WASHINGTON — Several high-profile former NASA officials and a half-dozen astronauts made an eleventh-hour plea to save the space shuttle fleet from becoming museum artifacts, even as NASA Administrator Charles Bolden publicly touted the agency’s plans for crewed exploration after the orbiter’s retirement.

The former NASA hands made their case in a June 30 letter to Bolden, warning the space agency’s chief that retiring the shuttle fleet “will create an unacceptable flight risk for maintaining safe and reliable operations of the International Space Station (ISS).”

If critical life- or mission-support systems aboard the station failed, the letter writers said, “shuttles are the only spacecraft that can provide independent spacewalks for critical ISS repairs.”

Other craft, they said, lack the power-generation and life-support capabilities needed to support such extravehicular repairs.

The letter was drafted by Christopher Kraft, former director of the Johnson Space Flight Center in Houston, and Scott Spencer, a transportation management consultant and one-time Democratic congressional hopeful from Wilmington, Del. Among those endorsing the letter were Apollo flight director Gene Kranz and six former astronauts, including Apollo 11 commander Neil Armstrong, Apollo 13 commander James Lovell and Robert Crippen, commander of NASA’s first space shuttle flight.

The former NASA officials called on Congress to immediately conduct emergency hearings on the issue of long-term ISS safety. The group also appealed to NASA’s international partners to help the agency develop “funding solutions” for more shuttle flights.

NASA is retiring the space shuttle fleet after Atlantis completes the STS-135 mission.

Meanwhile, Bolden, in a July 1 speech at the National Press Club here, reiterated the space agency’s strategy to outsource flying human beings to low Earth orbit.

“We have to get out of the business of owning and operating low Earth orbit transportation systems and hand that off to the private sector,” Bolden said in prepared remarks.

Bolden said that NASA now will focus on exploring destinations beyond Earth orbit with the shuttle’s successor: a government-owned “deep space crew vehicle and an evolvable heavy-lift rocket.”

The former is the Multi-Purpose Crew Vehicle, a $1.2 billion space capsule based on the Orion capsule Denver-based Lockheed Martin Space Systems has been working on since 2005. The latter is the Space Launch System, the congressionally mandated heavy-lift rocket that, according to government and industry sources, NASA has proposed the White House build using leftover space shuttle infrastructure and contracts awarded for the canceled Ares 1 rocket.

NASA has yet to unveil its proposed design  for the rocket. Bolden provided no details in his speech about the planned launch vehicle, saying only that the agency would reveal its chosen design “soon.”

The Space Launch System that NASA has been ordered to build would be capable of launching cargo to the space station but would not do so unless commercial transporters Space Exploration Technologies Corp. (SpaceX) and Orbital Sciences Corp. are unable to complete the cargo deliveries NASA has contracted them to carry out.

The same day that Bolden made his speech at the press club, a nonprofit group formed by the families of the astronauts killed aboard Space Shuttle Challenger put out its own statement in support of NASA’s human spaceflight plan.

“We, the families of the Space Shuttle Challenger crew … strongly support the continuation of human spaceflight under a new paradigm of commercially led efforts to low earth orbit, and government led efforts beyond,” the Challenger Center for Space Science Education said in a statement. The group, based in Alexandria, Va., promotes science and mathematics education.

The final shuttle mission draws the curtain on an era of human spaceflight that was defined in large part by the construction and maintenance of the international space station — Earth’s only remaining orbital outpost. The space shuttle, which can launch 24,400 kilograms to low Earth orbit, enabled the space station’s construction. It is one of two spacecraft to fly astronauts to the ISS. The other, Russia’s Soyuz capsule, will be NASA’s only source of crew transport to ISS until commercially developed alternatives arrive.

Under the terms of the current NASA authorization law, the ISS will remain in service until at least 2020. NASA plans to hand off responsibility for supplying the station to commercial cargo transporters. However, these commercial resupply runs cannot take place until the contractors, SpaceX and Orbital, prove that their spacecraft can make the trek to the ISS.

SpaceX, Hawthorne, Calif., completed the first of its three demonstration milestones in December when it launched its Dragon cargo capsule into orbit aboard its Falcon 9 medium-lift rocket. The company now must demonstrate that the Dragon can communicate and dock with the ISS. SpaceX wants to complete those milestones on a single test flight, and NASA has signaled that it will allow this, even though the milestones are currently scheduled to be demonstrated on separate flights.

Dulles, Va.-based Orbital Science is to launch its own cargo hauler, the Taurus 2 rocket and Cygnus space tug, from the Mid-Atlantic Regional Space Port at NASA’s Wallops Flight Facility on Virginia’s eastern shore, this October. A demo flight of the company’s Cygnus cargo capsule is to follow in December.

Orbital has held to those dates in spite of the June 9 test-stand fire that badly damaged one of the Taurus 2’s main engines.

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.