Nature: A demographic analysis shows that NASA’s cohort of experienced, highly qualified principal investigators (PIs) able to lead Discovery-class missions—a series of focused planetary science projects to explore the solar system—is nearing retirement age. Susan Niebur first noticed the demographic shift when she ran the Discovery program at NASA between 2003 and 2006. As an independent consultant she has continued to track the dwindling number of experienced PIs; by 2015, only 14 potential PIs out of a likely 30 will have experience as PIs, deputy PIs, or project scientists. If NASA doesn’t find a way to give younger scientists opportunities to develop the necessary experience to step into mission-leading roles, the program as a whole may suffer from PI burnout, missed launch windows, and cost overruns. However, an inexperienced PI doesn’t necessarily doom a mission to failure. Most of the program’s missions have succeeded, including those led by less experienced PIs.