Nasa poised to launch first Pluto probe

CAPE CANAVERAL, Florida - Nasa's first space probe to Pluto, set to lift off on Tuesday, will use radioactive plutonium pellets to power much of its expected nine-year journey to the far reaches of the solar system.

The US space agency, which is seeing protests against the plutonium load, will use the largest rocket in the US fleet as part of a plan that calls for bouncing the small probe, about the size of a grand piano, off Jupiter's massive gravity field en route to Pluto.

Congress authorised the relatively low-cost, US$700 million New Horizons mission after killing a proposal that the probe rely on nuclear-powered propulsion.

Instead, New Horizons will use conventional rocket-powered propulsion from a heavy-lift Atlas 5 rocket, built by Lockheed Martin.

For onboard electric power, the probe will rely on the same source as the Pioneer and Voyager probes of the 1970s, radioisotope thermoelectric generators.

Such generators go on probes that fly too far from the sun for solar-powered systems. They convert heat from the decay of radioactive plutonium pellets into electricity for the spacecraft's systems and science instruments.

The probe contains 11 kg of plutonium processed into fire-resistant ceramic pellets, which, if fractured, tend to break into large chunks, not hazardous microscopic particles.

EXPLORING THE KUIPER BELT

The plutonium use sparked a small protest at the Kennedy Space Centre earlier this month. A second protest was planned on Tuesday.

Nasa said there is a 1 in 350 chance of a mishap that releases plutonium around the Cape Canaveral launch site. Even so, the agency said, the chance of dangerous radiation exposure to workers and the public is low.

Nasa prefers to focus on a potentially rich scientific return from a mission to unexplored Pluto.

Scientists have come to understand, in recent years, that Pluto is not in strict terms a planet. It is one of thousands of planet-like objects in the distant region of the solar system called the Kuiper Belt.

Kuiper Belt Objects, which also include Pluto's largest moon, Charon, and two other recently discovered satellites, far outnumber the terrestrial inner planets - Mercury, Venus, Earth and Mars - as well as the gas giants of the outer solar system, Jupiter, Saturn, Uranus and Neptune.

Kuiper Belt Objects are believed to contain materials largely unchanged since they were formed at the birth of the solar system, some 4.6 billion years ago.

"Studying Pluto, Charon and the Kuiper Belt Objects are key to understanding the origin and evolution of the solar system," said science team leader Alan Stern of the Southwest Research Institute in Boulder, Colorado.

If Nasa launches New Horizons before February 2, the probe will be in position to fly by Jupiter within a year and pick up speed during a slingshot manoeuvre into Jupiter's gravity field.

That boost will allow the probe to fly by Pluto in July 2015. Otherwise, the journey will take until 2018 at the earliest.

- REUTERS