NASA had a program called the Space Experiment Module in which students all across the country, from K through 12, got a chance to place simple experiments aboard the space shuttle. These experiments mostly tested how various objects respond to spaceflight: One infamous SEM experiment determined how well cotton candy stood up to the radiation and weightlessness of outer space.

These kids were in my Saturday astrophysics class at the Rose Center. When I told them we had been selected for the SEM program, they refused to believe me. Fifth grade kids didn’t get to send stuff into outer space! I finally convinced them that it was for real, that they really could design experiments and put them on the shuttle. And the children lit up as if they were going into orbit themselves.

As did I. I’m a child of the Space Age. My first childhood memory is of the flight of Apollo 8. I collected astronaut GI Joes and watched every episode of “Star Trek,” and couldn’t wait for the day when I would go into space. I fought my grandfather, who was sure the moon landings were faked, and my uncle, who boasted that the Atlantean race had been there before. For me, the question of whether the benefits from manned spaceflight outweighed the risks, or the expense, was hardly worth bothering over.

But as the years went on and we got further from that dream, I began to doubt, and then ultimately to oppose with righteous fervor, the entire concept of manned spaceflight. If we wanted to discover life on Mars, we didn’t need people in space to do it. And if I wasn’t going, I childishly didn’t want anyone to go.

After the Columbia disaster, the debate over whether there is a point to subsidizing manned spaceflight has been opened up again, with a vengeance. I’ve found, perhaps because of my students, that I’ve returned to my youthful passions. And I’ve come to feel that the value of manned spaceflight isn’t found at the bottom of a balance statement or on a spreadsheet.

The modern fascination with space travel dates back to the early 1600s, when the invention of the telescope showed mountains and plains on the moon, and revealed the starlike planets to be worlds in their own right. The 17th century was a great age of exploration, and it was only natural to want to extend our discoveries beyond Earth. Lacking the means to do so, scientists and popular writers visited the other planets through the printed page, in what would come to be known as the science fiction novel.

In the 1880s, a Russian engineer, Konstantin Tsiolkovsky, published designs for the first practical interplanetary spacecraft. Eighty years later, Soviet cosmonaut Yuri Gagarin became the first person to orbit Earth, going into space in what was essentially Tsiolkovsky’s rocket. This Cold War coup, followed a few weeks later by the Bay of Pigs embarrassment, practically forced President Kennedy into declaring a race to the moon to save face. The future, that great undiscovered country, was going to be explored and understood by taciturn men in white suits and Snoopy hats, going in peace and taking along all mankind.

But once we reached the moon in 1969, mankind said thanks and lost interest. When we beat the Russians, we killed our only coherent, easily understood justification for having people in space. The public increasingly became aware of problems on Earth that needed fixing, and that fomented a backlash against space exploration from which NASA has never fully recovered. If they can put a man on the moon, the saying went, why can’t they [cure cancer, end racism, clean this goddamn snow off my driveway]? Others were more direct: “I just can’t for the life of me see voting for monies to find out whether or not there is some microbe on Mars, when in fact I know there are rats in Harlem apartments,” said then Rep. Ed Koch of New York in 1971.

NASA’s darkest days, as far as manned spaceflight is concerned, were probably the six years from 1974 to 1980. There was only one manned mission in that time, a single Apollo capsule launched into low Earth orbit to dock with a Soviet Soyuz. It was a propaganda display of the new era of détente between the superpowers, but the congratulatory handshakes with our old enemies might just as well have been a way of saying thanks, we couldn’t have done it without you.

The development of the space shuttle in 1981 didn’t bring back Apollo-era public support for manned spaceflight. The shuttle was designed to fulfill NASA’s grandiose post-Apollo plans for the conquest of the solar system. It would assist in the creation of a moon base, a 100-man space station, and various trips to Mars. When those plans died, all reason for the shuttle died with them. The unmanned Viking mission to Mars in 1976, and the spectacular pictures of Jupiter and Saturn sent back by the twin Voyager probes in 1980, showed that humans were unnecessary cargo when it came to exploring the planets. The first few shuttle missions were greeted with interest, but soon most of the public didn’t even know when a flight was in orbit. NASA was to spend more than a decade trying to find a purpose for its new toy — thus giving ammunition to those who felt that billions of dollars were being wasted.

