WASHINGTON — Getting astronauts to Mars in the 2030s could involve a pit stop at the planet’s largest moon, Phobos — a staging area where NASA could leverage both technology and lessons learned from its proposed asteroid retrieval mission, agency officials said here June 23.

“The moons of Mars pose a significant advantage as a waypoint on the way to the surface of Mars,” Jason Crusan, director of NASA’s Advanced Exploration Systems Division, said at agency headquarters here during a June 23 meeting of the NASA Advisory Council’s Human Exploration and Operations Committee. “If we choose to go through Phobos on our way to the surface of Mars, that environment of interacting with the asteroid will be nearly the same environment as interacting with Phobos.”

William Gerstenmaier, NASA associate administrator for human exploration and operations, clarified later in the meeting that between Mars’ moons, NASA considers only Phobos a potential waypoint for human explorers. The smaller martian moon, Deimos, has been ruled out, Gerstenmaier said.

During the meeting, NASA had on display a series of model spacecraft, including a crewed Phobos exploration vehicle that resembled a scaled-up version of one of the two uncrewed asteroid retrieval spacecraft concepts the agency will choose between later this year for the first stage of the Asteroid Redirect Mission — which would robotically retrieve an asteroid between 5 and 10 meters in diameter and haul it back to a distant lunar retrograde orbit. By 2025, astronauts aboard the Orion crew capsule, launched by the Space Launch System NASA is building, would fly to the asteroid to collect samples.

The Phobos exploration vehicle, Crusan said, could take advantage of some, if not all, of the technology NASA is developing for the robotic asteroid retrieval craft, including a bulked-up version of the high-power solar arrays the agency will use for the uncrewed leg of the asteroid retrieval mission. Operationally, however, the asteroid retrieval mission is a “direct analogue” for rendezvousing with Phobos, Crusan told the committee.

Crusan said a spider-like Phobos crew vehicle could be “tucked in” to the stony martian moon, which is roughly 22 kilometers in diameter and orbits only about 6,000 kilometers from the planet’s surface. That would provide Mars-bound astronauts with a radiation-shielded staging ground for descent to the planet’s surface.

The Phobos pit stop is only one of the elements of the strategy NASA trotted out for the NASA Advisory Council’s human spaceflight committee June 23. Crusan and Gerstenmaier also spoke of putting mini-space stations, comprising a Mars transfer habitat and pressurized crew vessel that could also be used for a Phobos-Mars astronaut taxi, in the same distant lunar retrograde orbit NASA’s eyeing for the asteroid retrieval mission.

The strategy outlined by Crusan and Gerstenmaier depends on technology and systems not yet a part of NASA’s budget. However, NASA’s strategy appears to satisfy one demand that House lawmakers made in a NASA authorization (H.R. 4412) bill awaiting consideration by the Senate Commerce, Science and Transportation Committee: explaining where the Asteroid Redirect Mission fits in a road map to Mars.

A road map to Mars is one of the key deliverables NASA will have to make to Congress should H.R. 4412 — which the House passed June 9 — become law. Even Democratic lawmakers on the House Science Committee are cautious about the mission and have declined to endorse it wholeheartedly until NASA shows how it helps get bootprints on Mars.

Gerstenmaier and Crusan filled in some of the blanks on NASA’s Mars road map only two days before a June 25 hearing of the House Science Committee, during which lawmakers discussed a report by the National Research Council’s Committee on Human Spaceflight. The 300-page report, unveiled June 4, concluded NASA’s so-called capabilities-based approach to human exploration — building adaptable, destination-agnostic hardware — will not get crews to Mars by the 2030s.

The co-chairmen of the NRC committee, Jonathan Lunine of Cornell University and former White House Office of Management and Budget Director Mitch Daniels, were the only witnesses summoned to the hearing.

Among other things, the NRC report concluded that returning to the lunar surface could help NASA reach Mars more quickly, by developing necessary technology and engaging international space agencies that already have designs on exploring the Moon.

NASA officials insist returning to the lunar surface is not a good use of taxpayer dollars. The House Science Committee’s chairman, Rep. Lamar Smith (R-Texas), disagrees and has been pushing for NASA to put lunar landing back on the road map to Mars.

Crusan, meanwhile, told the NASA Advisory Council that the projected 1 percent annual increase the White House has penciled in for the next five years for NASA’s roughly $4 billion human exploration account is not enough to reach the red planet by any road.

“We’ve been living in times of flat budgets,” Crusan said. “In the near-term that’s okay. In the long-term, that will be unsustainable.”

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.