By Eric Hand of Nature magazine

When NASA invites proposals in 2013 for its next round of low-cost planetary missions, ideas are sure to be plentiful -- but not the leaders crucial to the missions' success. That's the conclusion of a demographic analysis that shows that the number of highly qualified principal investigators (PIs) willing or able to take the driver's seat in NASA's Discovery-class missions is dwindling.

"We have to recognize that this is coming and this is a problem," says Susan Niebur, who presented her analysis on 21 June at an international conference on low-cost missions at Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland. As a cohort of former PIs nears retirement age, Niebur worries that burn out, budget overruns and missed launch windows will be the result if NASA doesn't find a way to get younger scientists the experience they need to step up into mission-leading roles.

Niebur first encountered the dilemma between 2003 and 2006, when she was the NASA official charged with running the fiercely competitive Discovery programme -- the small, scientist-led planetary probes that are often the most innovative in NASA's repertoire. At that time, she says, she kept getting proposals "from the same guys".

Not that they were unqualified. On the contrary, they were precisely the sort of scientists NASA wanted in charge of spacecraft worth hundreds of millions of dollars -- people who knew their science but had also dirtied their hands with instrument hardware and experienced the headaches of building a spacecraft. The problem was that there were so few of them -- and they were getting older.

Now an independent consultant based in Silver Spring, Maryland, Niebur has been tracking the situation and says that it is getting worse. By 2015, when the winning proposal is chosen, there will be only 14 potential PIs aged 65 and under who have previously been PIs, deputy PIs or project scientists (see 'Planetary shortfall'). This means that many of the roughly 30 proposals that the Discovery programme attracts at every round will be coming from relative rookies.

Started in 1992, Discovery came to embody a new idea: that a space mission could be led by a scientist rather than an engineer. Of the 16 PI-led missions in NASA's history -- mainly within Discovery -- most have been resounding successes. In some cases, the PIs of those missions stepped into their leadership roles with less experience than NASA anticipated, such as astronomer Michael A'Hearn of the University of Maryland in College Park, who led the Deep Impact mission that pierced a comet's nucleus in 2005.

But that's no reason not to demand more-experienced PIs, says Alan Stern, NASA's science chief from 2007 to 2008. During his stint at the agency, Stern began looking at Discovery, and found that virtually all the projects in development were bursting their budgets. Some of the PIs had come straight from the blackboard, and some were like "deer in the headlights", he says. "We're giving them the keys to the kingdom on projects that are worth hundreds of millions of dollars," he adds. "That's an awfully expensive on-the-job training exercise."

Stern implemented strict rules requiring the heads of all PI-led missions to have had experience as a PI, a deputy PI, an instrument PI, a project scientist or a deputy project scientist on a previous mission. The problem with this was that planetary scientists have few chances to build up such experience. Unlike NASA's other science divisions -- astrophysics, Earth science and heliophysics -- planetary science does not include balloon and sounding-rocket missions that can give younger researchers a chance to lead. The Discovery missions, currently capped at US$425 million and launched every few years, are as cheap and as frequent as they come. Stern's rule was seen as too onerous, and it disappeared when he left the agency.

Rather than ruling out PIs with less experience, Niebur says NASA should enlarge the pipeline of eligible candidates. In particular, she wants the agency to require younger deputy PIs on future proposals. But PIs, trying to keep proposals lean, sometimes exclude deputies.

Michael New, the NASA official now in charge of the Discovery competitions, says that of the 28 proposals being considered in the latest round, which began in 2010, 19 include a deputy PI. However, he says, the deputies are on average the same age as the PIs -- suggesting that the demographic problem is not going away.

Nor is it safe to assume that deputies will want to take on the notoriously gruelling task of being a PI. In her analysis, Niebur found that of all the PIs, deputy PIs, project scientists and deputy project scientists on all NASA planetary missions since 1977, none went on to lead another mission as a PI.

That's not too surprising, says Bruce Jakosky at the University of Colorado in Boulder, the PI for the Mars Atmosphere and Volatile Evolution Mission (MAVEN), which was selected in 2008 as the winner of NASA's Mars Scout competition -- a programme similar to Discovery. It's just too much work. "I have the heart for MAVEN," he says, "but not for another one."

This article is reproduced with permission from the magazine Nature. The article was first published on July 5, 2011.