December 13, 2004
Dec. 8, the MESSENGER spacecraft spent three hours looking
for a star, specifically α 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.
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 α 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.
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.
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.
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 on board 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.
Corner: MESSENGER is about 98.1
million miles (157.8 million kilometers) from the Sun and 26.7
million miles (43 million kilometers) 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