Mercury captured by MESSENGER spacecraft

The MESSENGER probe's third and final flyby of Mercury was completed Tuesday Sept. 29. Although the spacecraft was on schedule and completed the main goals of the flyby, an unexpected instrument glitch prevented some data from being collected.

MESSENGER's name - MErcury Surface, Space ENvironment, GEochemistry, and Ranging - also describes the questions it seeks to answer about the smallest, and least known of the rocky planets.

With two flybys in 2008 under its belt, the probe is the first spacecraft to observe Mercury since the Mariner 10 mission, 30 years ago. In 2011, MESSENGER will become the first probe to enter into orbit around Mercury.

The Hopkins Applied Physics Laboratory (APL) is heavily involved in the mission and has several members working on MESSENGER's different teams.

"The science goals of the third flyby were similar to those of previous flybys - image for the first time parts of the planet never seen before, take quick passes of data from the other instruments on the target planet, and practice methods and sequences for data gathering and analysis we plan to employ in orbit," Noam Izenberg of APL said. Izenberg is the instrument scientist for one of MESSENGER's seven instruments, and has been working on the project since he was a post-doctorate at APL.

Scientists originally planned for MESSENGER to take detailed images and observations of nine surface features, mostly impact craters, on Mercury. However, as MESSENGER passed into Mercury's shadow it lost solar power and its instruments entered safe mode. Although function was restored once the probe passed behind Mercury, not all the intended measurements were obtained.

Nevertheless, MESSENGER sent 150 megabytes of data back to earth, including pictures of the 5 percent of Mercury's surface that has never been imaged. On a whole, the mission was successful.

"The primary purpose for the third flyby was to change MESSENGER's orbit sufficiently for its final Mercury encounter in March of 2011, when we'll fire the main engine one last time to settle into Mercury orbit," Izenberg said.

MESSENGER will have time to make many detailed observations of Mercury when it enters its orbit in 18 months.

"From here it's partly a waiting game, and partly practice, practice, practice," Izenberg said. "Seven instruments all operating with their own requirements have to be coordinated very carefully, and we continue to plan out details of the entire one-year primary mission; to choreograph a solution that will get us all the science data we hope for."

- Additional reporting by Jeffrey Siegel