NASA's vaunted lunar program came to a close after the Apollo 17 mission of December 1972, and no man has set foot on the moon since.

Instead, NASA set its sights on different horizons: the planets that share our solar system, and the cosmos beyond.

One goal - bolstered by the space shuttle program and the International Space Station, now set for completion in 2004 - is a permanent human presence in space.

Others cited by NASA Administrator Daniel S. Goldin are even loftier: How did the universe form and evolve? How can our exploration of the universe and our solar system revolutionize our understanding of physics, chemistry, and biology? Does life in any form exist elsewhere than on planet Earth? Here are highlights of the U.S. space program in the last quarter of the 20th century. Apollo-Soyuz In 1975, NASA and the Soviet Union cooperated on the first international space flight.

After being launched separately from their respective countries, Apollo and Soyuz crews docked their spacecraft and orbited together for two days.

Space shuttle After a gap of six years, NASA returned to human space flight with the launch of the space shuttle on April 12, 1981. Its mission complete, it glided to an unpowered airplane-like landing on a runway at Edwards Air Force Base, Calif.

Sally K. Ride became the first American woman to fly in space when the seventh shuttle mission lifted off on June 18, 1983.

It all came to a halt on Jan. 28, 1986.

A leak in the joints of one of two solid rocket boosters attached to the space shuttle Challenger caused the main fuel tank to explode 73 seconds after launch, killing all seven crew members.

The Shuttle program was grounded for more than two years but returned to flight on Sept. 29, 1988. Since then, NASA successfully has launched 70 flights of its four shuttle orbiters, Atlantis, Columbia, Discovery and Endeavour.

Space station For centuries, man has dreamed of living on the moon, or on Mars, or elsewhere in space.

Such extended, or perhaps permanent, stays are a central mission of future space exploration.

It began with NASA's Skylab program in 1973. After Apollo, NASA used its huge Saturn rockets to launch a relatively small orbital space workshop. There were three human Skylab missions, with the crews staying aboard the orbital workshop for 28, 59, and then 84 days.

Skylab fell out of orbit on July 11, 1979 and scattered across the Indian Ocean and western Australia. In 1984, Congress authorized NASA to build a major new space station as a base for further exploration of space. After several incarnations, it evolved into the International Space Station, a joint project with Canada, Japan, Brazil, Russia and 11 nations of the European Space Agency. After several delays, it began last December and is scheduled for a 2004 completion. To prepare, NASA participated in a series of missions aboard the Soviet space station Mir. Seven American astronauts lived aboard the aging space station, maintaining a continuous American presence for nearly 27 months ending in May 1998. Beyond Earth's orbit With spectacular successes and disappointing failures, NASA set out to escape Earth's orbit and voyage to the outer reaches of the solar system and beyond.

The major missions were:

Pioneer. The unmanned spacecraft Pioneer 10 and Pioneer 11, launched on March 2, 1972, and April 5, 1973, escaped earth's orbit on voyages to Jupiter and Saturn to study the composition of interplanetary space.

Viking. In 1975, NASA launched two Viking spacecraft to look for basic signs of life on Mars; the spacecraft arrived on Mars in 1976 but did not find any indications of past or present biological activity there.

Voyager. Voyagers 1 and 2, launched on Sept. 5, 1977, and Aug. 20, 1977, conducted a "grand tour" of our solar system, exploring Jupiter, then Saturn, Uranus and Neptune.

More than 20 years after launch, reprogrammed by remote control, Voyager 1 passed Pioneer 10 in January 1998 to become the most distant man-made object in the solar system. NASA says both Voyagers have enough power and propellant to operate and continue transmitting data until about 2020.

Hubble Space Telescope. In 1990, NASA launched the Hubble Space Telescope into orbit around the Earth. But NASA scientists soon discovered that a microscopic flaw in the polishing of the Hubble's mirror limited the instrument's observing power.

In December 1993, a team of astronauts performed a dramatic series of spacewalks to install a corrective hardware that functioned like a contact lens to restore Hubble's sight.

Mars Observer. NASA suffered a major disappointment when the Mars Observer spacecraft disappeared on Aug. 21, 1993, just three days before it was to go into orbit around the red planet.

Mars Global Surveyor: The first of NASA's "better, faster, cheaper" spacecraft after the loss of Observer, Mars Global Surveyor was launched on Nov. 7, 1996, and has been in a Martian orbit mapping the planet since 1998.

Mars Pathfinder. On the Fourth of July, 1997, after a voyage of 211 days, the Mars Pathfinder spacecraft landed on the red planet. Rapt Americans watched via television and the Internet as Pathfinder sent back spectacular images of the surface of the planet seen through the eyes of its miniature rover, Sojourner.

Galileo. Launched in 1989, the Galileo spacecraft arrived at Jupiter on Dec. 7, 1995, plunged its atmospheric probe into Jupiter's atmosphere and relayed information on the structure and composition of the solar system's largest planet.

In 1996, Galileo revealed that Jupiter's moon Europa may contain ice or even liquid water - thought to be necessary to maintain life.

Mars Polar Lander. NASA's Mars Polar Lander reached Mars this month but fell silent as it began its descent to the planet. That disappointment followed the September loss of the Mars Climate Orbiter, which was attrributed to NASA failing to convert English measurements to metric.

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