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A Premium on Power

As Spirit and Opportunity slip deeper into their extended missions, the need for power becomes critical. Power to communicate, power to work, power to stay warm, power to stay alive. The sunlight that used to bathe solar cells is growing fainter in a cold, dusty sky.

The rovers are electric vehicles. Energy from the Sun falls on strings of solar cells on the rover deck. It is either used or stored in two rechargeable lithium-ion batteries tucked inside the rover’s Warm Electronics Box. Back when Spirit and Opportunity first landed on Mars, their solar panel arrays were capable of producing nearly 900 watt-hours of energy per martian day, or sol. The landing sites, Gusev Crater and Meridiani Planum, were chosen because they are close to the martian equator and would allow the rovers to capture as much sunlight as possible. By the end of the primary mission, each rover’s capability to produce energy was reduced to about 600 watt-hours per sol.

Power continues to drop as the rovers move through their extended missions. Each sol brings a weaker Sun and colder temperatures. This is because Mars is moving further from the Sun in its elliptical orbit, and winter is creeping toward the equator from the southern hemisphere. The effect is less sunlight on solar panels and that, in turn, means less power for the rovers.

In an effort to keep Spirit and Opportunity as healthy as possible for as long as possible, rover planners weave tactics of survival into daily commands. They include tilting solar panels toward the northern Sun; limiting daily activities; programming the rovers to take regular naps during the day; devoting entire sols to recharging batteries; and using a method called “Deep Sleep” (powering off main electronics at night to conserve energy) when needed.

Spirit and Opportunity require at least 280 watt-hours of power per sol to survive. As they sink into a bitterly cold martian winter, the Mars Rover team will try to keep them alive until the return of the Sun. Come January, 2005, sunlight will start to increase at the martian equator. So, too, will the rovers’ chances of survival.

Web content editor/writer: Pamela R. Smith

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