NASA can handle the across-the-board budget cuts it has received under sequestration in the current fiscal year, but if the cuts continue into the fiscal year that begins Oct. 1, the gap in U.S. human space exploration capabilities will widen.

William Gerstenmaier, associate administrator for human exploration and operations, told the Senate Commerce space subcommittee that sequestration cuts in the out years will mean “we can’t deliver the programs that we’ve committed to you we would deliver.”

“We can tolerate the [2013] sequester activity because we’re prepared, but if it continues into ’14, the programs I described to you, the timetables I described, I don’t believe we could continue to support at the levels we did, so this is really going to be tough for us moving forward,” Gerstenmaier told Sen. Bill Nelson (D-Fla.), chairman of the subcommittee.

That would mean the first flight test of the Orion crew capsule to gather data on its large heat shield might have to be postponed from 2014, as would the 2017 first flight of the heavy-lift Space Launch System (SLS), the first crewed flight of an Orion in 2021, and the new asteroid-capture mission outlined in the agency’s fiscal 2014 budget request.

Gerstenmaier repeated earlier descriptions of the asteroid mission as a way to advance the readiness level of the technologies — advanced solar-electric propulsion, life support, deep-space rendezvous and docking — that would be needed for exploration deeper into the Solar System.

Those follow-on missions also would slip, pushing an as-yet-unscheduled mission to Mars even further into the future.

Apollo 10 Commander Tom Stafford, a retired Air Force lieutenant general who headed the Synthesis Group that studied the requirements for a human mission to Mars at the behest of President George H.W. Bush, outlined some of those follow-on missions and the developments needed.

A prerequisite would be the large, liquid-hydrogen-fueled upper stage already on the books for advanced SLS variants.

Stafford, who orbited the Moon on Apollo 10 and later took part in the Apollo-Soyuz Test Project with the Soviet Union in 1975, said a return to the lunar surface “is by far the most interesting near-term challenge confronting mankind in space,” and would be a valuable training ground for human missions to Mars.

For that flight, Stafford said, the U.S. should resurrect the nuclear-thermal rocket technology it abandoned in 1973.

“It is possible to get to Mars without a nuclear rocket, but why would we try to do so,” Stafford said in his written testimony.

“Far from being an artifact of a science fiction movie, a nuclear upper stage is something we once had — a working, space-qualified nuclear rocket lacking only a flight test” that was intended to fly atop the Saturn V Moon rocket.