If mission specialist Michael Massimino is worried about performing death-defying repairs on the world’s most famous (and expensive) telescope, he does a convincing job of hiding it. Snug in the electric-orange space suit that he will wear aboard the space shuttle Atlantis, he riffs genially about his Brooklyn upbringing, the search for a great slice of New York pizza, and the absurdities of NASA lingo. He discusses some technical issues about the suit with his crewmates. He reflects on the history around him in this corner of Houston’s Johnson Space Center, where Apollo communications equipment was tested four decades ago. In short, he exudes effortless competence and exactly zero fear.

Massimino will need that moxie and know-how when he and his crew blast off this month from Cape Canaveral and rendezvous with the Hubble Space Telescope 360 miles above Earth’s surface. Once the astronauts secure the orbiting 12-ton observatory to Atlantis, they will embark on five arduous space walks to install a new camera and spectrograph and fix two other malfunctioning instruments to upgrade Hubble’s vision. The crew will also swap in new batteries and gyroscopes, attach a protective blanket, and repair the guidance system.

NASA has sent servicing missions to Hubble before, but this one is unique in complexity and finality. If the upgrades work, Hubble should keep scanning the cosmos until at least 2013. At that point, however, its gyros or batteries will fail and the heroic telescope will become so much orbiting junk. Three members of the Atlantis crew have visited Hubble previously. After this mission, no astronauts will ever return.

More significantly, this is the beginning of the end not just for Hubble but for space shuttles—indeed for the entire American manned space program as currently configured. NASA will fly the shuttle a few more times, mainly to complete construction of the all-but-forgotten International Space Station, but by the fall of 2010 the last shuttle will be retired. For the first time in 29 years, the United States will have no way to launch humans into space.

If all goes well, both the end of the Hubble and the end of the shuttle will create only temporary gaps. NASA’s next great space observatory, the James Webb Space Telescope (JWST), is well along in development, slated for launch in 2013. It might even overlap with Hubble, depending on how long its predecessor lasts. But JWST is not a true replacement. It is designed to study infrared rays—invisible radiation just beyond the red part of the spectrum. That makes it great for examining infant stars and distant galaxies, but it will not be able to take space portraits that capture what the human eye can see.

The space shuttle, too, has a successor waiting in the wings. The Ares rocket and Orion capsule, modeled on the Saturn rocket and Apollo module from the 1960s, should begin crewed flights in 2015. (Between 2010 and 2015, all manned American flights into orbit will take place aboard Russian spacecraft.) Ares and Orion will not have wings and will not be reusable, but they will be designed to carry humans beyond Earth’s orbit, as part of NASA’s planned return to the moon.

NASA’s future depends heavily on this month’s Atlantis mission. If the shuttle flight goes awry, Hubble is doomed and the already-precarious support for Ares and Orion might evaporate. (Last fall Sen. Barack Obama suggested delaying NASA’s next human space adventure to focus funds on more practical problems.) To ensure success, the astronauts have logged long hours in the Shuttle Mission Simulator at Johnson. In a nearby building, the spacewalking part of the team has practiced 13 rounds of Hubble repairs under water in the NASA Neutral Buoyancy Lab (NBL). And over at the God­dard Space Flight Center in Greenbelt, Maryland, the training has continued on a Hubble mock-up in the center’s 1.3-million-cubic-foot clean room.

In the practice sessions every step is plotted, every contingency accounted for. Yet space history is full of sad proof that humans cannot always master the complexities of their own inventions. Two of the space shuttle’s 123 flights have ended in fatal catastrophe; the Columbia disaster happened just a year after the previous Hubble repair mission.

One of the reasons Massimino and the rest of the Atlantis crew look so relaxed is that they know success does not ride on them alone. Everywhere they go they are supported by thousands of others—suit technicians and rocket engineers, NBL scuba divers and Johnson flight engineers—busting their behinds to make sure that the last great shuttle flight is an unmitigated triumph. The fate of the Hubble Space Telescope and the destiny of the next generation of space explorers lie in their hands.