The question that has evolved along with the space program itself is whether the scientific achievements are worth the risk of sending humans into space.

According to Marcia Smith of the resources, science and industry division of the Congressional Research Service in April 2003 testimony following the Columbia accident, even in the beginning of NASA's manned space program, when launch-related technology was still new, the public has endorsed sending humans into space.

"The risks were high in those early flights. We had little experience with launching rockets into space, and with the spacecraft that protected the astronauts," she told Congress. "Yet the nation was willing to accept those risks, and pay the cost, to ensure American preeminence. Indeed, only three weeks after Alan Shepard's flight, President Kennedy called on the nation to commit to the goal of landing a man on the moon by the end of the decade, and the nation said yes."

Since the first space shuttle launch in spring of 1981, more than 100 successful shuttle missions have followed, but two fatal accidents involving 14 shuttle crew members grounded the fleet for two years each time to correct the potentially fatal structural flaws.

After each accident -- and after the United States won the "moon race" with Russia -- Congress and NASA had to weigh the costs and benefits of continuing to human spaceflight, Smith said.

Congress considered the pros of ending the program, including saving $4 billion -- the annual operating budget of the shuttle program, and $2 billion -- the cost of the International Space Station, and the cons, such as losing the national prestige connected to the program and the $30 billion the United States already invested in the space station, she testified.

Each time, Congress has decided to continue to support manned missions, she said.

Nonetheless, whenever the space program experiences a loss of life, the debate rekindles over whether robots should perform most of the missions designed to advance people's knowledge of the universe.

"Robotic missions can only do things," former astronaut Mae Jemison said on ABC's Nightline on Feb. 3, 2003, two days after the Columbia accident. "They can only do things that you've already thought of, in terms of possibilities. Humans have the ability to be much more flexible, where you can change the possible experiments that are done. We see with a different eye."

Duke University professor and former NASA historian Alex Roland, however, said during the same program that although NASA has always believed that manned spaceflight is essential to any successful space program and that the American public and Congress wouldn't support a program without it, the space agency should consider sending more robots into space.

"I'm not saying we should get rid of all manned space flight, but I think the public would be tolerant of a lot more good space science and a lot less of astronauts floating around, tending experiments that don't amount to much," he said.

In her congressional testimony, Smith laid out the differences of opinion on the value of human spaceflight.

"Critics of human space flight believe that robotic probes can gather the needed scientific data at much less cost, and that humans contribute little to space-based scientific research. They point out that no ground-breaking scientific discoveries have emerged from 42 years of human space flight that can be uniquely attributed to the presence of humans in space," she said. "Proponents insist that human ingenuity and adaptability are essential for some types of basic research in space, and can rescue an otherwise doomed mission by recognizing and correcting problems before they lead to failures."

The debate bubbled up again when NASA was considering whether to fix the Hubble Space Telescope. Driven by public pressure, NASA agreed in August 2004 to send a robotic mission to service the telescope before its expected failure in 2007. The space agency deemed sending astronauts, which had been done in the past, as too risky in light of the Columbia disaster.

But the National Academy of Sciences in a December 2004 report backed the tried-and-true method of sending humans to repair the Hubble, rather than the untested use of robots, even in light of the Columbia disaster.

"A robotic mission is seen as just too risky given the state of technology and the time available to design, build and test the robotic craft," the report said.

James Van Allen, professor at the University of Iowa, expressed the misgivings people often feel over the issue. He wrote in Issues in Science and Technology that "the only surviving motivation for continuing human spaceflight is the ideology of adventure." But he cautioned, "I ask myself whether the huge national commitment of technological talent to human spaceflight and the ever-present potential loss of precious human life are really justifiable."

Despite these misgivings, the public has remained steadfast in its support of the space program, despite the fatalities. According to an Aug. 18, 2003 USA Today report, "Since the Columbia disaster, Americans have rallied behind the space program. A USA Today/CNN/Gallup Poll shows support for increasing NASA funding to levels not seen since the 1980s."

Bolstered by this public support, politicians have also continued their backing of the shuttle and other programs.

"Support for a manned space program has always been strong, and no president has been willing to cancel it," Roger Launius, a historian with the National Air and Space Museum and former senior NASA historian, told the Houston Chronicle.

But in the end it is the astronauts who must make the fateful decision whether to hurtle themselves into space. Former astronaut Rick Hauck, who spoke at the Woodrow Wilson International Center for Scholars, said 18 of the 430 humans who have flown in space have died -- 14 on two shuttle missions and four on two Soyuz flights -- which equals a fatality rate of 4 percent.

"Would I have flown if I had known there was a four percent chance of death? No, I don't think I would have flown.," he said.

But ultimately, he continued, it should be up to the astronauts to decide if the risks are worth it. "The crew will have to be convinced that all reasonable measures have been taken to minimize risk."

-- By Chris Han, Online NewsHour