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.

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.