Nuclear power would allow missions to orbit - not merely fly by

The US space agency (Nasa) is progressing with ambitious plans to explore the Solar System using nuclear power.

Their hope, eventually, is to use electricity generated by nuclear power to propel a space probe and power its instruments on a voyage to the icy moons of Jupiter, satellites that just possibly might harbour life beneath their ice.

Before then, nuclear technology could be proved with a less ambitious mission, perhaps a nuclear-powered probe to the Moon.

As long ago as 1907, just two years after Einstein discovered his famous equation E=mc² which hinted at the vast power locked within the atom, Robert Goddard, who was himself to go on to pioneer chemical rockets, wrote: "The navigation of interplanetary space depends for its solution on the problem of atomic disintegration."

Once the power of the atom bomb had been demonstrated and the Cold War set in in the 1950s, all sorts of amazing proposals were developed for nuclear power in space.

Among them was project Orion, a plan to launch and propel spacecraft weighing thousands of tonnes and carrying dozens of passengers by detonating nuclear bombs behind a pusher plate.

Weak solar

Orion was cancelled when a nuclear test ban treaty came into force but another project, Nerva, to use a nuclear reactor to produce a rocket jet was the front runner for a possible human mission to Mars after the Apollo moon landings.

That, too, was scrapped and it was left to the Russians to launch several nuclear reactors into space to power spy satellites during the 1970s.

Project Orion envisaged "pulsed" nuclear propulsion

Dreams of nuclear power in space did not die with the collapse of the Soviet Union and restrictions on the Nasa budget.

Many space scientists agree that nuclear power is the only viable way of exploring the outer Solar System.

Using chemical rockets to move between planets and their moons is not really practical because of the fuel mass a spacecraft would have to carry with it, and relying on solar power to drive instruments is problematic because of the distance from the Sun.

Travel out to Mars and there's only a 10th as much solar energy as reaches the Earth.

At Saturn, it is a hundredth of the power we are familiar with. Arrays of solar cells to produce really useful power there would be impossibly big.

So far, probes to the outer planets, such as the Cassini craft now orbiting the ringed planet, have used Radioisotope Thermal Generators (RTGs) - solid state electrical generators powered by the heat of radioactive decay. But that power is limited.

The RTGs on Cassini would not produce enough to run a hair dryer (three units produce about 700 watts).

Scientists would love more electrical power for their instruments - but there is another use for which nuclear electric power could make all the difference: the ion engine.

'Dustbin' size

By using electrical energy to ionise atoms and accelerate them in a jet, it is possible to propel them at more than 10 times the speed a chemical rocket jet can manage.

That means that you need less than a tenth of the propellant to travel a certain distance.

Artist's impression of the Jupiter Icy Moons Orbiter (JIMO). Courtesy NASA/JPL

Fifty years ago, US nuclear scientists were working on a top secret project to develop a three thousand-ton rocket that would take people to Mars propelled by atomic bombs.

Now there's Project Prometheus, a more modest but nevertheless controversial plan to use nuclear power for unmanned missions beyond the earth and eventually to Jupiter and beyond.

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A huge nuclear-powered space ship eases itself out of Earth orbit and heads off on a great voyage of discovery.

This is not a dream of science fiction, but the goal of a $3 billion project that aims to launch its first nuclear behemoth before the end of the decade.

If the spectacularly successful Galileo probe to Jupiter was the Pilgrim Fathers' "Mayflower", then, by comparison, Project Prometheus is the QE2.

One of the key missions to benefit from the technology would be JIMO - Jupiter Icy Moons Orbiter (pictured above), scheduled for launch around 2016.

Before that may come a technology demonstration mission to a nearer target such as the moon.

Nukes in Space

But is this a technological dream-come-true or will it turn into a nuclear nightmare? Critics fear that Project Prometheus is an attempt by the United States to introduce powerful weapons into outer space through the back door.

Proponents point out that it doesn't use atom bombs, but a small, safe nuclear reactor that generates power for an electric propulsion system. Such a space vehicle would escape the Earth's gravity on a conventional rocket before activating its nuclear reactor.

But after that it would have sustained power to drive it to Jupiter and flit from moon to moon with enough power left over for instruments such as radar that could penetrate their icy surfaces and look for liquid oceans beneath.

Leo travels to Albuquerque in New Mexico where nuclear rocket scientists from around the world are meeting to discuss their work. And to a test chamber in Hampshire, England, where the non-nuclear part of the technology - an ion engine - is under development.

But there are complex moral and technological issues raised by this new generation of nuclear-powered space probes. For some, who are passionate environmentalists, the trade-off between nuclear risk and scientific return poses a profound dilemma.

Mass Protests?

Scientists like Bruce Gagnon, coordinator of the "Global Network Against Weapons and Nuclear Power in Space" worry that not only could an accident at launch spread radioactive material across the surface of the earth but also that the reactors and nuclear systems that NASA claim will be used for exploration, could actually be used to power space weapon systems such as lasers and electromagnetic cannons.

He led the protest over the Cassini missions - the plutonium fuelled probe that has now arrived at Saturn and says the same mass protest could happen again.

However the proponents insist that we are on a threshold of a new capability in space, and the only safe and sustainable way to journey away from the sun and still have useful power is to use nuclear technology.