U.S. engineers have long wanted to fold up an airplane inside a rocket and send it on a mission to cruise through the atmosphere of Mars. They now have a new potential customer for the concept: the Pentagon's Defense Advanced Research Projects Agency (DARPA).

DARPA started a program earlier this year called Rapid Eye to demonstrate technology that would lead to development of a rocket-delivered unmanned plane to fly high over the site of a natural disaster or other "hot spot." The idea is to give the U.S. president a first, quick look at the scene, DARPA Director Tony Tether said, following his talk at the Geoint 2007 Symposium in San Antonio.

"We got this idea from NASA," Tether said.

Engineers from NASA's Ames Research Center in California and Langley Research Center in Virginia, who are working on competing concepts for "planetary aircraft," began speaking with DARPA officials earlier this year about adapting some of their concepts for needs on Earth.

Since the 1970s a succession of NASA engineers and scientists have dreamed about flying an unmanned airplane over the surface of Mars to take close-up photos and make other measurements. Dale Reed, a researcher at NASA's Dryden Flight Research Center, Edwards, Calif., who died in 2005, earned a patent in 1977 on his design for a foldable Mars airplane called the Mini-Sniffer. In 1999, then-NASA Administrator Dan Goldin revived the dormant idea by challenging NASA to fly a plane over Mars in time for the centennial of the Wright Brothers flight in 2003. Joel Levine of Langley twice proposed missions to deliver an airplane to Mars.

None of those proposals left the ground because of a combination of budget, scientific and technical challenges. Engineers at Ames and Langley have continued to work on their designs in wind tunnels and in high-altitude tests, and today the idea of folding a plane inside a rocket and deploying it once it reaches Mars no longer seems as technically radical as it once did.

"I think there's actually relative agreement now that it's technically feasible and not terribly risky," said Larry Lemke, a mission designer at Ames.

Tether agreed. "There's no reason why we can't do this," he said.

NASA might have inspired the idea, but DARPA has decided to approach the U.S. aerospace industry first through its Broad Agency Announcement process. That has led some NASA engineers to wonder whether or how they might be able participate.

"They will be involved," Tether said of NASA engineers. "But we're going out on our normal process of a broad agency announcement to industry [saying] that we want this capability."

On Mars, the airplanes that have been proposed would fly in the thin martian atmosphere at altitudes between 5,000 to 10,000 feet (1,524 meters to 3,048 meters). On Earth, that would be the equivalent of flying at 100,000 feet (30,480 meters), a perfect altitude for surveillance, Tether said.

The concept of using rockets to deploy sensors as rapidly as possible to any hot spot throughout the world also is a goal of the U.S. Air Force's Operationally Responsive Space initiative.

"We didn't have that in mind," Tether said, noting that DARPA's concept for Rapid Eye is to support "the president." Nevertheless, the same concept could be used to support military personnel in the field, Tether said.

In a real world scenario, an unmanned Rapid Eye system would provide quick surveillance until other sensors could be trained on the scene. Those could include sensors aboard the new Ultra Long Endurance Aircraft DARPA is designing, Tether said.

Those craft would be slow flying but they would able to stay aloft for up to five years and would be supported by an autonomous refueling system DARPA is developing, Tether said.

"It sounds funny, but imagine if we could do that. Who needs satellites?" he said.

A description of the program published in July said the goal of Rapid Eye is to develop a high-altitude, long-endurance, unmanned aircraft that can be rocket-deployed from the continental United States worldwide within one to two hours to perform intelligence, surveillance, reconnaissance, and communication missions.

The enabling technologies listed included inflatable, folding structures, stable and dense energy storage, and low-oxygen propulsion.