Solar Energy Provides Crucial Power for Mars Rovers

Despite the high failure rates of previous Mars missions, the first of two NASA rovers landed successfully on the red planet’s surface, ushering in a new stage of interplanetary exploration. From beaming back high-resolution pictures of the craft’s surroundings, to doing the complex work of a geologist in hopes of uncovering signs of life, solar photovoltaic (PV) power is essential to the rover’s functions.

Mars, the Solar System – January 7, 2004 [SolarAccess.com] The Mars Exploration Rover Spirit successfully sent a radio signal after the spacecraft had bounced and rolled for several minutes following its initial impact at 11:35 p.m. EST (8:35 p.m. Pacific Standard Time) on January 3. The second rover is expected to land on January 24 (PST). Members of the mission’s flight team at NASA’s Jet Propulsion Laboratory, Pasadena, California, cheered and clapped when they learned that NASA’s Deep Space Network had received a post-landing signal from Spirit. The cheering resumed about three hours later when the rover transmitted its first images to Earth, relaying them through NASA’s Mars Odyssey orbiter. “We’ve got many steps to go before this mission is over, but we’ve retired a lot of risk with this landing,” said NASA Jet Propulsion Laboratory’s (JPL) Pete Theisinger, project manager for the Mars Exploration Rover Project. On each Mars Exploration Rover, the core structure is made of composite honeycomb material insulated with a high-tech material called aerogel. The deck is populated with three antennas, a camera mast and a panel of solar cells. Additional solar panels are connected by hinges to the edges of the triangle. The solar panels fold up to fit inside the lander for the trip to Mars, and deploy to form a total area of 1.3 square meters (14 square feet) of three-layer photovoltaic cells (known in the solar industry as triple-junction). Each layer is of different materials: gallium indium phosphorus, gallium arsenide and germanium. The array can produce nearly 900 watt-hours of energy per martian day, or sol. However, by the end of the 90-sol mission, the energy generating capability is reduced to about 600 watt-hours per sol because of accumulating dust and the change in season. When fully illuminated, the rover solar arrays generate about 140 watts of power for up to four hours per sol (a Martian day). The rover needs about 100 watts (equivalent to a standard light bulb in a home) to drive. Comparatively, the Sojourner rover’s solar arrays provided the 1997 Pathfinder mission with around 16 watts of power at noon on Mars. That’s equivalent to the power used by an oven light. This extra power will potentially enable the rovers to conduct more science. 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 will degrade and will not be able to recharge to full power capacity. Also, by the end of the 90-sol mission, the capability of the solar arrays to generate power will likely be reduced to about 50 watts of power due to anticipated dust coverage on the solar arrays (as seen on Sojourner/Mars Pathfinder), as well as the change in season. Mars will drift farther from the sun as it continues on its yearly elliptical orbit, and because of the distance, the sun will not shine as brightly onto the solar arrays. Additionally, Mars is tilted on its axis just like Earth is, giving Mars 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 than in the beginning of the mission will mean less energy on the solar panels. Composite Optics (COI), San Diego, California built the lander structures and performed significant design and analysis for the project. In addition, COI manufactured the solar array substrates that provided power during the “cruise” stage to Mars and the solar array substrates mounted on the MER to provide power on the surface of Mars. With the help of PV power, the rovers will help to satisfy four major goals for NASA: to determine whether life ever arose on Mars, to characterize the climate of Mars, to characterize the Geology of Mars, and to prepare for Human exploration.