Spaceflight

At T-0, the shuttle's twin solid rocket boosters ignite, and the spacecraft lifts off. About 10 minutes later, the astronauts are in a low orbit, traveling at 25 times the speed of sound and lapping the planet once every 90 minutes. The astronauts' destination -- the space station -- is circling some 240 miles (384 km) above Earth, but the shuttle is behind the station and must catch up. To close the distance, the astronauts conduct periodic firings of the shuttle's on-board thrusters.

The astronauts reach the space station three days after launching from KSC. The final approach, from behind and below the station, is slow and methodic. Constant air-to-ground communications are maintained as the shuttle makes a giant loop around to the top of the station and, slowly attaches to the docking port. Once the shuttle and station are fully mated, the two crews can mingle. There's a brief celebration, but the astronauts have a very full schedule and get to work almost immediately.

Because of zero gravity (or microgravity, to be more precise), working in space is quite different from working on Earth. Astronauts must get used to being weightless, which causes bone and muscle deterioration and requires that everything loose -- including sleeping astronauts -- be tied down. Eating, drinking and using the bathroom are especially challenging activities for astronauts in orbit. Over the years NASA has designed ingenious solutions that make living in space as comfortable as possible.

While in orbit, astronauts spend most of their time in the relatively safe confines of the shuttle or space station. Many missions, however, require a spacewalk, perhaps to deploy a satellite or make repairs. During a spacewalk, an astronaut must wear a space suit -- what NASA calls an extravehicular mobility unit (EMU) -- to protect and sustain him or her in the vacuum of outer space. Each EMU has a hard upper torso, a lower torso assembly and legs. A portable life support system, or PLSS, integrates fully with the suit and is worn like a backpack. The weight of the EMU-PLSS assembly is considerable. The suit itself weighs about 110 pounds (50 kg), the PLSS about 310 pounds (141 kg). For this reason, NASA designed EMUs for work in weightless conditions only, where the weight of the suit itself is unimportant. The Apollo suit, by comparison, was much different. Including the life support backpack, the Apollo suit weighed about 180 pounds (82 kg).

Most shuttle missions last two to three weeks. Usually, one shuttle astronaut will trade places with one of the astronauts on the space station at the end of the mission. Those returning to Earth board the shuttle and prepare for departure. Before undocking, the shuttle commander will generally bid farewell to the station commander. Then the shuttle will spring loose from the docking port and back gently away from the station. A final lap allows shuttle crew members to snap pictures of the station. Then it's back to Earth.

Re-entry can be quite dangerous. On the next page, you'll read about the challenges astronauts face as they return home.