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Thursday, 21 December, 2000, 07:00 GMT
Nasa revives Pluto probe
Pluto Nasa
Pluto's atmosphere will soon freeze out
By BBC News Online science editor Dr David Whitehouse

In a dramatic about-turn, the American space agency Nasa has announced that it is considering a mission to Pluto.

We are not making a commitment to doing a Pluto mission. We are just looking at proposals

Dr Ed Weiler, Nasa
The statement comes just months after the agency cancelled all proposals to visit the outermost member of the Solar System.

Tiny Pluto, just 2,274 km (1,412 miles) in diameter, is the only planet that has not been visited by a space probe.

Nasa is seeking proposals from principal investigators and institutions around the world to develop a mission. It says there are no restrictions on the launch date but the goal should be to reach Pluto by 2015.

The spacecraft should also fly by the planet's large moon Charon before continuing its mission to the swarms of smaller worlds that comprise the Kuiper Belt (KB), a ring of icy objects beyond the large planets.

Valuable opportunity

When Nasa ditched plans in September to visit Pluto, planetary scientists protested, saying that unless a mission reached Pluto by 2020 at the latest, a valuable opportunity to study the tiny world's atmosphere would be lost for 230 years.

Pluto Nasa
This is the only image of Pluto taken by the Hubble Space Telecope
This is because Pluto has a thin atmosphere for only 20-30 years out of its 248-year orbit of the Sun. For the rest of the time, the atmosphere freezes out.

Scientists would dearly love to get a mission off the ground by 2004.

It would normally take about 12 years to travel the 6 billion km (3.7 billion miles) to Pluto but if a mission is launched soon that travel time could be cut to just eight years.

Last chance

"It is the last opportunity for a spacecraft to take advantage of a Jupiter gravity assist to give it the boost it needs to reach Pluto in reasonable flight times," said Louis Friedman, director of the Planetary Society based in the US.

Discovered in 1930
2,274 km in diameter
Orbits Sun every 248 years
6 billion km from Earth
"A later launch pushes the arrival date so far into the future that most of Pluto's tenuous atmosphere might freeze out by the time the spacecraft gets there," he added.

Nasa says that a mission to Pluto can only go ahead if it can happen swiftly and at the right price - less than $500m.

"We are not making a commitment to doing a Pluto mission. We are just looking at proposals," Nasa official Dr Ed Weiler said.

Planet of ice

Pluto is a strange and mysterious world. By far the smallest of the planets that orbit the Sun, it is made of ice and rock and was only discovered in 1930.

Pluto Nasa
The probe will go on to investigate Pluto's moon Charon
A large moon orbiting it, named Charon, was discovered in 1978.

Astronomers believe it is the largest member of a swarm of smaller worlds made of rock and ice that orbit the Sun in the cold, dark outer reaches of the Solar System called the Kuiper Belt (KB).

The technology to put a probe into orbit around Pluto is not available, so scientists intend to perform a fly-by and then hopefully target the probe at a KB object.

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See also:

13 Oct 99 | Sci/Tech
A planet beyond Pluto
20 Jul 99 | Sci/Tech
Natural gas found on Pluto
04 Feb 99 | Sci/Tech
Pluto stays a planet
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