[ RadSafe ] Nuclear-reactor spacecraft poor for astronomy

[Herren, Roy WS.]

The following article is from 


http://www.newscientistspace.com/article.ns?id=dn7928


Nuclear-reactor spacecraft poor for astronomy 


*	13:14 31 August 2005 
*	NewScientist.com news service 
*	Maggie McKee 

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's 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 "nuclear electric propulsion" 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.

But the NRC report finds that the reactors would be virtually useless
for - and could even hamper - observations of astrophysical phenomena
beyond our solar system.

"Reactors are messy things," says NRC panel member Gary Bernstein, an
astronomer at the University of Pennsylvania in Philadelphia, US. "They
generate huge numbers of radiation particles and gamma rays." 

He says these by-products of fission could effectively "blind" space
telescopes such as Hubble, Spitzer, and Swift if the reactors operated
near the Earth, as they did in the past. "We didn't see a benefit of
this technology for any kind of pure science that peers outside the
solar system or does fundamental physics tests," he says. 


Hot hydrogen


Nor did the panel find that NASA's 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 "adequate for either application". 

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 - without repairs - 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 - in a huge spacecraft called the Jupiter Icy
Moons Orbiter - was too ambitious. It is now considering testing the
reactor around Earth's Moon instead.


Trunk full of batteries


"There's an awful lot of technological development that's going to take
a very long time," says Louise Prockter, a member of the NRC's solar
system panel at Johns Hopkins University in Laurel, Maryland, US.

"There's still a lot we can do with the technology we have," she adds,
referring to the radioisotope thermoelectric generators (RTGs) that
"passively" 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 - the equivalent of using a "trunk
full" of flashlight batteries to start a car. "It's much easier at that
point to graduate from radioisotopes to fission reactors," he says.


Death knell


However one scientist, who wished to remain anonymous, told New
Scientist that public concern over the safety of nuclear reactors
resulted in a Catch-22 situation: "You're 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's a death knell for their programme."

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's 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's 2006 budget
request, the money was scheduled to come from "exploration systems" -
and not the science budget. 

But Bernstein says he is worried about the effect of the cost on NASA's
other missions. "If you're going to make this a priority, then what gets
deprioritised?" he asked New Scientist. "It's not free."