WASHINGTON -- NASA is still targeting a January 2006 launch of the New Horizons Pluto probe after the U.S. Department of Energy (DOE) confirmed it can deliver most of the nuclear fuel the spacecraft will need for its 10-year mission. A post-mission encounter with Pluto's mysterious Kuiper Belt neighbors, however, appears a likely casualty of the plutonium pinch.

NASA had considered postponing the New Horizons launch a full year due to a plutonium-238 shortage exacerbated by a security-related shutdown of the DOE lab that processes the radioactive material. The spacecraft will use the plutonium in a radioisotope thermal generator (RTG), a long-lived nuclear battery. RTGs transform heat from decaying plutonium pellets into electricity to power science instruments, computers and other flight systems.

Postponing the launch would have added three to five years to the probe's transit time and millions of dollars to the mission's cost.

With work halted at the DOE's Los Alamos National Laboratory following a security breach, processing of NASA's plutonium order has fallen far behind schedule. The department recently completed its investigation into the mishandling of classified information at the New Mexico nuclear weapons lab. Sensitive work is expected to resume there shortly. Los Alamos had about half of the plutonium-238 that NASA needs for New Horizons ready to go when the lab was shut down in July.

The spacecraft is in the middle of assembly at Johns Hopkins University's Applied Physics Laboratory in Laurel, Md., where it has already been outfitted with two of its three scientific instruments. The spacecraft still must undergo months of testing. It is scheduled to be shipped to Kennedy Space Center in late 2005 to be readied for launch aboard an Atlas 5 rocket.

The DOE still does not expect to deliver 100 percent of the plutonium-238 that NASA requested for the $600 million mission. However, DOE plans to provide at least 80 percent of NASA's order -- enough to permit the spacecraft's RTG to crank out the 182 watts of power New Horizons officials say is the minimum required for a successful encounter with Pluto.

Earl Wahlquist, associate director of the DOE's Space and Defense Systems Power Office, told Space News Sept. 28 that the department believes it can meet NASA's stated minimum by giving up flight-ready Plutonium-238 that the department had been using in long-term tests.

Orlando Figueroa, deputy associate administrator for programs in NASA's Science Mission Directorate, said the New Horizons team convinced him they could meet all the objectives of the Pluto flyby with less than full power.

The spacecraft's RTG generally loses 3 to 5 watts of power-generating capacity a year. If the DOE provides only the minimum amount of plutonium-238 required for a successful Pluto flyby, New Horizon's RTG might not be able to pump out enough wattage for a worthwhile encounter when the probe finally reaches the Kuiper belt, which rings the solar system, two to four years after zipping past Pluto.

Unless DOE comes through with closer to 90 percent of NASA's original Pluto order --- still a possibility, according to Figueroa -- an extended mission targeting Kuiper Belt objects might be beyond the New Horizon probe's capability.

"We would perhaps be giving up the Kuiper Belt objects," Figueroa said in a Sept. 23 interview here. "The power would not be there."

Alan Stern, New Horizon's principal investigator, said he and his colleagues would be able to get by on 182 watts of power during the probe's six-month encounter with Pluto without sacrificing mission objectives by keeping certain systems on cold standby until needed, for example.

Even if the department delivers only what NASA and Stern consider the bare minimum, NASA would have years to decide whether it should send the New Horizons probe in hot pursuit of a Kuiper Belt object anyway, according to Stern. New Horizons is not expected to reach Pluto until 2015, and the probe's RTG, he said, could always last longer than expected, making a brief Kuiper Belt tour at least a possibility.

But Stern said the Kuiper Belt is too scientifically important to leave to chance.

He said NASA could launch a New Horizons follow-on mission by 2008 or 2009 for a fraction of the cost of the original. Although NASA would have to pay for another launch -- an expense that accounts for about 40 percent of the mission price tag, according to Stern -- the second spacecraft could be identical to the first, saving the cost of designing the probe and writing software for it. Stern also said both probes could be flown by the same mission operations teams.

Stern is not alone in putting a premium on Kuiper Belt exploration. The National Academy of Sciences recommended in 2003 that exploring Pluto and the Kuiper Belt should be NASA's top priority for medium-class missions in the decade ahead.

Stern could get several million dollars from the space agency next year to make his case that a follow-on mission is the right thing to do.

In September, Senate appropriators, noting that the paucity of plutonium has jeopardized the Kuiper Belt tour, added $4 million to the NASA budget bill last month to pay for a study of the feasibility and likely cost of launching a so-called New Horizons 2 mission relatively soon. The bill cleared the Senate Appropriations Committee Sept. 21 and is awaiting action by the full Senate.