When space shuttle Challenger disintegrated during launch in January 1986, the naysayers came out again, and at the time I was one of them. Not only is space travel too dangerous for humans, we said, it’s also too expensive. In these days of limited space budgets, NASA spends far too much money on the shuttle. Space exploration could be done far more efficiently (not to mention safely) with unmanned robots, we said. The endless development of the space station Freedom, which spent its entire budget without launching a stick of hardware into space, only seemed to prove the point. By the end of the 1980s, it seemed that the shuttle’s only supporters were the people whose jobs depended on it and the politicians who were afraid to kill it completely.

But on Sept. 2, 1993, U.S. Vice President Al Gore and Russian Prime Minister Viktor Chernomyrdin signed an agreement for the two nations to cooperate in space. The international partnership seemed to breathe new life into the concept of human spaceflight. Suddenly the shuttle had something to do! America’s aborted space station was resurrected and renamed the International Space Station; it would be launched into an orbit accessible to both Cape Canaveral and Baikonur. Cosmonauts would fly on the shuttle and astronauts would fly to Mir, working under the control of each other’s space agencies, collaborating jointly on everything. And with a permanent human presence in space came a new category on the shuttle’s astronaut manifest: who was going up, and who was coming down.

By January 2002, it looked as if manned spaceflight had a rosy future after all. Shuttle missions to complete ISS and repair the Hubble Space Telescope had been planned out to 2006. Next-generation propulsion systems that could land humans on Mars were in the advanced planning stages. And NASA was getting ready to launch Columbia, its oldest shuttle, on an all-science mission, one that would contain experiments sent in by schoolchildren from all over the country.

It took some work to get the kids to design realistic experiments. A few kids wanted to send up personal items, like locks of their hair or pictures of their family, as souvenirs. One student wanted to pack the test tubes with sand and sell the individual grains on eBay.

We eventually came up with five ideas. Two experiments tested whether there was more ultraviolet light in space than on Earth. One experiment determined if photographic film was sensitive to space radiation. One experiment was designed to see if a tube full of Central Park air smelled any different after it had been in space. And the final experiment tested if a digital camera’s memory card would still contain pictures after being exposed to space radiation for two weeks. Of course, we already knew the answers to most of these questions, but that was OK; the official reason for the project was to help the students develop scientific thinking. The real lesson was to teach the kids that knowledge and curiosity could take them literally anywhere. It was a lesson I once knew but had forgotten.

What would become the last flight of Columbia, mission STS-107, was originally scheduled for July 2002. It finally launched on Jan. 16, 2003, almost a year after we wrapped the experiments. The mission ended last Saturday with a streak of light across the Texas sky.

The kids and I never got the chance to meet any of the astronauts on “our” flight, so I don’t know if their deaths will be anything more than abstract losses to these students, now in sixth grade. I think they’ll be most upset at the fact that their experiments won’t be coming back. I remember the students’ initial suspicion that the project was a sham, and I’m sure some of the kids will feel angry for letting themselves believe. Outwardly they’ll be little gangsta tough guys, pretending that they don’t care. Inside, though, some of these children will feel that their only chance to touch the stars has been forever taken away. Part of me also feels that way.

Imagining what might have been gives us some perspective. The Arthur C. Clarke and Stanley Kubrick timeline would have given us giant rotating space stations, moon bases, and manned missions to Jupiter two years ago. Early NASA timelines would have had us on Mars 17 years ago. Even a more modest timeline would have had ISS finished already.

