The initial estimate of the cost of putting a spacecraft into orbit around Europa was $4.7 billion. (NASA/STR/Reuters)

Europa, a moon of Jupiter first spotted by Galileo four centuries ago, has geysers spewing material from what appears to be a subsurface ocean. It’s not inconceivable that there are fish down there in that cold, dark sea.

Scientists have long dreamed of sending a robotic probe to Europa, and they have put such a mission at the top of their wish list. But Europa is a hard target: It’s very close to Jupiter, and a spacecraft and its instruments would need extra shielding to keep them from being fried in Jupiter’s harsh radiation environment.

The initial estimate of the cost of putting a spacecraft into orbit around Europa was a wince-
inducing $4.7 billion. Engineers then came up with a cheaper alternative, in which the spacecraft would go into an orbit around Jupiter that would send it past Europa dozens of times. During these flybys it could sample the material ejected by the geysers, looking for signatures of life in the ocean below the moon’s icy crust. That might cost on the order of $2 billion.

But budgets are tight at NASA. Officials have said there won’t be any new “Flagship” missions costing north of $1 billion in the coming years, other than ones previously approved.

Now comes the Obama administration’s 2015 budget request, which includes $15 million for studying a possible Europa mission. That’s a tiny fraction of the $17.5 billion requested for the agency. It’s less than what Congress has already allotted for Europa studies the last couple of years. But supporters of robotic exploration are grateful that, for the first time, the administration is signaling support for a Europa mission.

But will this mission really materialize? This is the dilemma for anyone writing about NASA these days: The agency sometimes starts programs that fail to survive the erosional forces of politics and constricting budgets.

It takes at least a decade to do anything significant in space, but our political cycle is faster than that. Even programs where the metal has already been cut can wind up canceled. That happened with the Constellation program of President George W. Bush, which would have returned astronauts to the moon. President Obama killed Constellation, and with it the Ares 1 rocket that had already burned through billions of dollars.

The administration last year proposed the Asteroid Redirect Mission, which would involve capturing a small asteroid, hauling it to an orbit around the moon and visiting it with astronauts. In the new budget request, the administration wants to boost funding for the ARM, to $133 million. But this is another mission that may never actually materialize. Technically, the ARM is hardly a slam-dunk, because scientists have yet to find a good target rocket. (It has to have just the right size, shape and composition, and it has to be spinning just so, etc.) Republicans have opposed funding for the ARM.

No one vocally opposes the Europa mission, and many people love the idea. But Europa’s ocean and its hypothetical life-forms must compete against other NASA priorities that have entrenched support among powerful senators and the corporate aerospace community. NASA, for example, is building a new jumbo rocket, the Space Launch System, and a new crew capsule, Orion. They cost about $3 billion a year. The international space station requires a similar pile of money.

The administration is particularly focused these days on boosting the “commercial” side of space. Officials hope that by 2017 American astronauts can go into space once again on American rockets, rather than relying on the Russians — to whom the United States is paying $71 million per seat on Soyuz rockets to get to and from the space station. The political crisis in Ukraine makes this reliance on the Russians all the more awkward.

Science is important at NASA, but it typically is a secondary priority after human exploration of space. And science has had its own cost overruns, specifically with the James Webb Space Telescope and the Curiosity rover on Mars. The Europa mission and a proposed space telescope known as WFIRST are the kind of expensive, Flagship-class projects that are in disfavor these days among the frowny-faced people at the Office of Management and Budget.

NASA officials have indicated that they’d like to go to Europa for less than $1 billion. But science by definition operates on the edge of the known, and space science, out there on the farthest frontier, is never cheap. Sometimes it costs a Saganesque sum: Billions and billions.