January 15, 2006

NASA poised to launch first Pluto probe

By Irene Klotz

CAPE CANAVERAL, Florida (Reuters) - 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 U.S. space agency, which is seeing protests against the
plutonium load, will use the largest rocket in the U.S. 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.

The Pluto mission was set to begin at 1:24 p.m. EST (1824
GMT) on Tuesday.

Congress authorized the relatively low-cost, $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

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 24 pounds (11 kg) of plutonium processed
into fire-resistant ceramic pellets, which, if fractured, tend
to break into large chunks, not hazardous microscopic


The plutonium use sparked a small protest at the Kennedy
Space Center 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 maneuver 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