Mars Rover enjoys its day in the sun

The scientists at the U.S. National Aeronautics and Space Administration (NASA) have given Opportunity a much-needed break—to literally recharge its batteries—as winter sets in on the Red Planet, the fourth in distance from the Sun in our solar system.

The rover and its “twin,” the Spirit, require power to operate. Without power, the vehicles cannot move, use their scientific instruments, or communicate with Earth. The main source of power for the rovers—both of which arrived on the Red Planet in April 2004— comes from a multi-panel photovoltaic (PV) solar array. The panels resemble "wings," but their purpose is to provide energy; not to lift the rovers above the planet’s surface.

When fully illuminated, the solar arrays generate about 140 watts of power for up to four hours per “sol” (the name for a Martian day). Each rover needs about 100 watts (equivalent to the energy emitted by a conventional light bulb) to drive. In comparison, the solar arrays on the robotic Sojourner provided the 1997 Pathfinder mission with around 16 watts of power at noon on Mars. That´s equal to the power of an oven light.

The power system for the Mars Exploration Rover includes two rechargeable batteries that provide energy for the rover when the sun is not shining, especially at night. Over time, the batteries are known to degrade and become unable to recharge to full power capacity—a problem attributable to three issues:

1. The surfeit of red dust on the planet coats the solar arrays and makes them increasingly impervious to light;

2. Mars drifts farther from the sun as it continues on its yearly elliptical orbit and— because of the distance—the sun does not shine as brightly onto the solar arrays; and.

3. Mars is tilted on its axis, just like Earth, which creates seasonal changes. Later in the mission, the seasonal changes at the landing site and the lower position of the Sun in the sky at noon will impact the amount of energy that hits the solar panels.

The scientists at NASA’s Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, who manage the Mars Exploration Rover Project for the NASA Science Mission Directorate–Washington, have opted to “park” Opportunity for the next several months at a site informally named "Greeley Haven." The name is a tribute to planetary geologist Ronald Greeley (1939-2011), who was a member of the science team for the Mars rovers and many other interplanetary missions. Greeley taught generations of planetary scientists at Arizona State University–Tempe, until his death two months ago.??

The site is an outcrop that provides a sun-facing slope to aid in maintaining adequate solar power during the rover’s fifth Martian winter. It also provides targets of scientific interest for the rover’s robotic arm to examine. ??Closer to the Mars equator than Spirit, the Opportunity rover did not need to stay on a sun-facing slope during previous winters. Now, however, Opportunity’s solar panels carry a thicker coating of dust than in the previous winters. Unless an unlikely wind cleans the panels in coming weeks, the team will use a strategy employed for three winters with Spirit: staying on a sun-facing slope. For several months of shortened daylight before and after the southern Mars winter solstice on March 30, 2012, the sun will appear relatively low in the northern sky from the rover’s perspective, and Opportunity will work on the north-facing slope.

??Plans for research at Greeley Haven include a radio-science investigation of the interior of Mars, which began during the first week of January; inspections of mineral compositions and textures on the outcrop; and recording a full-circle, color panorama. to be called the Greeley Panorama. ??The radio-science investigation studies tiny wobbles in the rotation of Mars to gain insight about the planet’s core. It requires many weeks of radio-tracking the motion of a point on the surface of Mars to measure changes in the spin axis of the planet. ??The winter worksite sits on the "Cape York" segment of the rim of Endeavour Crater. Opportunity reached the edge of this 14-mile-wide (22-kilometer-wide) crater five months ago after three years of driving from the smaller Victoria Crater, which it studied for two years.

Opportunity and Spirit completed their three-month prime missions in April 2004 and have continued on for years of bonus, extended missions. Both rovers have made important discoveries about wet environments on ancient Mars that may have been favorable for supporting microbial life. Spirit ended communications in March 2010 as its energy declined after losing the use of two of its six wheels, which prevented it from being able to gain a sun-facing tilt for its fourth Martian winter.