Messenger space probe about to orbit Mercury for year-long survey

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JHU Gazette
(Johns Hopkins University, Baltimore, Maryland USA)
Monday 14 March 2011

Messenger spacecraft is primed 
for orbit mission at Mercury





by Paulette Campbell 
Applied Physics Laboratory (APL)

On March 17, NASA’s Messenger spacecraft will execute a 15-minute maneuver that will place it into orbit about Mercury, making it the first craft ever to do so, and initiating a one-year science campaign to understand the innermost planet.

Antennas from each of the three Deep Space Network ground stations are on a round-the-clock vigil, allowing flight control engineers at the Johns Hopkins University Applied Physics Laboratory to monitor Messenger on its final approach to Mercury.

APL built and operates the Messenger spacecraft and manages this Discovery-class mission for NASA.

On March 7, the spacecraft began executing the last cruise command sequence of the mission. This command load will execute until today, when the command sequence containing the orbit-insertion burn will start.

“This is a milestone event for our small, but highly experienced, operations team, marking the end of six and a half years of successfully shepherding the spacecraft through six planetary flybys, five major propulsive maneuvers and 16 trajectory-correction maneuvers, all while simultaneously preparing for orbit injection and primary mission operations,” said Messenger systems engineer Eric Finnegan, of APL. “Whatever the future holds, this team of highly dedicated engineers has done a phenomenal job methodically generating, testing and verifying commands to the spacecraft, getting Messenger where it is today.”

The mission operations team last week turned its attention to the final preparations for the insertion burn and to establishing nominal operations for the primary mission. As with the last three approaches to Mercury, the navigation team and the guidance and control team have been successfully using the solar radiation of the sun to carefully adjust the trajectory of the spacecraft toward the optimum point in space and time to start the orbit-insertion maneuver.

As of the most recent navigation report, on Feb. 22, the spacecraft was less than 5 kilometers, and less than three seconds, from the target arrival point.

“These figures place the spacecraft well within the target corridor for successful orbit insertion,” Finnegan said. “Additional body and solar-array attitude alternations will further refine this approach and nudge the spacecraft closer to the optimum target location. This approach will require the spacecraft to spend extended amounts of time at attitudes that do not support transmission of telemetry from the spacecraft, so monitoring of the spacecraft will be conducted with both telemetry and carrier signals.”

The in-flight preparations for this historic injection maneuver began on Feb. 8, when several heaters on the spacecraft were configured to condition the bipropellant used during the maneuver.

“Similar to preheating the diesel engine of a truck or car prior to starting in cold weather to allow ignition and prevent damage to the engine, the Messenger team turns on and off different heaters on the spacecraft so that the pressures for each of the two propellant species -- hydrazine and nitrogen tetroxide -- are at the optimum ratio for safe and efficient maneuver execution,” Finnegan said.

On March 2, the engineering and operations teams convened the last detailed review of the injection command sequence. After three iterations of this command sequence, countless Monte Carlo simulations by the guidance and control team, numerous propulsion modeling simulations and more than 30 hardware simulations covering all manner of nominal and anomalous operating configurations, the sequence and the associated fault protection configuration were given the green light last week to start final preparations for upload to the spacecraft.

“The cruise phase of the Messenger mission has reached the endgame,” said Messenger principal investigator Sean Solomon, of the Carnegie Institution of Washington (DC). “Orbit insertion is the last hurdle to a new game level: operation of the first spacecraft in orbit about the solar system’s innermost planet. The Messenger team is ready and eager for orbital operations to begin.”

Messenger -- an acronym for 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 spacecraft launched on Aug. 3, 2004, and executed flybys of Earth, Venus and Mercury en route to its target planet.

For more on the Messenger mission, go to messenger.jhuapl.edu/mer_orbit.html.