In this fifth of five articles, space.com concludes its decade-by-decade look at the 20th century's most significant developments in space.

The 1990s began with a profound change in the world's political structure. In 1991 the Soviet Union was dissolved. The event marked the end of the Cold War that had spawned so many space accomplishments.

In Russia, which inherited most of the Soviet space industry, economic hardship loomed. Russian officials found themselves having to negotiate with Kazakhstan for access to Baikonour, their space program's main launch complex. And the Energia-Buran program, the Russian equivalent of the U.S. space shuttle, was officially scrapped in 1993.

For NASA, the 1990s got off to a shaky start. In 1990, the long-delayed Hubble Space Telescope was deployed from Space Shuttle Discovery. But soon after the $1.5 billion telescope reached orbit, astronomers realized its main mirror was flawed. Already, its stunning images were delighting astronomers. But without a difficult repair by shuttle astronauts, the telescope would never realize its full potential.

NASA's can-do image was bolstered once more in 1993, when spacewalking shuttle astronauts staged one of the most demanding space repair jobs ever -- to fix the Hubble Space Telescope. After receiving a set of corrective lenses, the telescope performed even better than originally planned. The repair came just in time for Hubble to record the crash of Comet Shoemaker-Levy into Jupiter in July 1994.

Then, in 1992, came a mishap that transformed NASA's planetary exploration program. That summer, scientists eagerly awaited the arrival at Mars of the Mars Observer spacecraft, which was slated to carry out a detailed study of the planet's surface and atmosphere from orbit. But ground stations lost contact with the probe shortly before it was due to go into martian orbit. Analysts concluded that the $1 billion craft was lost, probably because of a massive fuel leak.

For NASA administrator Dan Goldin, the loss of Mars Observer hit hard. It was clear to Goldin that the agency's long-standing penchant for big, expensive space missions, with decade-plus lead times, had to change. When it came to planetary exploration, NASA couldn't afford to put its eggs in a 1 billion-dollar basket. Goldin gave the agency a new motto: Faster, better, cheaper.

In 1993, NASA's space station program was revamped. Already plagued by endless delays, revisions and cost overruns, the International Space Station -- for both economic and political reasons -- would now include Russia (and Russian space hardware from a planned Mir 2 program) as a key partner.

On the planetary exploration front, Mars was the first target for the less expensive missions, with the Mars Surveyor program. For each new martian launch opportunity, 26 months apart, NASA would launch a new pair of Mars spacecraft. Another program, called Mars Pathfinder, would develop a network of small spacecraft to land on the Red Planet. And the New Millennium program would test new technologies for a variety of deep-space missions.

But it was a tiny spacecraft called Clementine -- a creation not by NASA, but derived from the ill-fated "Star Wars" ballistic missile defense program -- that first demonstrated the potential of the "faster, better, cheaper" approach.

Clementine's arrival in lunar orbit in 1994 was the first U.S. mission to the moon since the end of the Apollo program. While its scientific accomplishments didn't rival Apollo's, Clementine did send back high-resolution images and valuable data on the surface composition. And it discovered the first signs of water on our natural satellite, in the form of ice grains probably deposited over billions of years by comet impacts.

For Russia, Mars had long been an unrealized goal. Adding to a long string of unsuccessful missions to the Red Planet, a pair of orbiters called Phobos 1 and 2 were lost before completing their missions in 1988.

After the dissolution of the Soviet Union, Russian space scientists faced a new set of obstacles, namely the country's disintegrating space industry and dwindling government funds. They worked for months without pay to prepare a new pair of Mars spacecraft -- an orbiter and several landers outfitted with a suite of scientific instruments -- for a November 1996 launch. The blaze of a Proton rocket heralded the Mars '96 probe's departure. But a malfunctioning upper stage failed to send the craft out of Earth orbit, ending the Russian's last attempt at planetary exploration in the 1990s.

NASA scored a success in its Mars exploration program when the Pathfinder spacecraft arrived on the martian surface in July 1997, some 21 years after the twin Viking probes touched down. Unlike the Vikings, however, Pathfinder used airbags to cushion its impact. Shortly after landing, the craft deployed a diminutive rover called Sojourner, which roamed the landing site under remote control from Earth, examining rocks and patches of soil.

The success of Pathfinder and Sojourner, which were produced for a fraction of Viking's cost, seemed to vindicate NASA's faster, better, cheaper credo. But the failure of four Mars spacecraft in 1999 -- Mars Climate Orbiter, Mars Polar Lander and a pair of Mars Micro-probes -- forced the agency to reexamine the risks of its new approach.

Still, everyone at NASA understood that there was no turning back: the era of high-cost planetary missions was long over. And in the aftermath of the Polar Lander's failure in December, 1999, NASA planners focused on how to improve chances for success in future Mars missions, including a sample-return slated for launch in 2005.

The goal of returning a piece of Mars to Earth became even more alluring in 1996, when NASA scientists announced they had found evidence of fossil bacteria inside a meteorite from Mars. While their claims remain controversial, they moved the subject of extraterrestrial life from the fringe to the mainstream of space science. It was clear that future Mars exploration could gather clues to whether life exists, or ever existed, on the Red Planet.

Still farther from home, Jupiter's moon Europa yielded clues that it might be an abode for life. High-resolution images from the orbiting Galileo spacecraft supported theories that beneath Europa's icy crust might exist an ocean of liquid water. It wasn't long before scientists began thinking about 21st-century robotic missions to drill through the ice and search for signs of living organisms.

As the 1990s ended, China launched a prototype of a piloted spacecraft, with plans to send people into Earth orbit in the first years of the new century. The United States and Russia collaborated on a mobile, ocean-based satellite launcher called Sea Launch. Also, NASA's Cassini orbiter, the last of the high-cost planetary probes, was en route to Saturn.

A probe called Stardust was heading for a rendezvous with a comet, with the goal of collecting samples of its dust for return to Earth. Crews of shuttle astronauts deployed an orbiting X-ray telescope called Chandra, and restored the ailing Hubble telescope to its work helping to reveal the mysteries of the universe. The first two modules of the International Space Station circled 200 miles above the Earth, awaiting further construction. And 77-year-old John Glenn, America's first man in orbit, became the world's oldest space traveler when he logged a nine-day shuttle flight.

These were just a part of the universe of space activities at the end of the 20th century. And a host of space projects, in production or on the drawing board, offered evidence that in the 21st century, human beings and their robotic surrogates would continue to explore the final frontier.