Former astronaut Jim Voss raised sharp questions today about why a third crewmember will be aboard Boeing’s Starliner test flight unless it is essential. During a discussion at a NASA advisory committee meeting, NASA officials said that Boeing astronaut Chris Ferguson, who is assigned to the test flight, is also in training for an extended ISS mission, but will fly whether or not he is needed on ISS. Voss insisted there is no reason to risk a third crew member unless it is absolutely necessary to the continuity of ISS operations.

Boeing’s Starliner is one of the two commercial crew systems being developed through public-private partnerships to ferry NASA astronauts to and from the ISS. SpaceX’s Crew Dragon is the other.

Both systems must undergo first an uncrewed demonstration flight test and then a crewed demonstration flight test before they are certified for operational use. The original plan was for each to carry two astronauts. SpaceX does not have its own astronauts, so the two for Crew Dragon would be from NASA. Boeing hired former NASA astronaut Chris Ferguson and wanted to have him on board to flight test its new vehicle, with the second crew member from NASA. Ferguson flew three shuttle missions while at NASA.

Earlier this year, however, NASA revealed that it had modified its contract with Boeing to allow its demonstration flight to carry three people — two from NASA and one from Boeing. Last month, NASA announced the flight crew assignments for the commercial crew missions. For Starliner’s test flight: Ferguson and NASA astronauts Eric Boe and Nicole Mann.

NASA explained in April that the decision to fly three crew members on Starliner is a hedge against the possibility that it and Crew Dragon will not be certified before NASA’s contract with Russia to ferry astronauts back and forth ends next year.

At the meeting of the NASA Advisory Council’s Human Exploration and Operations (HEO) Committee today, Bill Gerstenmaier, the head of human spaceflight at NASA, said SpaceX’s contract similarly may be modified.

Operation of the ISS requires at least one Russian and one American to be aboard. NASA must have a way to get its astronauts there and home again, so waiting until the flight tests are completed and certification granted could be problematical if the flights are delayed beyond when NASA can use Soyuz for those purpose.

If the schedules are not delayed, however, and there is no need for Starliner crew members to remain aboard ISS, Voss wanted to know why NASA would fly more people than are necessary on a test flight. Why increase the human risk when only two people are needed to fly the vehicle? Voss is a former NASA astronaut who flew on the space shuttle six times and spent 163 days on the ISS.

NASA commercial crew program manager Kathy Lueders explained that the program looked at the risks and decided it was more a question of the capabilities of the vehicle. It is being designed for the post-certification missions when it must accommodate four people.

She said the contractual requirement to the companies was for a crewed test flight. NASA did not set a minimum crew size. Boeing proposed one NASA astronaut and one Boeing astronaut, the latter reflecting the company’s tradition of always flying its own test pilot on every new vehicle it builds. NASA’s flight operations directorate, however, really wanted two NASA astronauts.

She assured Voss that NASA would continue to look at the risks for each vehicle, but Voss was not swayed. “I don’t think a third crew member is required to fly the vehicle” and unless NASA can demonstrate that having a third person does not increase the human risk “I don’t understand the rationale behind it.”

Gerstenmaier reminded the committee that NASA does not own these vehicles anyway. They belong to the companies and NASA is purchasing services from them. NASA’s goal is to be just one of many customers and the companies may market the vehicles however they choose. He added that NASA will work with the companies if they want to fly commercial astronauts to the ISS, but they will have to pay for whatever resources they use. That would include air, water, food, power or other essentials. Gerstenmaier said NASA has not yet determined what it will charge, but his point was that commercial crew is an entirely new paradigm from the days when NASA owned the vehicles.

Lueders painted an optimistic picture of the Starliner and Crew Dragon schedules, which were just updated earlier this month. The dates for both SpaceX and Boeing have slipped again. The new date for the crewed test flight of Crew Dragon is April 2019 and for Starliner is mid- 2019.

Correction: During the meeting, only Ferguson’s training for an ISS mission was discussed and this article stated that he might remain aboard ISS if needed. However, NASA later informed us that all three Starliner crew members are being trained as ISS crew members. All three will either perform the baseline test flight or remain for a 6-month ISS “expedition” mission. This article has been corrected accordingly.