BOULDER, Colo. – While NASA’s plans to shut down some older missions like the two Voyager spacecraft has generated backlash from the public and the science community, the pressure on the space agency’s budget is increasing the likelihood that more spacecraft will be turned off once their budgeted lifetime is over, even if they still are producing interesting data.

A number of space probes are believed to be on the chopping block, including the Ulysses mission to the Sun and several Earth-oriented space physics satellites.

The cancellation that has gotten the most attention so far is a proposal to end the mission of the two Voyager spacecraft, which were launched in 1977. While both are far beyond the orbit of Pluto they have yet to reach an area of great interest to physicists — the heliopause, which is believed to be the boundary where the solar wind and the sun’s magnetic influence stop and interstellar space begins.

The mission currently employs the equivalent of about 10 full-time people at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., significantly less than the approximately 300 during the height of its legendary “Grand Tour” of the outer planets which lasted through 1989.

“It’s clear that in a limited resource environment, decisions and choices have to be made – none would argue with that,” said Stamatios Krimigis, head emeritus of the Space Department at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. “The key issue is the criteria used to arrive at such decisions.”

Principal among these, Krimigis noted, has to be the scientific productivity of a particular mission.

“This is the area where the Voyager decision is totally indefensible. Why? Because Voyager has been at the fringes of the solar ‘cocoon’ that separates us from interstellar space for nearly three years now,” said Krimigis, who was a principal investigator on Voyagers 1 and 2 and also serves in that role on NASA’s Cassini mission to Saturn. Though Voyager has generated impressive science for many years, the excitement hasn’t stopped, he said.

Voyager is “exploration at its finest,” Krimigis said. “NASA has no present plans for an Interstellar Probe, and even if they approved such a mission tomorrow it couldn’t launch until 2014 and wouldn’t catch up with Voyager for at least 15 years after that,” he said. Voyager is likely to run low on power by about 2020, he said, but could possibly continue in some reduced capacity for several years after that, doing so through power sharing and other procedures.

In his first press conference, Michael Griffin, NASA’s new administrator, promised to carefully review any plans to shut down productive spacecraft. “It certainly would not be done without a careful review,” Griffin said, adding: “That is not to say that all missions are of the same importance. Voyager may well outrank others whose time to be turned off really has come.”

NASA has a number of spacecraft on missions that have been extended beyond their initial planned lifetimes. An even bigger controversy has erupted over former NASA Administrator Sean O’Keefe’s decision to cancel a space shuttle mission that would have added new instruments and other equipment to the Hubble Space Telescope, which was launched 15 years ago, but continues to contribute to valuable scientific research.

In addition, the Mars Exploration Rovers are now well into their second year of operation on that planet’s surface, despite an initial mission plan of only 90 days.

Mars has become home to the extended orbital missions of Mars Global Surveyor and Mars Odyssey. Furthermore, those workaholic surface robots, Spirit and Opportunity, just got the go-ahead for up to 18 more months of operations — purportedly at roughly $3 million per month.

There also have been discussions of extending the mission of the Cassini mission to Saturn and the Deep Impact mission, which is supposed to culminate in July when it hurls an impactor at comet Tempel 1.

Steve Squyres of Cornell University in Ithaca, N.Y., is leader of the Mars rover science team. He’s quick to answer about the value of their recently renewed Mars driving license.

Searching for nickels everywhere

Long-lived spacecraft that receive mission extensions usually cost less to operate than during their prime time. However, they should be made to run cheaper too, which is not always the case, said Wesley Huntress, Jr., director of the Geophysical Laboratory at the Carnegie Institution of Washington in Washington, D.C. In a former post, he served as the chief of NASA’s space science missions.

Space is populated with a large collection of older spacecraft still cranking out data, Huntress said. That being the case, the accumulative cost of running them begins to become significant, he said. And that’s what is happening now, with NASA “searching for nickels everywhere,” Huntress stated. Budget pressure is omnipresent.

“It’s a limited resource. You want to do new things, but you have some old things that are still productive. How do you make that tradeoff? The advocates of these missions don’t have to worry about that…the folks who are writing the checks do,” Huntress advised. “And so you see that battle happening.”

Huntress said he worries that NASA at its upper levels has lost the understanding that they have stakeholders. “They can’t make precipitous decisions without consultation with their stakeholders. The idea of canceling Voyager, for example, seems ill conceived …it seems to me to make very little sense,” he said.

“One of the reasons that the Voyager discussion is so intense is because there is a potential for new science…even though this mission has been on the books now for more than 30 years,” said Sean Solomon, principal investigator for the now en route Mercury Surface, Space Environment, Geochemistry, and Ranging (Messenger) mission.

“On the other hand, there are many missions that are contemplating extended periods. So you probably can’t do them all,” Solomon said. “The question is how do you decide which are the most worthy of these missions to continue to invest in?” There are gauges for making such a determination: Health of the spacecraft’s instruments; remaining fuel onboard to maintain attitude and pointing accuracy; and the originality and importance of the scientific return.

“But at some point NASA is deciding between extending a mission and flying a new mission. And that’s a tougher call,” Solomon said. “There is a natural tendency for active scientific groups to continue to want to see data coming in. So that’s why any process of deciding on how to invest scarce dollars in extended missions should be an objective …one in which a broader community is looking at comparisons among different kinds of missions with different kinds of objectives,” he concluded.

Try turning off one of the Mars rovers today, Huntress said. “You’d get the same reaction that you’re getting from Hubble. So I don’t think it’s peculiar to Hubble. I think it’s peculiar to the way the American public takes ownership of these instruments,” he noted.

During his tenure at NASA, Huntress had to put to rest the Compton Gamma Ray Observatory. Compton was safely deorbited and re-entered the Earth’s atmosphere on June 4, 2000.

“That one had no public consequences. That was strictly an engineering and scientific decision,” he said, with gamma ray astronomers knowing that American and European follow-on missions were coming. “Compton’s time had come.”

Overall, NASA’s report card on turning off spacecraft is far from stellar, Huntress said. “I think NASA prepares poorly, if at all, for extended missions…and that includes my tenure there too. We did not plan well for extended missions because we never anticipated them. But we always knew there was the potential for them,” he said.

Striking the right balance

“It’s something I’m wrestling with,” said NASA’s Alphonso Diaz, NASA’s associate administrator for the Science Mission Directorate. “Somebody’s extended mission is another person’s prime mission,” he told SPACE.com.

Diaz said that a senior review process done a few years ago to rank the strategic importance of missions “may or may not be relevant to the discussion today.” NASA is presently engaged in strategic roadmapping, he said, with some missions perhaps having more significance now given the space agency’s Moon, Mars and beyond visionary agenda.

“I’m willing to listen…to sit down and talk about that,” Diaz said. “We’re currently developing a strategy to try and understand how to get to another review, one that’s based on more current understanding of the significance of those missions,” he said.

Reducing the cost of operating some spacecraft over a long period is on the table too, Diaz explained. Coming up with the right balance of continuing to operate extended missions versus new developments is the trade that has to be made, he said.

“It’ll be awhile before we have a solution,” Diaz added. “We can’t operate everything all the time forever.”