This full-resolution image taken by the panoramic camera onboard the Mars Exploration Rover Spirit before it rolled off the lander shows the rocky surface of Mars. Scientists are eager to begin examining the rocks because, unlike soil, these "little time capsules" hold memories of the ancient processes that formed them. The lander's deflated airbags can be seen in the foreground. Data from the camera's red, green and blue filters were combined to create this approximate true color picture. Image credit: NASA/JPL/Cornell A little bit of plutonium from the Laboratory is keeping NASA's Mars rovers warm and ready to rove despite the frigid Martian temperatures.
In fact, the Spirit and Opportunity rovers can stay warm and keep collecting data for nearly five times longer, thanks to about an ounce and a half of Los Alamos plutonium-238.
Los Alamos' Pu-238 Science and Engineering (NMT-9) Group made eight lightweight radioisotope heater units each for the Spirit and Opportunity rovers. Each of the 16 units contains just under one-tenth of an ounce of plutonium, and each pumps out a continuous one watt of heat as the plutonium decays.
Housed inside the rover fuselages, called Warm Electronic boxes because they provide a temperature-controlled environment, the heater units keep electronic and mechanical components warm enough to function reliably in the bitter cold of space. They transfer heat directly to the rover systems and instruments, without moving parts or electronic components.
The heater units are the latest in a long line of plutonium heaters and thermal batteries fabricated at Los Alamos for all of NASA's deep space probes, as well as for the Sojourner rover, which explored the red planet for three months as part of NASA's Pathfinder mission in the summer of 1997. The heat comes from plutonium-238, the shorter-lived and much hotter cousin of weapons-grade plutonium, or plutonium-239.
Temperatures on the Martian surface at the rover landing sites can vary from about 70 degrees Fahrenheit in the daytime to 146 degrees F below zero at night. Los Alamos designed the heater units to keep the rovers between 40 below and 40 above; temperatures inside the Warm Electronics boxes have remained higher than a toasty four below zero.
"The constant heat provided by the lightweight radioisotope heater units will allow both rovers to gather data on the surface of Mars for at least 90 days," said Liz Foltyn of NMT-9. "Without that supplemental heat, the mission could last only 20 Mars days."
Heating each rover's components are small electrical heaters, excess heat from the electronics and the eight Los Alamos heater units. At night, with solar panels shut down, rover heaters rely solely on rechargeable batteries for power. The constant heat from the plutonium units greatly extends battery life, because the electrical heaters don't need nearly as much battery power.
Each cylindrical heat source consists of a hot-pressed pellet of plutonium oxide, a platinum-rhodium vented capsule, a pyrolytic graphite insulator and a tightly woven, pierced fabric graphite aeroshell assembly that protects the fuel from impact, fire or atmospheric re-entry. The units are roughly one inch in diameter and one and one-quarter inches long. The Warm Electronics Box is double-walled with panels of alloy honeycomb and epoxy graphite laminate. Between the walls is an insulating foam called aerogel.
"Some of these materials wouldn't be out of place on a Formula One racecar," Foltyn said. "And the goal is similar: keeping temperatures within safe ranges in extreme conditions."
Radioisotope heater units made at Los Alamos maintain operating temperatures for instruments aboard the Galileo space probe and on the Cassini spacecraft and Huygens probe. Coupled with static electrical converter systems in a variety of radioisotope thermoelectric generators, plutonium-238 heat sources have helped provide electrical power for numerous other successful space instruments for more than three decades, including Apollo lunar surface scientific packages, several satellites and the Pioneer, Viking, Voyager, Galileo and Cassini space probes.
The heater units on the surface of Mars originally were fabricated at Technical Area 55 for NASA's Cassini mission, which is scheduled to arrive at Saturn in July. Support for NMT-9 salaries and operations comes from DOE's Office Of Space and Defense Power Systems, while NASA paid for fabrication of the heater units.
Details about the project are available in a 1996 technical report by Gary Rinehart, "Lightweight Radioisotope Heater Unit (LWRHU) Production for the Cassini Mission," LA-13143-MS, available at http://lib-www.lanl.gov/cgi-bin/getfile?00318474.pdf online. (Adobe Acrobat Reader required)
More information about the Spirit and Opportunity rovers is available at the NASA-Jet Propulsion Laboratory Web site at http://marsrovers.jpl.nasa.gov/mission/spacecraft_surface_rover.html online.