5.4 NUCLEAR Mx

news feature Breaking the nuclear taboo

Despite public fears, nuclear-powered spacecraft are back on the agenda. Tony Reichhardt looks into the latest plans for planetary exploration.

tudying the fringes of our Solar Sys- which accelerates the craft in the opposite tem can be a frustrating business. direction. The space shuttle’s main engines, SJupiter’s moon Europa, with its ice- for example, produce high-pressure gas by covered ocean, and Saturn’s cloud-shrouded combining liquid oxygen and hydrogen. A Titan are exciting places. The problem is nuclear thermal rocket would use the heat getting there. Not only are the outer planets generated by a small nuclear reactor to turn far away, but extra fuel is needed to brake a liquid hydrogen into high-pressure hydro- spacecraft into orbit around a tiny moon gen gas. Nuclear rockets have the edge circling a massive planet. The exorbitant because, in weight terms, they use less fuel to costs of launching spacecraft weighed down produce comparable thrust. with sufficient fuel rule out most missions. Billions of dollars have been spent on Delegates at a NASA meeting on new nuclear propulsion research since the 1960s, approaches to exploring the outer Solar but an engine has yet to be tested in space. System, held in February at the Lunar and Interest flared up in the early 1990s, when Planetary Institute in Houston, kept coming NASA was asked to consider sending astro- back to an old but controversial solution: nauts to Mars. But NASA’s proposals were nuclear-powered rockets.Giovanni Bignami, too expensive, and when the programme scientific director of the Italian space agency stalled,so did interest in nuclear rockets. Blasted: the small amounts of plutonium used in ASI, puts it bluntly:“You can’t seriously go to Now, for the first time in years, propul- Cassini’s Saturn mission provoked protests. the planets unless you use nuclear propulsion experts find themselves with modest sion. All the rest is junk.” But funding agen- funding to study engines powered by nuclear Ground tests are one thing, but building cies, sensitive to public opinion, will need fission. Researchers at NASA’s Glenn an engine and flying it in space is another. So convincing. Protests against the small Research Center in Cleveland, Ohio, believe will NASA make a serious go of nuclear amounts of onboard plutonium at the launch a 500-kilogram nuclear-powered spacecraft propulsion this time? US missions to the of NASA’s Galileo mission to Jupiter in 1989 could be flying past Pluto in as little as six and outer planets are currently in a confused LIBRARY PHOTO GENESIS SPACE and the Cassini Saturn mission in 1997 have a half years after its launch — around four state. NASA wants to launch a Pluto mission turned ‘nuclear’into a forbidden word. years faster than conventional rockets could in the next few years — proposals for the pro- Rockets produce thrust by forcing a high- manage. Glenn’s veteran nuclear rocket ject are due in this week and one group is pressure gas to escape through a nozzle, designer Stan Borowski puts costs for the known to be submitting a nuclear propul- ten-year development of a nuclear thermal sion option. But the new US administration engine at $1.5 billion. is keen for NASA to prioritize work on In Italy, Bignami’s agency is funding pre- propulsion technologies.NASA science chief NASA liminary work on a concept developed by Ed Weiler has said the propulsion research Nobel prizewinning physicist Carlo Rubbia, will “look at what’s really needed” to make now at the University of Pavia. Rubbia’s pro- the Pluto mission successful, including posal would use americium-242 rather than nuclear thermal rockets and NEP. uranium for fuel. Conventional nuclear Looming over discussions at last month’s thermal engines generate heat from fission, meeting was the question of how the public and then transfer the heat to a separate con- will react. Borowski’s group at Glenn is tainer of hydrogen. Rubbia proposes placing already preparing a public information cam- the americium in direct contact with the paign.Others hope that President George W. hydrogen, making the heat transfer more Bush will provide the backing that his prede- efficient (Nature 397, 374; 1999). Work on cessor,Bill Clinton,declined to give. ‘Project 242’is currently focused on bonding All of which creates a cautious sense that americium to a substrate. perhaps, finally, nuclear propulsion’s time Another possibility is nuclear electric has come. For some, it clearly feels like now propulsion (NEP), in which electricity gen- or never. As William Jeffrey of the US erated by a reactor is used to expel a stream of Defense Advanced Research Projects Agency ions from the back of the rocket.The thrust is said at the NASA meeting: “If we can’t make small, but even a small thrust will eventually this a compelling argument now, we proba- accelerate a spacecraft to high speeds in the bly can’t in our lifetime.” I near vacuum of space. Ground tests of the Tony Reichhardt is a contributing correspondent for Nature. Safe Affordable Fission Engine (SAFE, in Project 242 case anyone misses the point), a NEP engine ± http://ars.rm.asi.it/~webars/Project242/P242_main.html Out of reach? The weight of fuel needed makes it developed at NASA’s Marshall Space Flight Safe Affordable Fission Engine expensive to put a craft in Europa’s orbit. Center in Alabama,are already under way. ± http://stday.msfc.nasa.gov/presentations/IST3.pdf