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Taking Commands

While MESSENGER sits quietly in the Astrotech Space Operations facility, there is a flurry of activity back in the Mission Operations Center at the Applied Physics Laboratory, where the MESSENGER team is testing and finalizing spacecraft and ground-support software. A spacecraft needs well-understood software onboard its computer and on the ground, in the spacecraft simulator. The simulator is a key to mission success - to safeguard MESSENGER from an errant instruction all command loads are first run through the simulator.

As computer users know, it is easy to make one small mistake, such as a misplaced comma or a wrong sign (like a "+" instead of a "-"). In everyday situations such a mistake simply causes a program error; the user corrects the offending command and moves on. In the unforgiving space environment a wrong sign could (for example) turn MESSENGER's solar panels away from the Sun, resulting in an onboard emergency.

MESSENGER has several layers of protection to prevent such an occurrence. Command "scripts" are carefully prepared and reviewed even before they're run through the simulator. Errors caught in the simulator are corrected and re-tested. If a mistake were to sneak through and be executed by MESSENGER during flight, the onboard autonomy software would detect the error, reorient the spacecraft or take other action to keep MESSENGER safe, send back a "What's up with that command?" message to controllers, and await further instructions from Earth.

In this week's Webcam frame MESSENGER is roped off in its clean room at Astrotech, near Kennedy Space Center, Florida. The air duct keeps MESSENGER's battery cool and the red cover protects the phased array antenna until launch. In June, activities on and around the spacecraft will pick up as final preparations for MESSENGER's July 30-August 13 launch opportunity begin.

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Last updated: May 21, 2004


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