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In less than three years, NASA could be testing a nuclear rocket in space.

The space agency and the Defense Advanced Research Projects Agency, or DARPA, announced on Wednesday that Lockheed Martin had been selected to design, build and test a propulsion system that could one day speed astronauts on a trip to Mars.

BWX Technologies, based in Lynchburg, Va., will build the nuclear fission reactor at the heart of the engine.

The $499 million program is named DRACO, short for the Demonstration Rocket for Agile Cislunar Operations.

A Number That Sums It Up: 3 to 4 months to Mars

What if a spacecraft could get to Mars in half the time it currently takes?

Every 26 months or so, Mars and Earth are close enough for a shorter journey between the worlds. But even then it is a pretty long trip, lasting seven to nine months. For most of the time, the spacecraft is just coasting through space.

But if the spacecraft could continue accelerating through the first half of the journey and then start slowing down again, the travel time could be slashed. Current rocket engines, which typically rely on the combustion of a fuel like hydrogen or methane with oxygen, are not efficient enough to accomplish that; there is not enough room in the spacecraft to carry that much propellant.

But nuclear reactions, generating energy from the splitting of uranium atoms, are much more efficient.

The DRACO engine would consist of a nuclear reactor that would heat hydrogen from a chilly minus 420 degrees Fahrenheit to a toasty 4,400 degrees, with the hot gas shooting from a nozzle to generate thrust. Greater fuel efficiency could speed up journeys to Mars, reducing the amount of time astronauts spend exposed to the treacherous environment of deep space.

Nuclear propulsion could also have uses closer to home, which is why DARPA is investing in the project. The technology may allow rapid maneuvers of military satellites in orbit around Earth.

Background: Back to the future

Nuclear propulsion for space is not a new idea. In the 1950s and 1960s, Project Orion — financed by NASA, the Air Force and the Advanced Research Projects Agency — contemplated using the explosions of atomic bombs to accelerate spacecraft.

At the same time, NASA and other agencies also undertook Project Rover and Project NERVA, efforts that aimed to develop nuclear-thermal engines similar in concept to those now being pursued by the DRACO program. A series of 23 reactors were built and tested, but none were ever launched to space. Until the end of this program in 1973, NASA had contemplated using nuclear reactors to propel space probes to Jupiter, Saturn and beyond, as well as to provide power at a lunar base.

“The technical capabilities, including early safety protocols, remain viable today,” Tabitha Dodson, the DRACO project manager, said in a news briefing on Wednesday.

A key difference between NERVA and DRACO is that NERVA used weapons-grade uranium for its reactors, while DRACO will use a less-enriched form of uranium.

The reactor would not be turned on until it reached space, part of the precautions to minimize the possibility of a radioactive accident on Earth.

“DRACO has already done all of our preliminary analyses across the entire spectrum of possibilities for accidents and found that we’re all the way down in the low probability and all the way down in the teeny tiny amount of release,” Dr. Dodson said.

What Happens Next: A test flight in orbit

The DRACO development is to culminate with a flight test of the nuclear-thermal engine. Kirk Shireman, a vice president at Lockheed Martin, said the launch was currently scheduled for late 2025 or early 2026.

The demonstration spacecraft would most likely orbit at an altitude between 435 and 1,240 miles, Dr. Dodson said. That is high enough to ensure that it stays in orbit for more than 300 years, or long enough for radioactive elements in the reactor fuel to decay to safe levels, she said.

Kenneth Chang has been at The Times since 2000, writing about physics, geology, chemistry, and the planets. Before becoming a science writer, he was a graduate student whose research involved the control of chaos. More about Kenneth Chang

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