In space, there is no place for a spacecraft to plug in a power cord. Not even with an adapter or wireless source of electricity.
Traveling millions of miles from Earth, spacecraft must rely on the sources of power they carry onboard.
NASA's Mars Exploration Rovers are no exception.
Both rovers, dubbed Spirit and Opportunity, are dependent on solar panels for power during daytime operations and advanced lithium ion rechargeable batteries for operations during the night. These batteries are advanced versions of those used in laptops, camcorders and cell phones. NASA researchers specifically designed and developed these power sources to operate efficiently at temperatures as low as -20 degrees C (-4 degrees F).
On any given day, NASA's Mars rovers each require about 100 watts of steady power to operate -- the same amount that illuminates a standard living room light bulb. This is in contrast to the Sojourner rover, whose solar arrays provided only 16 watts of power, about the same as an oven light. Sojourner used non-rechargeable lithium-thioinyl chloride batteries that eventually ran dry, leaving only the solar panels to fill its power needs. So NASA developed a new generation of battery that would add extra power when needed and store unused power.
The advanced lithium ion batteries will allow Spirit and Opportunity to perform on-site scientific investigations during the course of their 90-day missions and trek up to 40 meters (131 feet) per day.
Building a Better Battery
MER rechargeable batteries are based on lithium-ion chemistry, and are considerably different than that of the more traditional nickel-cadmium or nickel-hydrogen batteries that are used in other space missions.
Beginning in 1994, researchers at the NASA's Jet Propulsion Laboratory (JPL) began to develop low temperature rechargeable lithium ion batteries for space applications that could survive in the low, harsh, cold temperatures of space. The batteries needed to be able to handle temperatures as low as -30 degrees C.
All batteries contain two electrodes -- one positive and one negative. In the new lithium-ion batteries, the positive electrode (called a cathode) contains high-voltage lithium metal oxide and the negative electrode (called an anode) contains lithium intercalation carbon, which prevents volatile reactions by the highly reactive lithium metal. The batteries also contain an advanced organic electrolyte. Developed by JPL, the electrolyte was specifically designed to handle low temperatures, and is composed of lithium salt dissolved in a mixture of organic solvents.
In 1998, Air Force Research Laboratory (AFRL), JPL and NASA's Glenn Research Center (GRC) formed a consortium to develop lithium ion cells for aerospace applications. Under this collaborative program, Lithion (a division of Yardney Technical Products) developed cells and qualified them for both aircraft and space applications.
In 2002, Lithion fabricated the rover batteries using the battery housings designed by JPL for rover applications. Each rover has two batteries, and they each have an energy content of about 300 WH and weigh about 8 pounds (3.1 kilograms). These lithium ion batteries offer 3-4 times mass and volume savings compared to the traditional Ni-Cd and Ni-H 2 batteries.
The Long, Cold Night
Solar panels can meet rover power needs during the day, but what happens when the sun goes down on Mars?
The sole source of power on a rover becomes its rechargeable battery, which allows the systems to survive the cold Martian night, and extreme temperatures of space. The Li-Ion batteries are also used during the day when a little extra juice is needed to add extra power to solar arrays when the rover is transmitting data from the Mars rover to Mars Odyssey orbiter.
To get the most out of the technology, the batteries are stored in warm "boxes," which contain heaters to maintain the temperature of the batteries in the extremely low temperatures of space and on Mars. The solar arrays on top of the box attract sunlight as a source of energy and generate up to 140 watts of power. This stored energy will power the rover as it explores the planet's surface.
The Mars Exploration Rover mission is the first major NASA planetary exploration mission to use the advanced lightweight rechargeable lithium-ion batteries, which are three to four times lighter than their nickel counterparts. In addition, the battery can last five times as long as the planned 90-day primary mission.
Because of the tremendous success of these batteries on the Mars Exploration Rovers, NASA plans to continue to develop these advanced lithium ion batteries for future space missions with more challenging environments, such as Venus (460 degrees C/860 degrees F) or the Neptune atmosphere (-170 degrees C/-274 degrees F), or to the icy moons of Jupiter.
Written by Samantha Harvey
Mars Exploration Rover Homepage
Jet Propulsion Laboratory
NASA Mars Exploration Program
Last Updated: 23 September 2013