On Dec. 8, the MESSENGER spacecraft spent three hours looking for a star, specifically Éø Leo (a 1.35-magnitude OB class star) to confirm that the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument was functioning as designed. These observations were part of the instrument's first post-launch calibration and maintenance operation since the commissioning checkout early in the mission that certified it had survived the intensity of launch. The MASCS science and engineering teams are now analyzing the results.

The goal of the first of two tests carried out during last week's exercise appears to have been achieved. It involved measuring the alignment of the Ultraviolet Visible Spectrometer (UVVS) detector by looking for Éø Leo and comparing where it appeared within the detector slit with where it was expected to be, given the commanded pointing of the instrument. Early analysis of the data clearly indicates that the star appears well centered in the spacecraft guidance and control target box. Also, spectral calibration of the UVVS detector, done by matching brightness readings from the instrument with the star's known properties, is nearly complete.

The second test was a 48-hour maintenance check that included prompting the instrument shutters, slits and grating drive to open and close to verify they were operational and that the detectors will have an unfettered view when their science mission begins. During the test, pre-set commands told UVVS to scan the environment around MESSENGER for emissions that arise from interplanetary hydrogen, and the Visible Infrared Spectrometer (VIRS) was turned on and its calibration lamps activated. A wakeup call was also sent to the first and second photo multiplier tubes inside the UVVS, which activated them for a 24 hour conditioning experiment, followed by similar testing of the third tube.

These tests mark the first time the MASCS instrument was taken, in-flight, through the rigors of a complete data acquisition exercise using stored commands rather than real-time command operations, and the MASCS science and engineering teams are pleased with the early analysis results. It was the culmination of several months of science and Mission Operations Center teams working together to test and debug software and make critical adjustments before uploading the commands prior to the test.

The instruments will receive routine calibration and maintenance exercises every few months throughout the cruise stage. Over the next few months instrument calibrations will include Mercury Dual Imaging System (MDIS) measurements of its onboard calibration target and X-Ray Spectrometer (XRS) calibration observations of Cassiopeia A. The first complex set of calibration observations involving multiple instruments will include observations of the Moon when the spacecraft swings back to Earth for a gravity assist next August.

MESSENGER is about 98.1 million miles from the Sun and 26.7 million miles from Earth. At that distance, the amount of time for a signal to reach the spacecraft from Earth is 2 minutes, 23 seconds. Since liftoff, MESSENGER's onboard computers have executed 20,254 commands from mission operators.