“All these worlds are yours except Europa. Attempt no landings there.”

Science fiction fans might recognize that line as the succinct alien warning delivered via the HAL 9000 computer in Arthur C. Clarke’s best-selling novel “2010: Odyssey Two.”

Well, fret not, aliens — the human race isn’t coming: not with robotic emissaries sent to merely orbit the jovian moon that has intrigued scientists for decades, never mind a lander to probe the oceans believed to lie beneath Europa’s icy surface.

That’s the blunt message from NASA in the wake of a 2012 budget request in which funding for planetary science would increase by $180 million next year, to $1.54 billion, but then decline over the next four years to $1.25 billion by 2016. This effectively rules out for the foreseeable future any NASA planetary probes on the scale of the Cassini spacecraft still orbiting Saturn or the Mars Science Laboratory, a car-sized, nuclear-powered rover slated to launch in November.

According to Jim Green, director of the Planetary Science Division at NASA headquarters, the best the agency can hope for is a $1 billion contribution to a joint mission with the European Space Agency (ESA) that would launch in 2018 to collect martian soil samples for possible return to Earth at a later time. If the scaled-back mission still requires a nuclear power source, up to half of NASA’s contribution could be absorbed by the Atlas 5 rocket needed to launch such a sensitive payload.

While it’s nothing to sneeze at, $1 billion in today’s dollars doesn’t buy the type of mission traditionally accorded flagship status. The total cost of Cassini, for example, which launched in 1997, is $3.26 billion, including launch and mission operations. For a sense of what NASA gets for $1 billion nowadays, consider Juno, a solar-powered probe that will spend a year studying Jupiter’s atmosphere from a polar orbit. The first in the New Frontiers series of medium-class probes, Juno originally was viewed as something that would tide scientists over until the next big outer planet mission.

Scientists naturally are dismayed at the situation, but things are tough all over. NASA’s budget is likely to remain flat at best through 2016 at $18.72 billion, which is considerably less than what was anticipated at this time last year. That budget must accommodate the president’s main priorities — commercial crew and cargo services for the international space station, space technology research, and Earth science — as well as cost growth on programs like the flagship-class James Webb Space Telescope.

Congress, meanwhile, has its own priorities for NASA, namely a deep-space crew capsule and a heavy-lift rocket based on the solid-rocket motors used on the soon-to-be-retired space shuttle. Just what NASA would do with that hardware is unclear; there’s no funding in current or projected budgets for the other systems astronauts would need to visit an asteroid or other deep-space destination.

In a recently released survey of planetary missions, the U.S. National Research Council recognized that NASA cannot afford even a scaled-back Europa orbiter absent an improved budget outlook, and thus ranked it second among flagship-class mission priorities. But the survey panel’s recommendations were based on a rosier budget outlook than NASA now faces, meaning even the No. 1 flagship-class priority, the Mars sample-collection mission, cannot go forward as currently designed.

As it now stands, NASA must take care to preserve sufficient funds to carry out two of the seven medium-class New Frontiers missions recommended by the panel. These proposed $1 billion missions, which include a probe to Jupiter’s volcanically active moon Io and a comet sample-return mission, would no doubt enlighten and even amaze, but none carries the discovery potential of the Jupiter-Europa Orbiter mission designed by NASA.

With an estimated price tag of $4.7 billion, such a mission clearly is unaffordable in these times. What’s galling is that NASA is being forced to indefinitely defer opportunities for truly awe-inspiring exploration even as it pours billions of dollars into programs whose main purpose appears to be job preservation in certain congressional districts.

For many if not most of those who have long wondered what lies beneath the ice of Europa, be they scientists or the intellectually curious, the answer won’t be forthcoming — not in this lifetime. The question now is whether future generations will dare to believe such discoveries are even possible.