Spacecraft powered by nuclear fission reactors are of limited use to astronomers, the US National Research Council panel has concluded. The report calls into question NASA鈥檚 multi-billion-dollar Prometheus project, which aims to develop such spacecraft for future missions to the Moon, Mars, and the outer solar system.
Early reactor technology was used in space once by the US in 1965 and a couple of dozen times by the Soviet Union from 1967 to 1988. Now, NASA hopes to improve on the technology, which releases heat by splitting uranium.
The 鈥渘uclear electric propulsion鈥� NASA is focusing on could provide up to a million watts of electricity to power instruments and propel spacecraft using a stream of ions. This could support many more scientific instruments, beam back more data, and allow spacecraft to visit more targets than current technologies.
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But the NRC report finds that the reactors would be virtually useless for 鈥� and could even hamper 鈥� observations of astrophysical phenomena beyond our solar system.
鈥淩eactors are messy things,鈥� says NRC panel member Gary Bernstein, an astronomer at the University of Pennsylvania in Philadelphia, US. 鈥淭hey generate huge numbers of radiation particles and gamma rays.鈥�
He says these by-products of fission could effectively 鈥渂lind鈥� space telescopes such as Hubble, Spitzer, and Swift if the reactors operated near the Earth, as they did in the past. 鈥淲e didn鈥檛 see a benefit of this technology for any kind of pure science that peers outside the solar system or does fundamental physics tests,鈥� he says.
Hot hydrogen
Nor did the panel find that NASA鈥檚 nuclear programme would support its planned human missions. The NRC acknowledged that fission reactors would be useful for both space travel and long-term human bases on the Moon or Mars. But it said it is not clear whether the nuclear electric propulsion NASA is pursuing is 鈥渁dequate for either application鈥�.
Another reactor technology that uses fission to heat hydrogen so it can be forcefully expelled to provide rocket thrust might get astronauts to Mars more quickly, the panel writes.
The NRC did identify several robotic missions where nuclear electric propulsion could be beneficial, including one to send probes and landers to Neptune and its largest moon, Triton.
But the NRC cautioned that significant hurdles remain for the technology to actually be practical. These include the ability to operate continuously 鈥� without repairs 鈥� for the decade or so that it would take to reach Neptune. In fact, earlier in 2005, NASA admitted that its first plan to use the technology 鈥� in a huge spacecraft called the Jupiter Icy Moons Orbiter 鈥� was too ambitious. It is now considering testing the reactor around Earth鈥檚 Moon instead.
Trunk full of batteries
鈥淭here鈥檚 an awful lot of technological development that鈥檚 going to take a very long time,鈥� says Louise Prockter, a member of the NRC鈥檚 solar system panel at Johns Hopkins University in Laurel, Maryland, US.
鈥淭here鈥檚 still a lot we can do with the technology we have,鈥� she adds, referring to the radioisotope thermoelectric generators (RTGs) that 鈥減assively鈥� produce electricity using heat released by the radioactive decay of plutonium-238. RTGs have been used to power dozens of space science missions, from the Voyager probes now at the edge of the solar system to the Cassini spacecraft around Saturn.
But Anthony Hyder, a physicist at the University of Notre Dame University in Indiana, US, points out that RTGs can only generate a few hundred watts of electricity. He says any missions requiring more than that would have to use many more RTGs 鈥� the equivalent of using a 鈥渢runk full鈥� of flashlight batteries to start a car. 鈥淚t鈥檚 much easier at that point to graduate from radioisotopes to fission reactors,鈥� he says.
Death knell
However one scientist, who wished to remain anonymous, told New 杏吧原创 that public concern over the safety of nuclear reactors resulted in a Catch-22 situation: 鈥淵ou鈥檙e not going to develop it until someone says they need it and no one is going to say they need it because they know it鈥檚 a death knell for their programme.鈥�
Space researchers generally believe spacecraft reactors can be used safely, for example by launching the reactor in pieces before assembling and starting it in space.
While reactors would definitely boost a mission鈥檚 power level, the technology does come at a heavy financial cost. NASA projects Prometheus will cost $3 billion between now and 2010. In the agency鈥檚 2006 budget request, the money was scheduled to come from 鈥渆xploration systems鈥� 鈥� and not the science budget.
But Bernstein says he is worried about the effect of the cost on NASA鈥檚 other missions. 鈥淚f you鈥檙e going to make this a priority, then what gets deprioritised?鈥� he asked New 杏吧原创. 鈥淚t鈥檚 not free.鈥�
