Human spaceflight is dangerous — and it’s about to get more so, according to former Johnson Space Center director Christopher Kraft, who says NASA is making a mistake by retiring the space shuttle.

Kraft has co-written a letter, endorsed by a number of Apollo-era NASA veterans and astronauts, contending that the international space station will become more hazardous for astronauts without the shuttle’s resources as an emergency backup.

“I think they’ve got their head in the sand,” said Kraft, who was NASA’s first flight director.

He said the shuttle’s robotic arm has no duplicate on the Russian Soyuz spacecraft, which NASA will need to rely on for several years as the private sector develops new vehicles for getting astronauts into orbit. Nor can the Soyuz permit two astronauts to conduct spacewalks simultaneously, Kraft said. Such spacewalks might be necessary if the station lost power or underwent decompression.

“In a worst-case scenario, deterioration and loss of systems on an abandoned ISS could result in an uncontrolled, catastrophic reentry with risks to populated areas around the world,” the letter states.

Robert L. Crippen, pilot of the first shuttle mission in 1981 and one of the letter’s endorsers, said the shuttle’s retirement will make it impossible to replace large components on the station should they fail. The Soyuz payload capacity is much smaller than the shuttle’s.

“If it runs into any significant problem, it could be the demise of the station,” Crippen said.

NASA strongly disagrees. The space station can support spacewalks if repairs are needed, and it has plenty of spare parts, the agency said. The Obama administration has funded a final shuttle mission — scheduled for Friday morning — to deliver a final batch of supplies.

“NASA has been planning for the retirement of the shuttle fleet for six years,” said William H. Gerstenmaier, associate administrator for space operations at NASA headquarters in Washington, “and a key part of that plan has been to ensure that the station is fully stocked with spare components. And for critical systems, more than one spare is prepositioned on orbit.”

Two of the three surviving shuttles have already been stripped of many components and are on their way to museums. Atlantis, the last shuttle, will be given the same treatment when it returns to Earth after its 12-day mission. The supply lines for crucial shuttle parts have been discontinued. Although Kraft and others are lobbying for the continuation of the shuttle program, the chances for such a reversal in policy are infinitesimal.

The hazardous nature of low-Earth orbit was driven home Tuesday morning when a piece of space junk — composition and origin unknown — hurtled past the space station at 29,000 mph. Astronauts took refuge in two Soyuz spacecraft that function as lifeboats, prepared to detach if necessary. The mystery object came within 1,100 feet of the station but caused no damage, and within minutes the astronauts resumed their normal activities.

The military has catalogued more than 19,000 pieces of space junk, including spent rockets, dead satellites and debris from orbital collisions. Because of the speeds involved, even a small object can pose a significant hazard to the station. Astronauts on the station had to briefly take shelter in a Soyuz in March 2009 when a tiny fragment of a rocket motor hurtled past within a few miles.

But risks go with the territory. After the space shuttle Columbia disintegrated in the atmosphere in 2003, the accident investigation report concluded that the shuttle, “and all human spaceflight, is a developmental activity with high inherent risks.”