Word that NASA will not be able to launch a Mars orbiter as part of a wider exploration effort with the European Space Agency (ESA) is a good indicator of the painful decisions that lie ahead for a U.S. space agency squeezed between a shrinking budget and massive cost growth on its flagship astronomy mission.

NASA doesn’t want to talk about these choices and that’s understandable; not only are such things unpleasant, the U.S. agency is in the midst of preparing its 2013 budget request, which, per tradition, will not be made public until February. But ESA must make programmatic decisions in preparation for the targeted 2016 launch date — which now might not take place — for the European-built orbiter, so NASA had no choice but to inform its partner that a U.S. rocket was no longer in the offing.

Such is the peril of international cooperation in trying fiscal times. In NASA’s case, the situation is made worse by continued cost growth on the James Webb Space Telescope (JWST), the designated successor to the Hubble Space Telescope. Here too, NASA and its White House overseers have information that people — in this case, U.S. lawmakers — absolutely must have to make funding decisions that could impact any number of U.S. civil space programs.

U.S. Rep. Frank Wolf (R-Va.), chairman of the House Appropriations commerce, justice, science subcommittee that funds NASA, requested that information in a Sept. 28 letter to White House Office of Management Budget Director Jacob Lew. In July, Rep. Wolf’s subcommittee recommended terminating JWST, whose cost has ballooned from an initially projected price tag of less than $2 billion to $8.7 billion, according to NASA’s latest estimates. That figure is some $2.2 billion higher than an independent panel estimated a year ago, when NASA was still clinging to a roughly $5 billion price tag.

Not surprisingly, the Senate Appropriations commerce, justice, science subcommittee, chaired by Sen. Barbara Mikulski (D-Md.), whose home state hosts the JWST development and operations facilities, recommends pressing ahead with the program. The Senate panel proposed adding $155 million to NASA’s $375 million request for JWST next year.

Setting aside the real possibility that U.S. government activities during 2012 will be funded at prior-year levels under a continuing resolution — a different topic for a different day — JWST’s budget is among the issues to be settled during a House-Senate conference that will yield a unified spending bill for NASA and assorted other U.S. federal agencies.

As Rep. Wolf explained in his letter, sparing JWST from the budget ax will necessarily impact other NASA programs. He asked which specific NASA programs and activities would see their funding cut in order to accommodate the giant observatory within the agency’s overall budget, which is expected to remain flat, at best, for the next several years.

The chairman has a point. Lawmakers have a way of asserting their own priorities when it comes to federal spending — a prime example being the Space Launch System, the giant rocket with no clear mission that Congress directed NASA to build — but they have a right and a responsibility to fully understand what’s at stake with JWST.

Canceling the project would be devastating. The money wasted and jobs lost can be quantified; the dulling of space technology’s cutting edge can be predicted. But the loss to science would be incalculable: Like Hubble — NASA’s most productive science platform ever — JWST has the potential to dramatically change astronomers’ understanding of the universe and its ongoing evolution. It is fair to say that if JWST is built and performs as advertised, it will document phenomena and objects never before seen, perhaps not yet even theorized.

Yet as painful as it is to even think about canceling JWST, this option must at least be considered given the unprecedented pressures on NASA’s budget. Even if lawmakers ultimately opt to stay the course — and they should — they have to understand the implications of that decision for NASA’s other activities.

In that regard, the Space Launch System, which per the House and Senate spending bills is slated to receive nearly $2 billion next year, is an appropriate bill payer for JWST. Given that NASA has no established exploration destination requiring the heavy-lift rocket on the schedule mandated by Congress, stretching out its development to help fund an observatory of undeniable scientific merit — its substantial problems notwithstanding — is a fair trade.