WASHINGTON — An independent safety panel warns that “a continuing and unacknowledged accretion of risk” in NASA’s human space exploration programs, caused by schedule pressures and flat funding, could put crews on future missions in jeopardy.

NASA’s Aerospace Safety Advisory Panel (ASAP), in its annual report published Jan. 13, stated it has growing concerns about a variety of issues in the development of the Space Launch System and Orion spacecraft that result in “an apparent erosion of safety” in those programs.

“Over the past year, the Panel has noted a continuing and unacknowledged accretion of risk in space flight programs that we believe has the potential to significantly impact crew safety and the safe execution of human space missions,” the report stated. “The Panel’s concern is not the result of singular action but the accumulated impact of decisions made and risks assumed — either explicitly or tacitly, in small or large steps — that have mounted up and led to an apparent erosion of safety.”

One specific area of concern ASAP raised in its report is the schedule for Exploration Mission 2 (EM-2), the second flight of the SLS and the first intended to carry a crew. In September, NASA announced that mission had a 70-percent chance of being ready for launch by April 2023, but that NASA would continue to work towards a launch in 2021, as originally scheduled.

ASAP criticized that approach. “NASA’s internal direction to the programs is to work to a 2021 EM-2 launch date, which has a schedule confidence level close to zero at requested funding levels,” the report stated, adding that NASA appeared to be making “safety trade-offs” to make that date.

The report cited several specific issues with SLS and Orion development and planning for the EM-2 mission, including development of a new SLS upper stage called the Exploration Upper Stage and testing of the life support system in Orion on that mission. The report suggested some of those concerns could be eased if NASA kept Orion in Earth orbit during that mission, rather than go into cislunar space as currently planned.

“This decision,” the report said of NASA’s current EM-2 plans, “reflects an aggressive development plan that takes the Exploration System from qualification testing to integrated human operations in cislunar space in just two missions.”

Other issues raised by ASAP include revisions to a planned in-flight abort test of Orion, changes to the spacecraft’s heat shield and “zero fault tolerant” systems in Orion’s service module, such as propellant valves, that could result in catastrophic failures of the overall spacecraft.

ASAP noted that SLS and Orion have received more money from Congress than what NASA has requested in recent years, but that a flat funding profile has contributed to development issues the panel found. “This distribution of resources reflects one more typically observed in ‘level-of-effort’ programs rather than a budget constructed to achieve the needed design efforts of a major program’s discrete and integrated requirements,” the report stated.

ASAP, which in past years has been critical of NASA’s commercial crew program, was more supportive of it in this year’s report. The panel cited “substantial improvement in openness and interaction” with program management, a concern it raised in its 2014 report. It also endorsed keeping two companies in the program, despite budget pressures.

However, ASAP warned that “significant challenges” still exist for the commercial crew program and the two companies, Boeing and SpaceX, currently developing crew vehicles. The panel concluded there is “a high likelihood of delays to the first test flights” scheduled for 2017, citing ongoing delays in completing reviews of the companies’ plans to certify their vehicles for carrying NASA astronauts.

“The Panel’s concern is that over time, schedule or budget pressure — or both — will lead the [NASA commercial crew] Program Office to accept more risk than desirable for crew and mission safety,” the report stated.

ASAP also cast a skeptical eye on NASA’s overall plan for sending humans to Mars, known by the agency as the “Journey to Mars.” The panel criticized a lack of technical details and mission architectures in a NASA report released in October about the Journey to Mars. Providing more details, the panel argued, “would go a long way toward gaining the needed support from future administrations, the Congress, and the general public.”

“If not,” the panel added, “then perhaps NASA should be working on a different mission, or at least using a different approach for the current mission.”