The country's shuttle fleet is to be retired this year or next, leaving NASA without a spaceship of its own for years.
Glenn, who became the first American to orbit Earth in 1962, told NBC News that flying the space shuttle fleet beyond its currently scheduled retirement date in November would be the best use of taxpayer dollars.
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"We'll spend almost as much buying our astronauts seats on Russia's Soyuz as we would to keep the shuttles flying," Glenn said. "The cost of continuing shuttle is really very tiny compared to the $100 billion investment we've made in the station, and keeping shuttle flying, we'll have the biggest spaceship ever to carry seven [astronauts] and tons of cargo."
The 88-year-old Glenn speaks from the perspective of a seasoned politician as well as a spaceman. The Ohio Democrat served as a U.S. senator from 1975 to 1999, and made an unsuccessful run for the presidency in 1984. He flew on the space shuttle for a widely publicized mission in 1998, becoming the oldest human in orbit at the age of 77.
The retired senator-astronaut penned his concerns about America's future space effort in an eight-page open letter released on Monday.
"Why terminate a perfectly good system that has been made more safe and reliable through its many years of development?" Glenn asked.
Fear of failure
Glenn fears that a failure involving Russia's Soyuz craft, the only ship besides the shuttle capable of bringing astronauts to the space station, would almost certainly result in the abandonment of the station. In a worst-case scenario, the station would lose altitude and break apart in the atmosphere before it could be reoccupied. Debris from the multibillion-dollar complex, spanning twice the width of a football field, could endanger people on Earth.
"We're putting ourselves in line for a single-point failure ending the whole manned space program, and I don't think we should be putting ourselves into that position," said Glenn, a supporter of President Barack Obama.
The current schedule calls for the final shuttle flight to lift off in November. However, that last scheduled launch to the space station could conceivably be delayed until next year, depending on how NASA works out the logistics.
NASA would like the White House and Congress to fund yet another shuttle mission in mid-2011 to bring more supplies to the orbital outpost. Sen. Bill Nelson, an influential Florida Democrat who flew on the shuttle himself in 1986, signaled this month that provisions for an extra shuttle flight would be included an authorization bill now under consideration.
Glenn's comments suggest that still more shuttle flights could be scheduled to narrow the gap in America's spaceflight capability even further.
Glenn is not the first former astronaut to voice doubts about the plan to retire the space shuttles. Over the past couple of months, more than 20 astronauts have signed statements protesting NASA's current policy.
That policy, formulated by the Obama administration, calls for grounding the shuttles permanently, canceling the development of a crew-worthy Ares 1 rocket and relying instead on the Russians and commercial transports to service the space station.
The retirement plan dates back to 2004, when President George W. Bush laid out a vision to replace the shuttles with next-generation rocket ships capable of sending astronauts back to the moon by 2020. Obama sought the cancellation of the back-to-the-moon plan, also known as the Constellation program, after an independent panel concluded that it was unworkable based on current budget assumptions.
Instead, Obama is calling for a step-by-step development of NASA's deep-space capabilities, leading to a crewed mission to an asteroid by 2025 and flights to Martian orbit in the mid-2030s. In the meantime, operations on the space station would be extended to 2020.
Moonwalkers oppose policy
One of the best-known opponents of Obama's space policy is Apollo 11 commander Neil Armstrong, the first man to walk on the moon, who told two congressional hearings that the current plan would cede America's pre-eminence in spaceflight to other countries.
Commercial space companies such as California-based SpaceX are on track to start delivering cargo to the space station as early as next year, and they've said they could provide crew-worthy space vehicles for NASA's use in three years or so. But Apollo 17 commander Gene Cernan, the last man to walk on the moon, doubts those claims and has told Congress it could take 10 years to produce a private-sector spaceship safe enough for astronauts.
Many in Congress have been critical of the current spaceflight plan, in part because it could mean the dislocation of thousands of aerospace employees.
Current legislation bars NASA from canceling work on Constellation unless Congress approves, but the space agency is citing a legal provision known as the Anti-Deficiency Act to force cutbacks on Constellation-related work. Some NASA contractors say they will shift or lay off workers in response.
This report includes information from msnbc.com's Alan Boyle.