When considering how to power vehicles and equipment in the regions of space there have only really ever been two serious schools of thought; solar or nuclear. Solar power has been the preferred option for a long time, but the power outage is not constant and lithium-ion batteries are required to store the energy so that power remains during non-daylight hours. Also, due to the limitations on size and weight, the solar panels are not the same as those used on earth. Solar panels on earth are made from silicon and are relatively easy and cheap to produce, unlike the gallium-arsenide panels used on the Mars rovers Spirit and Opportunity, which have very high production costs. Solar panels are still, and I assume will continue to be into the future, the preferred option for providing energy to satellites and equipment which remains in space, but for vehicles intended to land on and explore alien planets nuclear power provides more options.
Dust storms on Mars have been known to blot out up to 90% of the sun (a serious problem for objects depending on solar based energy), and the residing dust which remains on the solar panels can seriously reduce their efficiency. Due to the unreliability of the power supply Mars rovers in the past have been small to keep the energy requirements low. NASA’s Spirit and Opportunity were only 400 pounds each, and ran on a similar power level to a desk top computer. The new rover, Curiosity, is powered by a nuclear power pack, delivering a consistent power supply. This consistency allows Curiosity to be much larger, it weighs 2,000 pounds, and also carry more advanced and heavy duty science equipment, such as a laser for breaking up rocks in order to analyse their constitution. “This is a much larger vehicle, the size of a small car,’’ said Stephen G. Johnson, the director of the space nuclear systems and technologies division at the Idaho National Laboratory, which prepared the nuclear power pack for the new probe.
The idea of a nuclear power pack heading off into space to an untouched planet has naturally riled many environmentalist and anti-nuclear parties. However the power pack is not the same as a nuclear reactor here on earth. Energy is not produced by splitting the atoms of a highly radioactive isotope. Instead it runs off Plutonium-238, an artificially manufactured isotope with a half life of just 88 years. The radioactive decay is so fast that it glows red-hot and this heat is then converted directly into electricity. Dangers are minimal because the radiation released is in the form of alpha particles which can be easily confined, and Plutionium-238 cannot be used to create bombs.
The major problem that NASA faces when pursuing this form of technology is that, as Dr. John M. Logsdon, a space expert at George Washington University, said “It’s really only possible with plutonium-238 to do what it’s intending to do,” and the United States stopped making Plutonium-238 in the 1980’s. Since then they have bought it from Russia, but now they no longer make it either. A 2009 report by the National Academy of Sciences called for restarting production, but this has not been done, mostly for cost reasons.
A proposed alternative to the Plutonium reliant nuclear power pack is a Stirling Engine system which could produce five times as much electricity from each unit of heat (NASA’s current method is only 6% efficient). Although the obstacle still to overcome in the development of this technology is that it has many moving parts and has not yet been adapted to space use.
Therefore, solar cells have always been used where possible. Steven W. Squyres, a professor of astronomy at Cornell who is the chief scientist behind the Opportunity and Spirit rovers, said: “You always use solar when you can; it’s simpler, cheaper, just easier to do. You only use nuclear when you have to.’’ This thought was obviously prevalent when NASA launched their Jupiter-bound, Juno space shuttle, as that too relies upon solar cells; even though Jupiter is five times as far from the sun as Earth, and therefore the sun’s intensity is 96% lower.
NASA needs to urgently develop a more efficient method of powering their rovers. At the moment they are simply abiding to the simple, cheap solar solution, but at the expense of the explorative abilities of the vehicles. The adage “If you are going to do a job, you might as well do it properly” comes to mind. If NASA are going to spend billions of dollars sending probes to Jupiter and Mars, they should send the best probes possible, not settle for the cheap, reliable option.
By. James Burgess of Oilprice.com