MILAN — NASA is rethinking its desire for a “continuous human presence” in low Earth orbit during the transition from the International Space Station to commercial platforms to include options where, at least temporarily, the agency may not always have astronauts in orbit.

In a talk at the International Astronautical Congress here Oct. 16 on NASA’s development of a low Earth orbit microgravity strategy, Deputy Administrator Pam Melroy mentioned what she called the “elephant in the room” of a desired continuous human presence in LEO. NASA has had astronauts in orbit on the ISS continuously since 2000.

“Continuous human presence: what does that mean?” she said in a discussion of feedback NASA received to a draft of that strategy published in August. “Is it a continuous heartbeat or a continuous capability? While we originally hoped that this would just emerge from this process, we’re still having conversations about that.”

That comment puzzled some attendees, who had assumed that continuous human presence meant having people in space continuously. In a later briefing, she called that concept “continuous heartbeat.” There are reasons to have people in orbit continuously to conduct research, she said, and concerns science would be lost if there was a gap in human presence.

“In addition, there’s a national posture element to it, to not have humans on orbit after what would be nearly 30 years of continuous presence,” she said, referencing the planned retirement of the ISS in 2030.

An alternative approach, “continuous capability,” would mean retaining the ability to have humans in orbit even without maintaining a continuous presence. That could be an option if initial commercial space stations have limited resources.

“We know that our partners are going to evolve. We didn’t build the space station overnight and they won’t either, so they will have limited capabilities to start with,” Melroy said.

That could support concepts where commercial stations were crew-tended initially but could later expand to having crews on board continuously as the stations, and their business cases, evolve.

“Do we need continuous heartbeat to achieve our objectives, or could we live with something like a crew-tended capability and maybe evolve to a continuous heartbeat?” said Robyn Gatens, ISS director at NASA Headquarters, at the briefing.

Melroy said agency thinking was “starting to converge more towards continuous heartbeat,” which would also sustain the business models of companies providing crew and cargo transportation to commercial stations. “It doesn’t matter if you have a space station if there’s no way to get there.”

That analysis will support not just NASA’s LEO microgravity strategy but also plans for the next phase of the Commercial LEO Development (CLD) program at the agency to support development of commercial space stations. That second phase will fund one or more companies to certify their stations for NASA astronauts and to purchase services on those stations.

NASA plans to issue a request for proposals for phase two of the CLD program in 2025, Gatens said, with awards in 2026. How many companies win awards will depend on budgets and the proposals the agency receives, she added.

The agency still desires a phased transition from the ISS to commercial space stations where there is an overlap between the deployment of the first commercial station and the retirement, and deorbiting, of the ISS.

“This gets to the continuous heartbeat discussion,” Melroy said at the briefing when asked if there was flexibility in the retirement date of the ISS. She noted that the U.S. Deorbit Vehicle (USDV), the spacecraft that will conduct the final maneuvers to reenter the station, has a “timer clock” that limits its on-orbit lifetime to one and a half years. “There’s no way we should launch something that has a ticking clock until you have another capability that you’re sure of.”

“As soon as we have that minimum capability that we need, we will deorbit the ISS,” Gatens added. That minimum capability, she said, was the USDV and at least one commercial station. “Those two conditions need to be met.”

The discussion about continuous human presence is one part of a broader evaluation about feedback to the strategy, which outlines 42 goals in areas such as science, technology development and international cooperation that NASA wants to pursue in LEO even after the end of the ISS.

The strategy uses the same approach that NASA took in the development of its Moon to Mars Architecture, including soliciting feedback from stakeholders. Melroy said that NASA received more than 1,800 responses to the draft LEO microgravity strategy and conducted two workshops, one for international partners and one for those in industry and academia.

NASA is reviewing those comments and incorporating them into a final version of the strategy, to be published by the end of the year. Those comments, she said in her talk, included a desire to provide more emphasis on space sustainability and to better explain why humans need to be in LEO.

“Everyone wants to see themselves front and center, in these goals and objectives,” Gatens said at the briefing, with feedback from some international partners and scientists who said the goals didn’t emphasize their areas of interest enough. Others noted a desire to better explain what NASA meant about its goal for “sustainable destinations” in LEO included in the draft report, such as the cadence of missions or their cost.

“People really appreciated us doing this,” she said of the feedback on the strategy. “The feedback that we’ve gotten has been very constructive.”