It was another brilliantly sunny day for NASA astronaut Tom Jones. In orbit on his fourth space shuttle mission in February 2001, Jones was outside the International Space Station, installing a new laboratory module. He remembers the moment with great clarity: Gerhard Thiele, another astronaut, called from the ground to relay the news that the robotic NEAR Shoemaker probe had just made the first-ever landing on an asteroid, 433 Eros.

“There I was, turning bolts on the ISS,” says Jones. “I was thinking: What a cool job this is. But how much cooler would it be if I were doing this on an asteroid!”

The idea that astronauts might visit an asteroid and explore it up close had long intrigued him. Today, Jones is more convinced than ever that it would be a grand and worthwhile journey. “The asteroids,” he says, “are begging for a visit.”

By “the asteroids” he doesn’t mean one of the rocks circling out in the main belt between Mars and Jupiter, but something a lot closer to home: An Earth-crosser, or NEO (near-Earth object). A rogue.

Jones is part of an unofficial group of NASA actives and alums who have been studying, mostly on their own time, the particulars—engineering requirements, mission trajectories, scientific payoffs, and costs—of a human trip to an asteroid. Like the Mars Underground, a larger group of enthusiasts who for the past 20-plus years have been pushing for a voyage to Mars, the asteroid agitators are trying to build support for a mission. The two groups are far from mutually exclusive: Plenty of Mars Undergrounders share the desire to see Constellation, NASA’s human exploration program, send astronauts rock-hopping first.

The operational lessons learned from such an expedition would be crucial. “There’s no way a Mars program could take shape without a crewed mission to an asteroid,” says Jones. Aerospace engineer Robert Zubrin, who, as head of the international advocacy group the Mars Society, is one of the leading proponents of an expedition to the Red Planet, likes the logic of a shakedown flight to an asteroid. “I think it’s a valuable idea. It would help validate the Constellation hardware within a meaningful time frame,” he says. “Basically, it takes us farther out into space, and that’s good. Sort of like Columbus getting out there and saying, ‘There aren’t dragons out here after all.’ ”

Constellation’s primary destinations are the moon and Mars, but the asteroid hopefuls are lobbying to insert a third stop in the itinerary. For the record, NASA has no plans to send astronauts to a near-Earth object, and agency officials describe it as highly improbable given current budgets.

The Asteroid Underground is unfazed. According to Jones, “When you talk to an audience of taxpayers, they see the stepping stones: moon, asteroid, Mars.”

The idea of a mission to an asteroid is not new. In 1966, Eugene Smith, an engineer with Northrop Space Laboratories, conducted a study for NASA on the use of Apollo hardware, including the giant Saturn V rocket, to carry six astronauts on a flyby of Eros. The trip would have been scheduled for 1975, when the asteroid came within 13 million miles of Earth, more than 50 times the distance to the moon. The round trip would have been 500 days.

More recently, NEOs came to public attention in July 1994, when the comet Shoemaker-Levy 9 broke up and the pieces slammed into Jupiter, the largest packing a wallop equivalent to 400,000 times the power of the largest U.S. nuclear bomb ever exploded. Anyone who read a newspaper that summer imagined the same thing happening to Earth, and within two years an international organization called the Spaceguard Foundation was established to coordinate the tracking of asteroids and comets that might collide with the home planet.