House appropriators appear intent on forcing a day of reckoning for NASA’s James Webb Space Telescope, a flagship-class astronomy mission whose price tag has soared well above initial estimates and threatens to climb still higher. Whether or not to cancel Webb at this stage — with some $3 billion already invested and key elements of the observatory coming together — is a monumental decision; it must not be taken before all the facts are in and weighed, including the latest estimates of the money and time needed to complete development.

Webb is marked for termination in a 2012 spending bill that would provide $1.9 billion less for NASA than the $18.7 billion requested by the Obama administration. Members of the House Appropriations Committee approved the measure July 13, setting up a likely showdown with their U.S. Senate counterparts. Sen. Barbara Mikulski (D-Md.), who chairs the Senate Appropriations subcommittee that oversees NASA spending, has already come out swinging for Webb, which is managed by Goddard Space Flight Center in Greenbelt, Md., and whose science operations would be managed by the Space Telescope Science Institute in Baltimore.

Aside from Webb, the accounts hit hardest in the NASA spending bill are space technology, robotic exploration precursor missions and support for commercial vehicles to transport crew and cargo to the international space station. This wasn’t surprising: Rep. Frank Wolf (R-Va.), chairman of the House Appropriations subcommittee that funds NASA, has been a vocal critic of the administration’s bid to commercialize human spaceflight operations in low Earth orbit, while technology development and robotic precursors are the type of large yet amorphous programs that make irresistible targets for both budget hawks and those seeking to siphon off funding to their preferred programs. Thus the bill recommends giving the Space Launch System — the heavy-lift rocket Congress has directed NASA to build — more funding than was requested despite questions about its affordability and mission.

House lawmakers clearly are looking to make an example of Webb, which when conceived was supposed to cost in the neighborhood of $2 billion but whose official price tag now exceeds $5 billion. An independent panel led by the Jet Propulsion Laboratory’s John Casani last year pegged the final cost at $6.5 billion assuming a launch in 2015 — more than a year later than its official launch date. Rep. Wolf, during the full committee markup of the appropriations bill July 13, cited the Government Accountability Office in projecting a total cost of $7 billion to $8 billion for the observatory, the designated successor to NASA’s Hubble Space Telescope.

Despite its problems, there are very good reasons not to kill this program: The National Research Council’s latest decadal survey of astronomy missions listed Webb’s completion as its No. 1 priority and prime contractor Northrop Grumman says more than 75 percent of the observatory’s hardware is either built or under construction. As everyone knows, developing Hubble was neither cheap nor trouble free but since beginning operations with corrected optics the telescope has proved itself a scientific treasure-trove; abandoning its successor would leave a huge void in the astronomy community. It also would force the dismissal of hundreds of highly skilled engineers, whose expertise in many cases would be lost to the U.S. space industry forever.

On the other hand, there has to be a cost threshold above which it no longer makes sense to continue investing in a program. Walking away from a $3 billion investment is an extraordinarily difficult and painful thing to do, but if it’s going to cost another $5 billion or more to complete the mission NASA might not have another choice, particularly in the current budgetary environment. As it is, Webb’s cost growth is forcing NASA to defer most other astronomy projects and threatens to eat into funding for agency activities in other disciplines such as planetary and Earth science. Any assertions that the program’s technical difficulties are behind it must be taken with a grain of salt; the cost of the U.S. Air Force’s already over-budget Space Based Infrared System missile warning system, for example, rose by billions more after service officials all but declared victory on the development front.

For a group of lawmakers determined to establish their budget-cutting bona fides, the James Webb Space Telescope program has made itself a target that is simply too tempting to pass up. Continuing uncertainty about Webb’s cost to complete makes it that much more difficult to defend, regardless of its scientific merit.

However, NASA is still holding onto a card in the form of a revised program plan devised internally in the wake of the Casani report. NASA Administrator Charles Bolden told lawmakers during a House Science, Space and Technology Committee hearing July 12 the plan has been presented to an independent review board and will serve as the basis for future funding requests for the program. Lori Garver, Mr. Bolden’s deputy, said July 7 that the agency would lay out the revised plan in the next budget cycle — presumably meaning next February — and that it would permit the observatory to launch within the decade.

That’s simply not good enough. NASA is asking Congress for $375 million in 2012 for Webb — not nearly enough to put it on track to launch on the schedule projected in the Casani report but nonetheless a substantial sum to an increasingly cash-starved agency. Congress should not be asked, much less expected, to approve that request in the absence of NASA’s latest plan and cost estimates. It is possible that whatever NASA reveals will only serve to reinforce the recommendation by House appropriators. But it’s also possible that NASA has a compelling case for soldiering on. Either way, it is better to have that information while lawmakers are still deliberating the 2012 budget request. Congress should demand to see the plan immediately.