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

[Ron Baalke]

http://messenger.jhuapl.edu/news_room/details.php?id=280

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 
Mercury."

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.
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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 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.