Scientists and managers on NASA’s Wide-field Infrared Survey Explorer (WISE) project were no doubt disappointed by a review panel’s decision to recommend against funding a three-month mission extension, which would have enabled the celestial-object-hunting spacecraft to make a second complete sweep of the sky. Advocates said the extension would have confirmed the findings of the originally planned mission while possibly leading to the discovery of more stars, galaxies, asteroids and comets.

Launched Dec. 14 for a planned 10-month mission, WISE appears so far to be an unqualified success. Equipped with a suite of four infrared cameras, the spacecraft has already detected more than 50 previously undiscovered near Earth objects, including comets, asteroids and centaurs, the latter being objects that exhibit characteristics of both comets and asteroids.

The spacecraft and sensor, built by Ball Aerospace & Technologies Corp. and the Space Dynamics Laboratory, respectively, appear to be in excellent health. Program officials now estimate that the liquid hydrogen that keeps the infrared sensors cool enough to detect faint sources of heat will last one month longer than originally expected, which would have covered one-third of the proposed mission extension.

The coolant’s exhaustion would have left the sensor suite with degraded capability for the remaining two months of the second-round observations. But proponents said WISE would retain two operating infrared channels, enabling it to detect objects visible at those wavelengths as well as to help astronomers track movements of already-observed objects such as dead stars.

The estimated cost of the extension was $6.5 million, a modest sum compared with the $320 million price tag for the original mission, which itself is reasonable for Explorer-class probes. Separately, program officials had asked for $8 million to expand the WISE data catalog to include data on things like galaxy clusters and cosmic dust clouds that were not part of the mission’s primary target set.

In short, the case for extending WISE was a reasonable one. Unfortunately, it ran up against several competing priorities that were examined by NASA’s Astrophysics Division Senior Review for Operating Missions. These included three flagship-class missions: the Chandra X-ray observatory, the Spitzer Space Telescope and NASA support for the European Space Agency’s Planck observatory. In findings released May 6, the panel, which meets every two years, unanimously endorsed continued support for each of those three missions based on the expected value of the scientific return.

WISE, along with several others, did not make the cut; in fact, WISE ranked at the bottom of the proposed extensions in terms of scientific value, owing to the depletion of its onboard sensor coolant.

This seems unfortunate, particularly for officials and scientists who have devoted considerable time and energy to WISE, but the fact is that NASA cannot afford to extend every successful mission, even if the spacecraft in question remains healthy with the potential to continue yielding useful data.

It’s no secret that money’s tight at NASA — now, as always — and astronomy budgets in particular could come under more pressure with the continued cost growth on the flagship-class James Webb Space Telescope. Just recently NASA officials acknowledged that the Hubble Space Telescope successor likely will need additional testing that could delay its launch by four months, and that issues related to the program’s overall technical complexity could lead to even lengthier delays if Congress does not provide additional funding beyond what NASA requested in 2011.

Slated to launch in 2014, the Webb telescope is already behind schedule and, at $5 billion, well over budget. But the reality is that substantial cost growth and delays are par for the course when it comes to large, one-of-a-kind space missions; NASA’s focus needs to be on getting Webb right, which means doing whatever needs to be done to reduce risk.

Assuming all goes well with Webb in the years following its launch, NASA’s Astrophysics Division Senior Review for Operating Missions will be weighing whether to extend the mission come 2019. If Webb comes anywhere near to living up to its billing as Hubble’s successor, the answer will of course be yes, probably at the expense of one or more smaller missions that were no less successful in proportion to their cost. That lower-cost missions tend to fall to the bottom of the priority list when extensions are being weighed — flagships do more; that’s why they cost more — does not in any way diminish their scientific contributions.