuver Puts MESSENGER Right on Course

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 10 Apr 2015 16:35:57 -0700 (PDT)
Message-ID: <201504102335.t3ANZvK9014174_at_zagami.jpl.nasa.gov>


MESSENGER Mission News
April 9, 2015

Nothing but Helium: Correction Maneuver Puts MESSENGER Right on Course

The MESSENGER team is pulling out all the stops to give the spacecraft
life far beyond its original design. On April 8, mission operators at
the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel,
Md., successfully conducted a contingency orbit-correction maneuver (OCM-15a),
to supplement the April 6 burn (OCM-15) that concluded early when the
last drops of hydrazine fuel were expended.

Had there been a little more hydrazine, OCM-15 would have raised MESSENGER's
periapsis altitude a full 25 kilometers.

"The team couldn't be sure precisely how much liquid hydrazine remained
onboard, and how much of that was accessible," explained APL's Karl Whittenburg,
MESSENGER's Deputy Mission Operations Manager. "Onboard fault-protection
software was designed to transition autonomously to use of gaseous helium
for propulsion, should hydrazine depletion occur during this maneuver.
Although the transition occurred as designed, our post-maneuver analyses
indicated a shortfall in the desired trajectory change."

"To our knowledge, this is the first-ever use of a pressurant for a planned
propulsion of a spacecraft, so we could only theorize how it might perform,"
Whittenburg continued. "OCM-15 gave us performance data on this technique,
and we are now fully confident that future use of gaseous helium will
continue to provide MESSENGER with a unique vantage point for studying

Wednesday's contingency maneuver -- this time designed to use gaseous
helium exclusively -- raised the spacecraft's minimum altitude above Mercury
from 18.2 kilometers (11.3 miles) to 29.1 kilometers (18.1 miles). During
the operation, a velocity change of 1.94 meters per second (4.34 miles
per hour) was imparted, releasing the pressurant through the four largest
monopropellant thrusters. Implemented when the spacecraft was at nearly
the farthest point in its orbit from Mercury, today's maneuver increased
the spacecraft's speed relative to Mercury and also increased the spacecraft's
orbit period to 8 hours, 20.3 minutes.

This view of MESSENGER shows the spacecraft orientation at the start of
OCM-15a. During the maneuver, the sunshade protected heat-sensitive components
from direct sunlight. OCM-15a was planned and executed in a record two
days' time and will keep MESSENGER on its aggressive course to make never-before-seen
observations of the planet, made possible only during this final "hover
campaign." The next maneuver, on April 14, will once again use gaseous
helium to give MESSENGER and its science payload a bit more time to reveal
more of the mysteries of the innermost planet in our solar system.
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 was launched on August 3, 2004, and entered orbit
about Mercury on March 18, 2011, to begin a yearlong study of its target
planet. MESSENGER's first extended mission began on March 18, 2012, and
ended one year later. MESSENGER is now in a second extended mission, which
is scheduled to conclude no later than April 30, 2015. Dr. Sean C. Solomon,
the Director of Columbia University's Lamont-Doherty Earth Observatory,
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.
Received on Fri 10 Apr 2015 07:35:57 PM PDT

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