In this fourth of five articles, space.com's Andrew Chaikin continues a decade-by-decade look at the 20^th century's most significant developments in space.

On April 12, 1981, 20 years to the day after Yuri Gagarin became the world's first space traveler, a new type of space vehicle stood ready for launch at the Kennedy Space Center. To the eye it appeared as an unlikely collection of forms: Something resembling an airplane, standing on its tail, was attached to a huge cylinder the size of a grain silo, which in turn was accompanied by a pair of slender rockets.

When the moment of launch came, however, everything worked perfectly. Seconds before liftoff, Columbia's three main engines ignited, followed by the two solid rocket boosters, and the shuttle roared toward orbit. In space, the astronauts discovered that the orbiter had lost some of its heat-protective silica tiles, raising concerns for the craft's fiery reentry into the Earth's atmosphere.

But Columbia performed, in Young's words, "like a champ," and the mission ended on April 14 with a picture-perfect landing on a desert runway at California's Edwards Air Force Base. Addressing a crowd of well-wishers Young declared, "We're really not too far, the human race isn't, from going to the stars."

Later in 1981,Columbia proved its reusability by flying a second mission, and in 1982 it flew three more. The orbiter's fifth flight, STS 5, was also the first operational mission of the shuttle program, and inaugurated the shuttle as a satellite launcher, leaving two commercial communications satellites in Earth orbit.

STS 5 also featured the first so-called mission specialists -- astronauts whose job was not to fly the orbiter but to carry out experiments and other tasks in orbit. Up to six mission specialists -- for a total crew of eight people -- could fly on the shuttle at once.

In February 1984, spacewalking shuttle astronauts tested a Buck Rogers-style jet-pack called the Manned Maneuvering Unit. In April of that year, spacewalkers Pinky Nelson and Ox van Hoften became the first on-site satellite repairers when they fixed the ailing Solar Maximum Mission astronomical satellite.

By late 1985, NASA was operating a fleet of four shuttle orbiters and setting a record pace of launches. That year nine shuttle missions were flown, and even more flights were planned in 1986. There was even a program in the works to fly ordinary citizens in space.

There was reason for the break-neck pace. The shuttle had not lowered the costs of access to Earth orbit. And for commercial launches, NASA was facing stiff competition from Europe's Ariane rocket, which enjoyed government subsidies and lower operating costs. NASA's original forecasts had called for a shuttle flight almost every week, and while no one at the agency believed that goal was attainable, some still talked of flying 24 missions per year. But that was not to be.

On January 28, 1986, the shuttle Challenger exploded 73 seconds after liftoff, killing its seven-member crew, including schoolteacher Christa McAuliffe. The disaster stunned the nation and shattered any illusions that spaceflight had become routine. For NASA, it brought the realization that the agency had been living too close to the edge.

Months of investigation by a presidential commission traced the Challenger accident to a bad seal in a solid rocket booster. And there was also criticism of the decision-making process that had cleared the shuttle for launch in unusually cold conditions. As NASA struggled to recover from the disaster, the United States scrambled to shift its satellite launching tasks to expendable rockets. Many payloads slated for shuttle launches, including the Hubble Space Telescope, would face years of delay.

For the Soviet Union, 1986 was the beginning of a new era in long-duration space missions with the launching of the first modular space station, called Mir ("Peace"). Mir, whose crews would make even longer stays, offered somewhat more room and more "creature comforts" than Salyut. It was also the first space station meant for continuous occupation.

Already, cosmonauts had set a new space endurance record of 237 days -- almost eight months -- aboard the Salyut 7 space station. In December 1987, Vladimir Titov and Musa Manarov arrived at Mir; they returned to Earth a year later.

Around this time, the Soviets were talking openly of plans to send humans to Mars. Before that could happen, however, long-duration missions in Earth orbit would have to find ways of keeping space travelers healthy -- physically and psychologically -- during months or even years in space.

To break the monotony of their space marathons, Salyut and Mir crews were visited occasionally by teams of cosmonauts, including a number of "guest cosmonauts" selected from various Soviet-bloc nations. They also received shipments of mail, supplies and gifts via un-piloted Progress spacecraft.

On Earth, meanwhile, Soviet space planners, who had feared the U.S. Space Shuttle might be used to drop nuclear weapons, were ready to inaugurate a shuttle of their own. Called Buran ("snowstorm"), it closely resembled the American shuttle orbiter, but had the ability to fly under remote control from Earth, and to make powered landings.

Buran was mated to a giant new booster called Energia for launch. But the high costs of military and civilian space programs were catching up with the Soviets, and Buran made only a single, un-piloted mission, in November 1988.

Funding was also an issue for the U.S. space program, especially in the area of space science. With most of NASA's budget going to support the space shuttle program, relatively little was left for robotic missions.

Although the twin Voyager probes continued to provide stunning data -- after surveying Jupiter they flew past Saturn, and Voyager 2 continued on to Uranus and Neptune -- few successors were in the works. When Halley's Comet made a rare appearance in the inner solar system in 1985-86, the U.S. was not among the nations who launched space probes for close encounters with the comet.

And in the aftermath of the Challenger accident, delays and cancellations meant that only two new planetary missions were launched during the 1980s: the radar-equipped Magellan Venus orbiter and the Galileo Jupiter orbiter, each dispatched by shuttle astronauts in 1989.

At the same time, NASA was working toward another of its cherished goals, a permanent space station in Earth orbit. By the time the space shuttle was flying again in September 1988, the agency had gained approval for the project. The station became an international venture as NASA joined forces with the European Space Agency, Canada and Japan. At the close of the decade, however, the station was mired in bureaucracy, its design and completion date uncertain.

The space station would continue to draw much of NASA's energies in the 1990s, as the agency struggled to redefine itself and its goals in space.