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How rovers weather the winter on Mars

Catching precious sunlight requires precise planning to maximize the rover’s capabilities and safety.
639pxOpportunity_in_Endurance_Crater
A simulated view of NASA's Opportunity Mars Exploration Rover in Endurance Crater, near where it will be spending this winter.
NASA/JPL-Caltech/Cornell
The shortest, coldest days of winter are approaching for those who live in Earth’s Northern Hemisphere. But on Mars, the Opportunity rover is already in the midst of the Red Planet’s doubly-long winter in its southern hemisphere, with the period of least sunlight having passed in October and November. During winters on Mars, the rover must rely on a smart strategy that employs tilting its solar panels northward to maximize energy for its ongoing mission.

Mars, like Earth, is tilted on its rotation axis with respect to its orbit around the Sun. Its tilt is about 25 degrees, while Earth is tilted about 23.5 degrees, so the two planets experience similar seasons — but Mars, with its 1.88-Earth-year-long year, has seasons that last almost twice as long. That makes planning for power acquisition (via solar panels) and usage (through driving and data collecting) essential during the winter season, when sunlight is scarce and the Sun takes a northern path through the sky from the rover’s point of view. To gain power for mission-necessary tasks, the rover must tilt itself northward to catch as much sunlight as possible during the long winter season.

Opportunity is currently exploring a valley known as “Perseverance Valley” on the western rim of Endurance Crater. It’s been there for about five months, and the location puts it in an ideal spot to catch the northern Sun. As it stops to collect valuable photons at planned sites nicknamed “lily pads,” the rover takes the time to explore the immediate area, studying rocks along the crater’s rim and imaging the valley in great detail.
PIA15959_hires
A 2007 martian dust storm captured from above by the Mars Color Imager instrument on NASA's Mars Reconnaissance Orbiter. Such storms can cover the rovers - and their solar panels - in dust. The next dust storm season is expected in 2018.
NASA/JPL-Caltech/MSSS
The energy available to a solar-based Mars rover depends on more than just the position of the Sun and the length of the martian day. It also depends on the condition of the solar panels, which can be easily covered in dust kicked up by everyday winds and during storms. Sometimes those winds pile dust onto the panels, limiting their ability to absorb sunlight. Other times, the winds clean the panels, revealing a smooth surface to boost the rover’s power-collecting capabilities.

Currently, Opportunity’s panels are relatively clean, but there’s a potential dust storm season approaching next year, so mission planners are remaining optimistic but watchful. “If Opportunity's solar arrays keep getting cleaned as they have recently, she'll be in a good position to survive a major dust storm. It's been more than 10 Earth years since the last one and we need to be vigilant,” said Jennifer Herman, who leads Opportunity’s power subsystem operations team at NASA's Jet Propulsion Laboratory, in a press release.

This coming January will mark the 14th (Earth-year) anniversary of the Opportunity and Spirit rovers, which were designed to last about three months. Instead, Opportunity is still going strong, though Spirit became stuck in a sand trap and was unable to tilt northward during its fourth winter in 2009. Spirit’s mission was ultimately ended in May 2011, but Opportunity persists — as does the Curiosity rover, which doesn’t rely on solar power, but instead has a radioisotope thermoelectric generator to provide power.
PIA21711_hires
A view of “Perseverance Valley,” which Opportunity is now exploring.
NASA/JPL-Caltech/Cornell/Arizona State Univ.
For now, Opportunity will continue “leaping” from lily pad to lily pad, exploring the valley for clues as to how it was formed, and whether water (and how much) was involved. Its ultimate goal is the bottom of the valley, but that’s an area with no lily pads available for soaking up wintertime sunlight — so it’ll have to wait for the seasons to change.
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