Astronauts revisit the shuttle debate Retired senator-astronaut John Glenn is surrounded by other space veterans in front of the space shuttle Discovery during its handover to the Smithsonian at the Udvar-Hazy Center in Chantilly, Va., on Thursday. Glenn says the shuttles were "prematurely grounded" but accepts the shuttle program's end. For some veteran astronauts, today's transformation of the shuttle Discovery into a museum exhibit is a cause for celebration. For others, it's a reminder of their regrets. But for John Grunsfeld, the one-time "Hubble Hugger" who is now NASA's science chief, the dominant feeling was a sense of relief. Discovery's handover to the Smithsonian's National Air and Space Museum has re-ignited questions about the end of the 30-year space shuttle program. Why did they have to be retired? The short answer is that in the wake of the 2003 Columbia tragedy, policymakers decided that once the job of building the International Space Station was finished, it would just be too risky and expensive to keep the shuttles flying. Instead, President George W. Bush decided to re-target the space program on destinations beyond Earth orbit. For Bush, the first focus was going to be the moon. President Barack Obama shifted that initial focus to near-Earth asteroids, but the endpoint is the same: eventually getting to Mars. And the shuttles could never do that. They weren't built to go beyond Earth orbit. Nevertheless, some of America's best-known astronauts think the shuttles should have been kept around a while longer - particularly because NASA will be dependent on the Russians for rides to the space station for the next three to five years. 'Unfortunate decision' "The unfortunate decision eight and a half years ago to terminate the shuttle program, in my opinion, prematurely grounded Discovery and delayed our research," retired senator-astronaut John Glenn said during today's handover ceremony at the museum's Steven F. Udvar-Hazy Center in Virginia. Former senator-astronaut John Glenn speaks as the Smithsonian formally accepts space shuttle Discovery for permanent exhibition. Another retired astronaut who rode on Discovery, Tom Jones, voiced similar frustration during an interview conducted before today's ceremony. "I'm reliving the disappointment that the shuttles are retiring without a rapid successor," he told me. Jones wishes that the White House and Congress had revved up NASA's plan for new spaceships capable of going to the space station and beyond: the Constellation Program, which initially aimed to put U.S. astronauts back on the moon by 2020. Instead, Constellation was so cash-starved and technically challenged that the Obama White House scrubbed the program and reworked elements of it into the current plan to visit an asteroid by 2025. "We dropped the ball on this," Jones said. If we just went from 0.5 percent of the federal budget to 0.6 percent, this would all be a non-issue. The benefit of retaining an American system for resupplying the space station is what motivated Glenn's call to keep the shuttles flying. Glenn made his pitch to the White House in 2010 - but Obama didn't go for it, and the former Democratic senator told me today that he accepts the verdict. "No need crying over what happened in the past," Glenn said. Let's get on with the future. The 'Hubble Hugger' and his pin Grunsfeld thinks the White House made the right call, at least on the question of grounding the shuttles. He's best-known for his role as a spacewalker on Hubble servicing missions in 1999, 2002 and 2009. During that last mission, Grunsfeld was the one who bade the Hubble Space Telescope goodbye forever. Now he's NASA's associate administrator for science. The way Grunsfeld sees it, keeping the shuttles flying might have led to another disaster like the 1986 Challenger explosion - or the loss of Columbia and its seven STS-107 crew members in 2003. "There's a possibility we could have flown them for a little bit longer, or extended them at some cost," Grunsfeld told me. I'm actually extremely thankful that we are rolling Discovery into the Air and Space Museum, and not burying its parts. We flew out the space shuttle program gracefully. We didn't lose another one. It would have been tragic. The fact is that the space shuttle program was ended with dignity - it was an amazing accomplishment, and I'm just thankful for that. Then he shared what he called a "small, personal story." Just this morning, on my flight suit for the first time since the loss of Columbia, I took my STS-107 pin off. I felt like this was an apt celebration, that we flew out the program safely after Columbia, and that affected me very deeply," Grunsfeld said. Now that we are where we are, I'm looking forward to getting the next space vehicle going. The end ... and the beginning Retired astronaut Eileen Collins, who became NASA's first woman shuttle pilot during a 1995 mission on Discovery and went on to command shuttle missions in 1999 and 2005, has some firsthand knowledge about the risks associated with flying the shuttles. The 2005 mission on Discovery marked NASA's "return to flight" after the Columbia tragedy. She and most other people at NASA had thought they had solved the foam-loss problem that led to the Columbia's doom - but mission managers were shocked to see that the fuel tank shed a substantial piece of foam insulation during Discovery's ascent. No significant harm was done, but it took another year for NASA engineers to rework the problem to their satisfaction. This week, retired NASA shuttle manager Wayne Hale recounted the episode in a blog item headlined "How We Nearly Lost Discovery." Today, Collins noted that each shuttles was originally designed to fly for 100 missions or 10 years, whichever came first. Discovery, the most traveled of the shuttles, flew 39 missions ... over the course of 28 years. She recalled that she agreed with the shuttle retirement plan that was announced in 2004, but was disappointed when the Constellation Program was canceled in 2010. At that time, I would say yes, we should keep the shuttles flying - with one major exception. Back in 2006, we at NASA made major decisions to start shutting down the pipeline for parts. In 2010, to reverse the decision and continue flying the shuttles was going to be very expensive and take a very long time. So it wasn't realistic to fly them again," she told me. The worst thing we can do to our people is to constantly change things ... so in the end, the right thing to do was to fly out shuttle. I am personally very sad to see it go. But the big problem is, we don't have anything to follow on right now. We're going to get there. It's just that right now, we don't have it. It's not the end of the shuttle program that bothers Collins. Rather, it's the possibility that NASA won't be able to follow through on the beginning of the next program. "I don't want to see any more canceled programs," she told a school group after today's ceremony. If we have problems, we need to fix those problems and press on. We can't just cancel and walk away from them. I go to schools, and I talk to kids, and I say, 'If you have problems, stick with it, fix it, don't give up.' We don't want to continue to give up on programs that are going to be taking us out into space, whether it's with robots or with people. We need to keep working on those programs. Here's your chance to weigh in on the end of the shuttle program and the beginning of the next chapter in exploration. Just leave a comment below. More about what's next for NASA: