Critical Deep-Space Maneuver Targets MESSENGER for Its First Mercury Encounter

The MESSENGER spacecraft delivered a critical deep-space maneuver on Wednesday – 155 million miles (250 million kilometers) from Earth – successfully firing its large bi-propellant engine to change the probe’s trajectory and target it for its first flyby of Mercury on January 14, 2008.

“Completing this maneuver was a huge milestone for the mission,” offered MESSENGER Principal Investigator Sean Solomon. “We are now en route to the closest glimpse of Mercury that anyone has ever seen. Over the next three months the suspense about what we will find will steadily build.”

The maneuver was executed in two parts from the MESSENGER Mission Operations Center at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. At 6 p.m. EDT on October 17, the probe fired its large main engine for just over five minutes, using about 70 kilograms (154 pounds) of propellant to change its velocity by 226 meters per seconds, or just over 505 miles per hour.

Then, at 6:30 p.m. EDT, the small thrusters were fired for about two minutes, changing the probe’s velocity by an additional 1.4 meters per second. This burn redistributed the propellant in the main tanks to manage location of the probe’s center of mass, putting the spacecraft in a more stable mode of operation. “This action lowers the risk of having to do momentum correction maneuvers during November, when interference from the Sun will prevent communication with the spacecraft,” explained APL’s Jim McAdams, who helped design this maneuver.

“Everything went as planned, and we are now on target for a flyby of Mercury in January 2008,” said Mission Operations Manager Andy Calloway of APL, adding that this maneuver was the most critical of the mission other than orbit insertion, primarily because of the timing. “Deep- Space Maneuver-2 (DSM-2) was executed just nine days prior to the start of the longest solar conjunction communications outage period of the mission,” he said. “So there was limited opportunity to correct problems and to obtain good orbit determination data for the navigation team.”

“The MESSENGER team is breathing a lot easier now that we’ve seen the successful completion of this most important course-correction maneuver before Mercury orbit insertion,” McAdams said. “Not only did DSM-2 put MESSENGER on target for the first spacecraft encounter with Mercury in nearly 33 years, it was completed with the least margin for error of all five DSMs before Mercury arrival in March 2011.”

This was the second of five deep-space maneuvers that will help the spacecraft reach Mercury orbit. The first, on December 12, 2005, positioned the probe for its October 2006 flyby of Venus. DSM-3 on March 17, 2008, will position the probe for the second flyby of Mercury on October 6, 2008. DSM-4 on December 6, 2008, positions MESSENGER for Mercury flyby 3, scheduled for September 30, 2009. And the final deep-space maneuver on November 29, 2009, targets the probe for Mercury orbit insertion on March 18, 2011.

“Now that we are past DSM-2, we will complete our solar conjunction preparations and begin testing our final version of the Mercury encounter sequence,” Calloway said. “Once we exit the solar conjunction, we will finalize plans for the two December trajectory-correction maneuvers – TCM-19 and TCM-20 – as we correct any propagated errors from DSM-2 so we can put MESSENGER right on the flyby aim point. It has been over three years of densely-packed cruise operations, and we are finally about to fly by and begin collecting data at Mercury.”

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 launched on August 3, 2004, and after flybys of Earth, Venus, and Mercury will start a yearlong study of its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, 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.