WASHINGTON — Two separate groups, one made up of former astronauts and the other of members of the board that investigated the 2003 Space Shuttle Columbia accident, have written a key U.S. lawmaker to endorse NASA’s plan to rely on commercially developed vehicles to ferry crews to and from low Earth orbit after the shuttle fleet retires next year.

In letters to Sen. Barbara Mikulski (D-Md.), both groups said commercial astronaut taxi services do not pose inordinate risks and may in fact have safety advantages by virtue of their inherent simplicity. Mikulski chairs the Senate Appropriations commerce, justice, science subcommittee, which is responsible for funding NASA.

In February, Mikulski said she would put astronaut safety first in considering U.S. President Barack Obama’s plan to scrap the Constellation program, which aims to replace the shuttle with new rockets and capsules that eventually would take astronauts to the Moon. Obama’s plan would rely on commercially operated vehicles to ferry astronauts to and from the international space station and focus NASA’s efforts on developing technologies for deep space exploration.

The plan has encountered considerable resistance from lawmakers and others invested in the Constellation program, many of whom have suggested that commercial services are not yet mature enough to be entrusted with astronaut safety. The Constellation program features rockets derived in part from the space shuttle’s giant solid-rocket motors and an Orion crew capsule capable of going to the space station and to deep space destinations.

In one letter, 24 retired astronauts, including Buzz Aldrin, the second man to walk on the Moon, said “the private sector, working in partnership with NASA, can safely develop and operate crewed vehicles to low Earth orbit.” The astronauts cited the simplicity of the mission as well as the fact any rockets that would be used to launch astronauts either have proved themselves already or will have proved themselves by the time they are used for that purpose.

“By focusing on a simple spacecraft intended only to service low Earth orbit, commercial providers will avoid both the complexity of the Space Shuttle and the more extreme environments encountered by vehicles designed for exploration beyond,” the astronauts said in the undated letter, a copy of which was provided to Space News July 14.

The letter also said NASA would exercise strong oversight of any commercial crew taxi service. “Industry is now working with NASA on a human-rating plan that will optimize safety and specify NASA insight and oversight, and we are confident NASA will finalize these human rating requirements in a timely fashion,” the letter states.

The letter closely follows another one sent to Mikulski by five of the 13 members of the Columbia Accident Investigation Board (CAIB) that makes some of the same points. That group said Obama’s plan is not at all inconsistent with the CAIB’s findings, one of which was that NASA’s culture contributed heavily to the tragedy.

“In particular, we have been somewhat surprised to learn that some people, both within and outside of the Congress, have interpreted the new White House strategy for space which gives a greater role to the commercial sector in providing crew transportation services to the International Space Station, as being not in line with the findings and recommendations of the CAIB,” states the July 12 letter. The letter was signed by: Scott Hubbard, former director of NASA’s Ames Research Center in Mountain View, Calif.; John Logsdon, professor emeritus at the George Washington University here; Doug Osheroff, a Stanford University physics professor and Nobel laureate; Steven Wallace, former director of the Federal Aviation Administration’s Office of Accident Investigation; and Sheila Widnall, a former U.S. Air Force secretary.

The letter asserts that commercial spacecraft have the potential to be safer than Constellation’s Ares 1 rocket and Orion crew exploration vehicle.

“In contrast to Ares 1 and Orion, intended to both bring crews to the Space Station and to launch complex exploration missions, the commercial services under consideration as part of the President’s new plan are focused exclusively on the mission of safely transporting humans to low Earth orbit,” the letter states, adding that NASA is likely to use proven U.S. Air Force launchers to deliver privately developed spacecraft to orbit in addition to new privately built rockets.

“We anticipate that newer commercial vehicles such as Falcon 9 will also build up a strong history of cargo-carrying launches over the next few years,” the letter states, a nod to NASA’s 2006 agreement with Space Exploration Technologies Corp. under which the Hawthorne, Calif.-based company is developing the Falcon 9 rocket and Dragon space capsule to deliver cargo to the international space station.

In the letter, copies of which were sent to Senate Commerce Committee Chairman Jay Rockefeller (D-W.Va.) and Sen. Bill Nelson (D-Fla.), who chairs that panel’s science and space subcommittee, the CAIB members also dismiss the notion that only a NASA-led effort could provide the safety assurance needed for launching astronauts.

“We must note that much of the CAIB report was an indictment of NASA’s safety culture, not a defense of its uniqueness,” the letter states.

Although the CAIB members commended efforts by NASA leaders to avoid the kinds of organizational failures that led to both the Columbia accident and the 1986 Space Shuttle Challenger tragedy, “one might argue that the similarities in the organizational cause of both Challenger and Columbia suggest that it is very difficult for a single organization to develop, oversee and regulate such a complex human-rated spacecraft for an extended period of time,” the letter states. NASA’s operational experience with the shuttle does not preclude others from successfully developing and operating a human spaceflight capability, as evidenced by the Russians and Chinese, the letter states.

“We see no reason why a well-crafted NASA-industry partnership cannot match, or perhaps exceed, past performance in ensuring astronaut safety,” the letter states.