Looking Up – Bob Eklund

NASA’s Mars Reconnaissance Orbiter (MRO) has begun extra stargazing to help the space agency accomplish advances in Mars exploration over the next decade.

The spacecraft already has worked more than double its planned mission life since launch in 2005. NASA plans to keep using it past the mid-2020s. Increased reliance on a star tracker, and less on aging gyroscopes, is one way the mission is adapting to extend its longevity. Another step is wringing more useful life from batteries. The mission’s extended service provides data relay from assets on Mars’ surface and observations with its science instruments, despite some degradation in capabilities.

“We know we’re a critical element for the Mars Program to support other missions for the long haul, so we’re finding ways to extend the spacecraft’s life,” said MRO Project Manager Dan Johnston of JPL. “In flight operations, our emphasis is on minimizing risk to the spacecraft while carrying out an ambitious scientific and programmatic plan.”

In early February, MRO completed its final full-swapover test using only stellar navigation to sense and maintain the spacecraft’s orientation, without gyroscopes or accelerometers. The project is evaluating the recent test and planning to shift indefinitely to this “all-stellar” mode in March.

From MRO’s 2005 launch until the “all-stellar” capability was uploaded as a software patch last year, the spacecraft always used an inertial measurement unit—containing gyros and accelerometers—for attitude control. At Mars, the orbiter’s attitude changes almost continuously, in relation to the Sun and other stars, as it rotates once per orbit to keep its science instruments pointed downward at Mars.

The spacecraft carries a spare inertial measurement unit. The mission switched from the primary unit to the spare after about 58,000 hours of use, when the primary began showing signs of limited life several years ago. The spare shows normal life progression after 52,000 hours, but now needs to be conserved for when it will be most needed, while the star tracker handles attitude determination for routine operations.

The star tracker, which also has a backup on board, uses a camera to image the sky and pattern-recognition software to discern which bright stars are in the field of view. This allows the system to identify the spacecraft’s orientation at that moment. Repeating the observations up to several times per second very accurately provides the rate and direction of attitude change.

“In all-stellar mode, we can do normal science and normal relay,” Johnston said. “The inertial measurement unit powers back on only when it’s needed, such as during safe mode, orbital trim maneuvers, or communications coverage during critical events around a Mars landing.”

To prolong battery life, the project is conditioning the two batteries to hold more charge, reducing demand on the batteries, and is planning to reduce the time the orbiter spends in Mars’ shadow, when sunlight can’t reach the solar arrays. The spacecraft uses its batteries only when it is in shadow, currently for about 40 minutes of every two-hour orbit.

The batteries are recharged by the orbiter’s two large solar arrays. The mission now charges the batteries higher than before, to increase their capacity and lifespan. It has reduced the draw on them, in part by adjusting heater temperatures before the spacecraft enters shadow. The adjustment preheats vital parts while solar power is available so the heaters’ drain on the batteries, while in shadow, can be reduced.

MRO continues to investigate Mars with all six of the orbiter’s science instruments.

 

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