As NASA funding continue to dissipate, space agency officials are left trying -- frustratingly -- to figure out what's worth paying for. And the Spitzer Space Telescope is just the latest in a growing list of defunded and soon-to-be-extinct projects.

Since the Apollo missions to the moon ended in 1973, NASA's budget has steadily declined -- from 1.35 percent of the federal government's total expenditures to less than 0.6 percent today. Although the 2014 budget recovered a small portion of previously cut funds, the writing has been on the wall for some time now: do more with less.

Or just do less. So far, that's been the approach -- a run of tough decisions, a run of cuts. The Space Shuttle program was retired. More recently, Congress cut funding for NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA), a 747 jetliner outfitted with a telescope that climatologists use to study the infrared-absorbing properties of the atmosphere's water vapor.

"It turned out that we had to make very difficult choices about where we go with astrophysics and planetary science and Earth science, and SOFIA happened to be what fell off the plate this time," NASA administrator Charles Bolden said shortly after the latest budget proposal came out.

Even more recently, a NASA review panel -- convened every two years to determine the merits of various NASA programs -- decided to decommission the Spitzer Space Telescope, the the first telescope to directly capture the light from extrasolar planets. A last ditch effort to save Spitzer looks likely to be unsuccessful.

Yet as NASA funding shrinks, the possibilities of scientific exploration continue to grow, leaving NASA's task of project decision making tougher than ever. Go back to the moon, some say. How about a mission to Jupiter's moon Europa? What about deep-space exploration? Or Mars?

The House is expected to soon vote on a spending bill that would fill some gaps in NASA's funding. Even so, NASA's purse strings are likely to remain tight and tangled for the foreseeable future.