Yet other timelines would have had us not going into space at all, or going for a little while and then stopping. Apathy, shortsightedness, isolationism or a host of other problems could have kept us out of space for good. As modest as our current program is, it is literally better than nothing.

Now the drums are beating again, calling once more for a withdrawal from manned spaceflight. The same old arguments — money, safety, other more pressing problems — will be dredged up. A new area of opposition to manned spaceflight will probably be centered on the fear of terrorism: Spending money in space, instead of on homeland security, will lead to more dead astronauts and more dead citizens!

But like all the other economic reasons to neglect space travel, this is a false dichotomy. We have the money to find Martian microbes and eliminate Harlem rats, if we really want to do both. If there were to be a march on Washington to redirect NASA’s entire budget to find a cure for AIDS or cancer, I would be at the head of the line. But that bargain will never take place; money diverted from the space budget will never be spent en masse to rectify anyone’s favorite cause. There’s no logical reason we can’t achieve a balance, spending an appropriate amount of money on homeland security, on eliminating AIDS, on unemployment insurance, and on manned spaceflight. The Senate is discussing this week the question of NASA’s future, and while there will no doubt be a shakeup at the agency, no draconian cuts are planned. The shuttle and the International Space Station will remain part of NASA’s mission.

There are some who ask what are the here-and-now, bread-and-butter reasons for building a space station. The answer is that there are none. Or at least none that could be achieved only by building a space station. The Apollo program didn’t give us Tang or Velcro, but at least it paved the way for cordless tools, scratch-resistant eyeglasses and all-weather radial tires. We won’t even get that from ISS. There is talk that ultra-pure drugs can be produced in orbit, and they can, but in a practical sense it’s just talk. New alloys, crystals grown in microgravity, even orbital nanotechnology will make for great experiments, but not great spinoff products anytime soon.

This is a problem because people continually try to compare our current system of human space exploration with the voyages of discovery made in the 1500s. They couldn’t be more wrong. Those voyages were commercial expeditions, intended to produce stupendous profits by obtaining tradable goods for a small number of shareholders. That’s not going to happen in space for some time to come. The only way manned space will show a profit for the foreseeable future is by taking multibillionaires and boy-band singers up for $10 million joy rides.

Contemporary manned spaceflight is more comparable to the sea voyages of Captain Cook, or of Charles Darwin’s Beagle, at the height of the British Empire. Those expeditions existed solely to gather scientific data from all over the world. Cook and others collected information on geology, geography, astronomy, botany, oceanography, magnetism; nearly every branch of science was explored. If their researches eventually showed a profit, that was icing on the cake. It’s no accident that the shuttles Endeavour and Discovery are named after Captain Cook’s ships. As Cook taught us how to travel over the Earth, and eventually lost his life in doing so, the astronauts and cosmonauts are teaching us how to travel through space.

Regarding safety, here’s a news flash: Space is dangerous. Inherently. While we will do everything we can to minimize the danger, traveling in space will never be as secure as sitting in your office. But that’s all right. We know it’s not safe, and some people are prepared to take that risk. We didn’t stop flying in space when three American astronauts died in a launch-pad fire in January 1967. We didn’t stop when a Soviet cosmonaut, Vladimir Mikhailovich Komarov, died a few months later as his spacecraft tumbled out of control during landing. We didn’t stop in 1971 when three more cosmonauts died during reentry. We didn’t give up after Challenger was destroyed. Throughout human history, we’ve never been content with just looking at far-off hills; there’s always someone in the village who wants to know what’s on the other side.

Soon after Columbia broke apart on Saturday, the following message appeared in my e-mail. It’s a quote from “Star Trek: The Next Generation,” and it’s been making the rounds. Its message is not to minimize the death of seven astronauts, but to remind us of what the point of human space exploration really is:

“If you can’t take a little bloody nose, maybe you ought to go home and crawl under your bed. It’s not safe out here. It’s wondrous; with treasures to satiate desires both subtle and gross. But it’s not for the timid.”