On low power operations during Spirit's third winter on Mars

The Mars Exploration Rovers (MER) Project is operating two Martian robotic rovers, Spirit and Opportunity.
<sup>1

2</sup>
The primary mission duration included 90 Martian days (or ldquosolsrdquo as the team calls them) for each rover to explore the Martian surface. The project is currently enjoying a fifth successful year of operating both rovers on Mars, as several extended missions have been approved. Both of the rovers get their electrical power from solar arrays and store excess energy in batteries. Both operate in the southern hemisphere of Mars. Both share similar seasons, but Spirit is further away from the equator than Opportunity, and Spirit has collected more dust on its solar array panels. Opportunity gathers so much solar array power each sol that the trade-offs to be considered in daily operations focus around science gathering versus data storage and telemetry downlink limitations. Opportunity's ability to gather science on Mars is data bound, as it shunts excess power every sol. Spirit, on the other hand, gathers less solar array energy on its solar panels, because of its position relative to the equator and because of the relatively worse dust accumulation on top of its solar panels. For Spirit, daily operations are also typically data bound. But in the deepest of winter, the daily operations trade-offs pit science versus power, and telemetry downlink versus power. As Spirit's solar array power falls lower and lower, survival of the instruments and spacecraft become the primary focus during winter operations.