The test gives the XRS team its best look yet at the instrument's detectors, allowing them to measure how well its anti-coincidence and pulse shape discrimination circuits eliminate background signals from x-ray measurements.

"Background rejection is important because it improves the signal-to-background ratio, thereby increasing the sensitivity of the XRS measurements," says Richard Starr, XRS instrument lead scientist, from NASA Goddard Space Flight Center.

"We expect this observation of Cas-A to provide more reliable data than ground measurements on how well the background rejection is working."

The team will spend the next several weeks analyzing the test data and hopes to observe Cassiopeia A again during the cruise to Mercury to track the X-Ray Spectrometer's performance.

XRS wasn't the only part of the science payload to get a workout last week; the team also turned on the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) Feb. 7 for two days of maintenance operations.

Smooth Steering

On Feb. 4 the spacecraft team performed a successful steering test on the back phased-array antenna. Messenger is the first deep space mission to use an electronically steered phased-array antenna.

Two phased arrays, mounted on opposite sides of the spacecraft, provide Messenger's high-gain downlink coverage.

Stat Corner

Messenger is about 91 million miles (146.5 million kilometers) from the Sun and 30.6 million miles (49.2 million kilometers) from Earth. At that distance, a signal from Earth reaches the spacecraft in 2 minutes, 44 seconds.

The spacecraft is moving around the Sun at 67,981 (109,404 kilometers) per hour. Messenger's onboard computers have executed 28,608 commands from mission operators since launch.

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