NASA ENERGY SOURCE BLASTED.

Byline: Andrew Bridges

As NASA pushes deeper into space, it's likely it will continue to do so with the help of the mighty atom.

Anywhere from six to eight missions the Jet Propulsion Laboratory is planning for launch by 2015 will make use of plutonium 238 as an energy source, including one to the radioactive element's namesake, Pluto.

Although it may not placate critics of NASA's use of the expensive radioactive isotope, which can cost more than $1 million a pound, future missions may use far less plutonium than in the past.

JPL Director Ed Stone said there are few alternatives to plutonium when it comes to dependable energy sources millions - and sometimes billions - of miles from the sun.

``While we can't eliminate its use, we can reduce the amount,'' Stone said.

Since 1961, NASA has launched 25 spacecraft carrying radioisotope thermoelectric generators, devices that convert the heat produced by decaying plutonium into electricity. The latest, the 1997 launch of Cassini, used a record 72 pounds of plutonium.

The only real alternative to plutonium are solar panels, which use the sun's rays to produce electricity.

However, as spacecraft make their way farther out into the solar system, the power of the sun's rays diminishes. On Mars, the brightness of the sun is about 45 percent of that on Earth. By the time you get to Jupiter, it's 4 percent, trailing off to about 0.001 percent on distant Pluto.

``At that point, the only practical source of electricity is from a plutonium heat source,'' Stone said.

By improving the efficiency of how heat is converted to electricity, as well as the efficiency of the spacecraft's instruments, Stone said future missions would likely require much smaller amounts of plutonium. A trip to distant Pluto could require as few as 10 pounds of plutonium.

Critics still say that is too much.

``It's like being a little pregnant. A little plutonium is a lot of plutonium,'' said Karl Grossman, a New York journalism professor who has been one of the most outspoken critics of NASA's use of radioactive material. ``The issue of minuteness of quantity doesn't matter. The basic issue to me is whether it's needed.''

Officials insist plutonium is safe, even if a mission fails on launch.

A Government Accounting Office report on power sources for deep-space probes issued in May said future missions may pose less of a health risk than past missions.

``It's been safely used for over 35 years on a number of NASA space missions,'' said Earl Wahlquist, associate director for space and defense power systems for the Department of Energy's office of nuclear energy.

The plutonium used on space missions is pressed into ceramic pellets encased in a heat- and impact-resistant shield designed to survive intact in any re-entry through the Earth's atmosphere, Wahlquist said.

Of the 25 missions that used radioactive material, however, three did fail, sending the plutonium aboard back to Earth.