The Obama administration wants to send humans to Mars in the 2030s. Of course, such a mission requires a lot of advance engineering, and as a first step, nasa plans to send astronauts to a small asteroid that would be brought into a stable orbit around the moon. To achieve that mechanical feat, a solar-powered robotic probe is being designed to capture a space rock and slowly push it into place. A target asteroid has yet to be announced, and the robotic space tug has yet to be built, but the parties involved hope to have the rock relocated to the moon's vicinity as soon as 2021. nasa calls this concept the Asteroid Redirect Mission (ARM) and is marshaling resources across the entire agency to support it.

Michele Gates, the agency's program director for ARM, says that its advanced propulsion technology and crew activities would give nasa the capability and experience needed to someday reach Mars. The trip would demonstrate spacecraft rendezvous procedures and establish protocols for sample collection and extravehicular movements. And it would do all of this while keeping astronauts relatively safe, staying sufficiently close to home so that if something went wrong, the crew could potentially make an emergency return to Earth.

ARM's critics are loud and legion, however. In June the prestigious National Research Council issued a report stating that the mission could divert U.S. resources and attention from more worthy space exploration, highlighting parts of ARM as dead ends on the path to Mars. The harshest criticisms have come from asteroid scientists. Mark Sykes, director of the Planetary Science Institute in Tucson, Ariz., ridiculed ARM last September while testifying to a congressional committee, saying that the agency's tentative cost estimate of less than $1.25 billion for the concept's robotic component strained credulity.

“It doesn't advance anything,” Sykes says, “and everything that could benefit from it could be benefited far more by other, cheaper, more efficient means.”

The mission's detractors miss the point that it represents the nation's best opportunity in the foreseeable future to maintain its momentum in human spaceflight, says Louis Friedman, a space policy expert who helped to conceive ARM.

To this point, planetary scientist Richard Binzel of the Massachusetts Institute of Technology argues that NASA needs to look for more asteroids before it leaps into ARM. A robust asteroid survey, he says, would discover suitable targets for a crewed mission that would not require an expensive orbital relocation. “By the time we would tow a tiny rock into lunar orbit, we could be discovering more attractive, larger objects passing through the Earth-moon system that are easy to reach,” Binzel notes.

NASA plans to conduct a formal review of the ARM concept in February, and the Obama administration's next budget proposal is expected to request more funding for ARM. But the redirect's fate may have already been sealed by 2014's midterm elections, in which Republicans, who are largely opposed to the mission, took full control of Congress. With this latest blow to nasa's post–Space Shuttle plans for human spaceflight, the agency's astronauts may end up boldly going nowhere for many years to come—regardless of the approach.