Last year, after a lengthy, circuitous journey through the solar system, a NASA probe known as MESSENGER entered into orbit around Mercury. No spacecraft had visited the innermost planet in more than three decades, and none has paid an extended visit. With MESSENGER's arrival, NASA and its international counterparts now have spacecraft stationed at Mercury, Venus, Mars and Saturn—not to mention Earth and the moon. Two more NASA craft are en route to Jupiter and Pluto; yet another ought to reach the dwarf planet Ceres in 2015. Humankind's presence has never stretched so far.

It could stretch farther still, with robots spying down on bizarre moons that might harbor alien life or on the little-understood outermost planets. An even more novel campaign would ferry Martian rocks back to Earth for analysis. NASA had been on track to begin such an ambitious project, but alas, political maneuvering recently forced the space agency to scrap its plans.

The president's proposed budget for 2013 includes drastic cutbacks to planetary science of more than 20 percent that could derail many future missions. Such erratic handling of NASA threatens the nation's steady progress of solar system exploration, which is hypersensitive to the vicissitudes of budget politics.

Sending robotic missions out into the solar system requires years of preparation. Interplanetary probes depend on cutting-edge technologies that are developed and tested over time. And flight plans often demand a well-timed launch during a brief planetary alignment. Nurturing these complex missions calls for patience and a steady hand. That is why a group of planetary scientists draws up a blueprint for exploration every 10 years or so under the auspices of the National Research Council. This advisory panel issued its most recent report last year, which prioritizes the missions and objectives that will yield the most science per dollar. Shaking up the planetary science division now, for a relatively meager savings of $300 million, would force NASA away from these sensible, well-defined goals.

The most severe cuts were to Mars exploration, long a U.S. specialty. NASA was to begin the process of returning samples from the Red Planet during a joint 2018 mission with the European Space Agency (ESA). That campaign, perhaps the most important flagship project this decade, appears to be dead. With the release of the president's budget request, NASA had to concede that it would withdraw from the 2018 Mars mission, as well as from a 2016 launch, also in collaboration with ESA, of an orbiter that would have sought out the origins of trace gases in the Martian atmosphere. Both missions would have made significant progress toward answering the question of whether Mars was ever habitable.

The budgetary ax also threatens to push other top targets for exploration further into the distance. Foremost among them is Jupiter's moon Europa, which scientists suspect holds an internal ocean that could harbor life. The ice giants Uranus and Neptune have only been investigated in fleeting flybys. These worlds will remain unsolved puzzles without a reversal of regressive policies.

In a fraught fiscal climate, NASA should focus on what it does best and on what offers the best return on investment. Solar system exploration meets both criteria: the U.S. has long led the interplanetary charge, and the resulting scientific benefits have come at a relative bargain. This year NASA's planetary science program cost about $1.5 billion—less than what NASA spent designing a congressionally mandated rocket, the Space Launch System, which appears more likely to satisfy aerospace contractors than to aid the cause of space exploration. Such directives from lawmakers all too often land in NASA's lap without the funds to carry them out.

A mere fraction of a cent from every tax dollar seems a small price to pay for the extension of humanity's robotic reach to distant worlds—one of our greatest accomplishments as a nation, not to mention as a technological species. If planetary science must suffer, the reduction should be phased in gradually so that scientists can try to soften the disruption to long-term plans.

As this issue went to press, an appropriations bill was pending in the U.S. House of Representatives that would restore some funds to planetary science. Congress should take this first step toward stabilizing the program—and toward investing in planetary exploration at a level commensurate with its proven value.

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