NASA expects to decide by mid-September between postponing the launch of a nuclear-powered Pluto probe a full year — adding millions of dollars and three years of travel time to the mission — and staying on schedule with a less capable spacecraft.
It is not a choice NASA officials relish making. But with all work halted at the U.S. Department of Energy’s Los Alamos National Laboratory following a security breach, it is almost certain that the lab will not be able to guarantee all of the nuclear fuel needed to power the Pluto-bound New Horizons spacecraft by the time NASA needs it.
The plutonium-238 fuel is used to power a long-lasting spacecraft battery known as a radioisotope thermal generator, or RTG.
Only about half of NASA’s plutonium order was complete when the Department of Energy halted all work at the nuclear weapons facility in mid-July after two computer disks containing classified information were discovered missing. Some administrative work had resumed at the New Mexico lab by July 29, but Los Alamos Director Pete Nanos told reporters Aug. 3 that it could be two months before the lab is back to full operations.
Orlando Figueroa, director of NASA’s solar system exploration division, told Space News Aug. 6 that he no longer expects Los Alamos to be able to provide all the plutonium-238 that NASA needs in time to keep the mission on track for a January 2006 launch.
Half on hand
Los Alamos already has about half of the plutonium-238 that NASA needs for the mission ready to go. This material was left over from a spare RTG built years ago for NASA’s Galileo and Cassini missions.
Prior to the lab shutdown in July, Los Alamos scientists had been working to fill the rest of NASA’s order by turning plutonium bought from Russia into tiny pellets and then packaging them into modules roughly the size and shape of hockey pucks.
Los Alamos must deliver the modules to the Argonne National Laboratory’s Idaho Falls facility by February if NASA and the Department of Energy hope to keep the project on schedule. Argonne’s job is to integrate the modules with the RTG. Three months of rigorous testing will then follow, including vibration and exposure to intense cold and vacuum, according to Argonne-West spokeswoman Sara Catherine Foster.
Figueroa said Los Alamos was already scrambling to make up time lost during a previous security-related work stoppage and the latest situation has only made matters more difficult.
"Getting past the first shutdown took a tremendous effort," Figueroa said. "The latest shutdown took the threat from Code Orange to Code Red real quick."
Figueroa said NASA could launch the New Horizons probe with a less-than fully fueled RTG, but that would mean less electrical power for its seven instruments and other systems. RTGs work by transforming heat from decaying plutonium into electricity. A fully fueled RTG is designed to deliver about 200 watts of useable power for the mission.
An RTG running on half the required plutonium would generate only about half as much power.
Department of Energy officials are expected to tell Figueroa within two weeks just how much of the requested plutonium can be delivered in time for a 2006 launch schedule. Meanwhile, Figueroa has directed the New Horizons team to propose a less power hungry mission that launches on time.
Figueroa said he would decide by mid-September whether to go ahead with a scaled-back mission in January 2006 or a fully powered spacecraft one year later. Waiting a year would actually delay the probe’s arrival at Pluto for four to five years, however.
The 35-day launch window that opens in January 2006 is the last opportunity for nine years to take advantage of a time-saving Jupiter gravity assist that would enable New Horizons to reach Pluto as early as late 2014. The next favorable launch window, which opens in February 2007, would add three to four years to the probe’s transit time and between $60 million and $80 million to its $600 million cost.
Given all that can go wrong with a spacecraft in the unforgiving space environment, adding 1,000 days of cruise time also adds risk.
Alan Stern, the New Horizons principal investigator, said in an interview that he is evaluating a range of options for making due with less power and keeping the mission on schedule
"We’re looking at where we can save a few watts here and there," said Stern, of the Southwest Research Institute. "One Christmas tree bulb worth of power — about 15 watts — could make the difference between launching on time, or waiting a year."
In their hunt for ways to save power, Stern and his team will consider everything from keeping certain backup systems inactive during the cruise phase to cutting down on the probe’s data storage capacity, Stern said.
Getting there quicker
Also under consideration are different trajectories that could get the spacecraft to its destination months sooner and enable it to complete its core mission up to a year earlier. Because RTGs weaken over time, time saved translates into more available power. The tradeoff, Stern said, is that arriving later could actually be better for one of the mission’s experiments.
Stern said if Los Alamos can only guarantee delivery of the half of NASA’s plutonium order the lab had on hand when it shut down, the mission would have to wait.
"I cannot run at half power," Stern said. "The answer is probably somewhere between 170 watts and full power."
In addition to the 36 finished plutonium modules Los Alamos has on hand from the Cassini and Galileo programs — half of the 72 modules needed for full power— the lab had another 18 modules in various states of completion when it stopped work. If a 2006 launch is to remain a possibility, Stern said, Los Alamos would need to guarantee delivery of enough plutonium to provide at least 170 watts of power — about 61 or so modules in all.
NASA is expected to find out from the Department of Energy in the weeks ahead if Los Alamos can deliver even that many.
Figueroa, meanwhile, said the Los Alamos shutdown is not the only thing putting the January 2006 launch schedule in jeopardy. NASA officials at Kennedy Space Center in Florida also are facing a very tight schedule for qualifying the solid-rocket strap-on boosters that will give the Atlas 5 rocket the added lift it will need to send New Horizons on its way to Pluto.
Further, the spacecraft is encountering the usual developmental hiccups, Figueroa said. And because it will use a nuclear power source, the mission must undergo a rigorous safety and environmental review before it is cleared to launch.
"From the get-go we knew that making 2006 was going to be very tight," he said. "Across the board we knew this was going to be a heck of a challenge and it has proven to be so every step of the way."
After several abortive attempts during the 1990s to design an affordable Pluto mission, NASA made a fresh start in 2001 by picking the Southwest Research Institute from among a number of competing bidders to design the challenging mission. Its projected $600 million cost is considered modest by NASA standards.
Any added expense from postponing the mission, Figueroa said, could probably be accommodated within the budget NASA has set aside for the broader New Frontiers outer planets program. "Since this mission was so risk-loaded from the get-go, we have always been playing what-if games and placed a lien against the New Frontiers program because we thought that anything at any time could cause a delay."