GRAHAM WARWICK / WASHINGTON DC
New leadership has revitalised US efforts to develop nuclear-powered spacecraft and give fresh impetus to solar system missions. But will the public buy the idea?
To ancient Greeks, Prometheus was the titan who gave the gift of fire to humans. For NASA, Prometheus is the key to exploration of the solar system. It remains to be seen whether the US space agency's gift will be as well received by mankind.
Project Prometheus is the new name for NASA's Nuclear Systems Initiative, launched last year but just getting under way because of delays in approving the agency's 2003 budget. Whereas the Nuclear Systems Initiative was a $1 billion programme to develop nuclear power and propulsion technology, Prometheus is a $3 billion venture that includes a nuclear-powered mission to explore the moons of Jupiter.
Since becoming NASA administrator at the end of 2001, Sean O'Keefe has frequently railed at the limits conventional spacecraft impose on solar system exploration. Chemical propulsion restricts the size of space probes, which then require long, complex gravity-assist trajectories to reach their distant targets and, once there, can only perform a brief fly-by.
He uses the example of a conventionally powered mission to Pluto which, if started today, could be launched by 2007, would arrive at the outermost planet around 2017 and would provide at most a few weeks of fly-by observations. "I certainly hope there are still folks around in the next 16-20 years who care about the results," O'Keefe says.
An onboard nuclear reactor powering an efficient electrical propulsion system would allow the spacecraft to take a more direct route to a target such as Pluto, where it would enter orbit. At that distance from the sun, solar power is not viable, so the fission reactor would also power the spacecraft's payload, allowing a large array of power-hungry experiments, such as radar, to be carried out.
"Our goal is to enhance the ability of our robotic spacecraft to perform complex scientific investigation of the planets, and at some point in the future to enable human explorers to live off the land of planetary bodies," says O'Keefe. "Nuclear propulsion will enable exploration conditions that are inconceivable with current, conventional chemical propulsion."
Project Prometheus has two basic elements: resurrection of US production of radioisotope power systems for spacecraft; and the development of space fission reactor power systems. The proposed Jupiter Icy Moons Orbiter (JIMO) mission, which could be launched by 2011, would be the first to use nuclear power and propulsion.
Not nuclear
Radioisotope thermoelectric generators (RTGs) have been used on 25 USspace missions since 1991, including Apollo, Pioneer, Viking and Voyager. The last mission to be launched, in 1997, was the Cassini probe, which is to begin orbiting Saturn in July. Only two RTGs remain in the US inventory, spares for the Cassini probe, and they are earmarked for use on the planned Pluto-Kuiper Belt mission.
RTGs are not nuclear reactors. Instead heat generated by the decay of plutonium-238 dioxide is converted into electricity by reliable, but inefficient solid-state thermocouples. RTGs come in one size only, generating 300W electricity from three general-purpose heat source (GPHS) modules. The Cassini orbiter carries three RTGs - almost 6kg (13lb) of plutonium - to provide the power required.
Under Prometheus, the US Department of Energy (DoE) is developing an improved multi-mission radioisotope thermoelectric generator (MMRTG). Designed to operate on a planet's surface, as well as in the vacuum of space, the MMRTG is based on the silicon germanium thermoelectrics used in later RTGs, in a more flexible modular design suitable for a wider range of missions. The unit will generate at least 100W electricity from eight GPHS modules, or 4kg of plutonium.
The DoE has also begun development of the more-efficient Stirling radioisotope generator (SRG). Where the thermoelectrics used in the RTG and MMRTG are about 5% efficient, Stirling-cycle power conversion promises to be 20-25% efficient, allowing more electricity to be generated from less plutonium, which the USA has to purchase from Russia.
The Stirling converter is a free-piston machine that turns heat from a GPHS module into reciprocating motion, with a linear alternator generating around 60W of AC electric power that is converted into about 55W of DC electricity to power the spacecraft. The Lockheed Martin developed-SRG will use two heat sources (1kg of plutonium) and two converters to produce 110-120Wof electric power.
Alan Newhouse, Prometheus programme manager, says NASA plans to choose either the MMRTG or the SRG to power the Mars Smart Lander, planned for launch in 2009. Radioisotope power will allow this mobile surface laboratory to operate for more than 1,000 days, compared with 180 days for a solar-powered vehicle.
The second, and more ambitious, element of Prometheus is the development of space fission power systems. Nuclear reactors can generate 100kW or more of electricity, far more than RTGs and enough to power both the spacecraft's propulsion system and science instruments. This will allow heavy payloads to be carried to distances from the sun where solar power is no longer viable.
The USA has only ever launched one nuclear reactor into space, the SNAP-10A, in 1965. The 45kW liquid-cooled thermal nuclear reactor used thermoelectrics to generate 650W of electrical power, and operated in orbit for 43 days before it was shut down by a spacecraft fault. SNAP-10A is still in orbit. Joint NASA, DoE and Department of Defense development of the 2MW SP-100 was cancelled in the early 1990s without the advanced liquid-metal reactor ever having flown, and after almost $500 million had been spent.
Russia, meanwhile, has flown 32 fission reactors on radar ocean reconnaissance satellites. Thirty used thermoelectric converters generating 3kW and two used more advanced Topaz reactors with thermionic conversion, producing about 5kW of electricity. Two more-powerful Topaz reactors were purchased by the USA in 1993 for a nuclear electric propulsion programme that was cancelled.
After studying other potential applications, NASA has elected to build its nuclear power and propulsion programme around a mission to orbit three moons of Jupiter - Callisto, Europa and Ganymede - to search for evidence of vast saltwater oceans beneath their icy surfaces.
Power to spare
Without nuclear power and propulsion, the mission would require two conventional spacecraft, says Newhouse. Each would require 1kW of power, demanding smaller, more powerful RTGs "that we've not developed yet", he says. Even then, the two craft would not provide the same science output as a single orbiter with a nuclear reactor producing 100kW of electric power. "That's enough electricity to power a ground-penetrating radar, a laser ablator, or any other instrument they can think of," says Newhouse. Ample power will also allow precise navigation, onboard processing and megabit per second data communication rates, he says.
NASA plans to award 10-month contracts to Boeing, Lockheed Martin and Northrop Grumman to develop mission proposals, with one team to be selected a year from now to begin developing the orbiter. The JIMO will be the largest space probe ever built, weighing in at 20,000kg and measuring 40m (130ft) long when its mast is fully extended and radiators fully deployed.
The mission is ambitious. "We have three long poles in the tent: the nuclear reactor, power conversion, and the ion engines - four if you count the radiation-hard electronics needed to survive the environment around Jupiter," says Newhouse. NASA's Galileo probe has suffered several radiation-induced failures during its brief "swoop-bys" of Jupiter and its moons, and the JIMO is expected to be in orbit for several years.
Each contractor will propose an integrated reactor, power conversion and ion engine solution. Reactor options include liquid-metal cooling, like the SP-100, as well as gas and heatpipe cooled. Power conversion options include Brayton-cycle mini-turbines, as well as Stirling converters, thermoelectrics and thermionics.
Propulsion options include ion, Hall-effect and magnetoplasma dynamic engines, but all need to be scaled up dramatically from the low-power thrusters available today. NASA has baselined the JIMO with 32 10kW ion engines, but wants to use fewer 20-25kW power units. By comparison, the Deep Space 1 spacecraft's pioneering ion engine produced 3kW.
"There is a lot of work to be done," says Newhouse, and a long way to go before nuclear electric propulsion is ready for manned spaceflight. NASA sees the $3 billion Project Prometheus as a "downpayment" on a long-term nuclear propulsion programme, but first it must overcome the US public's concerns about the safety of nuclear spacecraft.
In more than 30 years of launches, RTGs have never been the cause of a spacecraft accident, but the generators have been on three missions that failed for other reasons. In all cases, NASA says, the RTGs performed as designed, containing their plutonium.
Executive decisions
Before any nuclear-powered spacecraft is launched, says Newhouse, there will be an extensive independent safety review and the launch will require presidential approval. The reactor, which would be packaged in a survivable aeroshell, would not be activated until the craft is in a high, safe orbit, and solar-electric power may be used initially. One safety benefit, Newhouse says, is that the more direct powered trajectory possible with nuclear electric propulsion avoids the need for Earth flybys, which have been used on several RTG-equipped missions. This eliminates the risk of the spacecraft colliding with the Earth, and could ensure that Prometheus's gift never comes home to roost.
Source: Flight International
