By Dr David Whitehouse BBC News Online science editor

...a tremendously positive step that will greatly enhance the prospects for human exploration and settlement of the Solar System Robert Zubrin, Mars Society

He is expected to back the US space agency's recent nuclear propulsion initiative, Project Prometheus, either in his State of the Union speech, due on 28 January, or later this year when he submits his 2004 budget to Congress.

It is believed he will give the initiative $1bn over five years, arguing that nuclear propulsion represents an essential technology for the manned and unmanned exploration of space.

Supporters say nuclear power could change the nature of space exploration, but add that it will take many years and significant resources to develop.

'New vistas'

Jim Garvin, Nasa's lead scientist for Mars Exploration, told BBC News Online that the space agency was very committed to "pursuing a vision in which access to unexplored territories in the Universe is possible" and that included "technology to open up new vistas and approaches".

Considerable investment would be needed

Central to this new approach is the agency's Nuclear Systems Initiative (NSI) which was launched last year following comments made by Nasa's chief, Sean O'Keefe, that nuclear propulsion was the only way to explore deep space and send astronauts to Mars.

O'Keefe is reported to have said to the Los Angeles Times: "We're talking about doing something on a very aggressive schedule to not only develop the capabilities for nuclear propulsion and power generation but to have a mission using the new technology within this decade."

However, Nasa public relations officials have played down this statement.

Nuclear propulsion is theoretically capable of achieving much faster speeds than conventional rockets using far less fuel.

Stolen fire

The technology was studied in the 1950s and 1960s in initiatives such as Project Orion, but it was subsequently neglected, partly for political (1963 nuclear test-ban treaty) and financial reasons.

However, a space advocacy group, the Planetary Society, has said: "In the long run, nuclear power and propulsion will likely be needed for missions to carry humans to Mars and back."

Using current rocket technology, it would take at least six months to cruise to the Red Planet.

Project Prometheus - named after the Greek god who stole fire and gave it to man - could cut this travel time to two months.

Advocates say that the nuclear option would make a manned Mars mission much easier, as it would reduce the need to carry so much food, fuel and oxygen, as well as relying on yet to be perfected recycling technology.

Distant moons

Nuclear power would also mean that Martian launch windows would be longer, allowing a more flexible choice of launch and return times, leading to a manned landing mission that could last as little as three or four months, as opposed to about three years.

Plutonium power: The Cassini-Huygens mission is headed to Saturn and its moon Titan

Robert Zubrin of the Mars Society and author of The Case For Mars told BBC News Online: "The decision by Nasa to revive its nuclear rocket development programme is a tremendously positive step that will greatly enhance the prospects for human exploration and settlement of the Solar System.

"Nuclear power reactors are essential for Mars base surface power, where they provide the power for reliable life support, ultra-high data-rate communications, and the in-situ production of ascent and return propellants, thereby increasing mission science return and cutting launch costs even more."

Nuclear power could also revolutionise the unmanned exploration of the outer Solar System.

Expected opposition

Current rocket technology does not allow a spaceprobe to reach distant Pluto and go into orbit around it - only a flyby is possible.

Chemical rockets also make it very difficult to get a probe to orbit such significant bodies as Jupiter's ice-crusted moon, Europa, a possible abode of life, or Titan, Saturn's major moon.

But it is inevitable that the development of the new propulsion systems will spark controversy from anti-nuclear groups.

There was major opposition to the launching of Nasa's Cassini probe to Saturn in 1997. The spacecraft uses plutonium to generate electricity for its onboard instruments.

Protestors feared a launch failure or an accidental re-entry when the probe swung by Earth could have led to widespread contamination.

Radioisotope generators have been used on a number of deep Solar System spacecraft.