Cold, lonely and isolated at the edge of the solar system, Pluto is still waiting for visitors.

The veil of mystery may soon be lifted from this littlest, most distant and most inscrutable of the sun's nine planets, the only one in the solar system that hasn't been surveyed by spacecraft.

Two small, unmanned craft could begin the 2.8-billion-mile journey to Pluto as early as 1998. They would reach the planet seven or eight years later and beam back the first detailed pictures and information. This is the scenario envisioned by scientists at Caltech's Jet Propulsion Laboratory in Pasadena.

Pluto's closest approach to the sun--about 2.7 billion miles--occurred more than two years ago, and the planet is again headed toward the edge of the solar system. As it moves away from the sun, colder temperatures will condense its atmosphere.

"It's essential we see the planet before the atmosphere is frozen onto the surface, making it inscrutable for the next two centuries," says astronomer Richard J. Terrile, mission scientist for the Pluto flyby.

Given this urgency, the investigative spacecraft will be light and small--each weighing 350 pounds and measuring about four feet in diameter.

"We'll send them off at the highest velocity that anything's ever been launched," says Robert L. Staehle, Pluto team manager at the laboratory. "This will enable us to get back the first pictures as early as 2006, while the planet still has an atmosphere and some light."

In contrast, the Voyager 2 spacecraft that sent back images from Jupiter, Saturn, Uranus and Neptune was the size of a compact car and weighed 1,800 pounds. It traveled 12 years before beaming back its first pictures of Neptune, Pluto's nearest neighbor, in 1989.

The two Pluto-bound spacecraft may be launched as much as a year apart. Besides serving as a backup, the second probe would allow scientists to map Pluto's entire surface.

"We'll be going by the planet so fast that we'll only have an opportunity to map the side facing the sun," Staehle says. "By sending a second spacecraft, we can view Pluto's other face when it is rotated toward the sun and dimly illuminated."

There's one big caveat: The $400-million mission, although a lot less expensive than previous planetary probes, must be added to already tight federal budgets.

"Each time we've visited a new planet, we've learned a great deal about our own world, and particularly the origin of the solar system," Terrile says.