NASA goal to snag an asteroid and bring a space rock close to Earth is a distraction from the effort to send humans to Mars, a top asteroid expert says.

In a commentary in the journal Nature, planetary scientist Richard Binzel of the Massachusetts Institute of Technology calls NASAs Asteroid Redirect Mission (a.k.a. ARM) a multibillion-dollar stunt. The strongly worded text comes after a National Research Council report in June offered multiple routes to get to Mars -- including the asteroid-snagging plan -- but didnt recommend one over any other.

Binzel, whose work has helped to evaluate the hazard from various near-Earth asteroids, said its time to choose an option that doesnt detract from the ultimate goal of reaching the Red Planet.

Over the next two months, Obamas 2015 budget will be shaped. NASA needs to make a clear choice about its priorities, Binzel wrote. It should abandon the ARM mission concept and make an asteroid survey its top priority to provide a basis for future crewed missions.

The Obama administration has set goals to send astronauts to an asteroid by 2025 and to Mars by the mid-2030s. But trying to retrieve part of an asteroid is a waste of resources, Binzel argued.

It will require an ancillary spacecraft deploying either a huge capture bag or a Rube Goldberg contraption resembling a giant arcade-game claw, Binzel wrote. Neither technology is useful for getting humans to Mars.

Instead, rather than hunt an asteroid down, lasso it and lug it back, NASA should use near-Earth asteroids as stepping stones on the way to Mars, he said. Thousands of sizable space rocks pass by Earth as close as the moon each year at least one 10-meter-wide asteroid comes within that distance each week.

Near-Earth asteroids are the most accessible interplanetary stepping stones to Mars, Binzel wrote. Why retrieve an asteroid when we can wait for one to come near us?

Ideally, NASA would aim for increasingly distant targets. The first missions could cross short distances and last a few weeks, and build to months-long missions sending humans farther into space. Each would be a learning opportunity to test technology that could eventually transport astronauts to the Red Planet.

But if asteroids were to be used as stepping stones, their movements would need to be predicted well ahead of their Earth flybys, Binzel said. A 2005 federal law requires that NASA find 90% of hazardous asteroids (140 meters or larger) by 2020. That effort would require $200 million per year which may seem like a lot, but is a fraction of the billions that the Asteroid Redirect Mission would cost, he said.

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NASA's plan to capture an asteroid? Bad idea, scientist says