A new postcard of Pluto is whetting the appetites of scientists planning the first mission to the solar system's last planet.
Using the Hubble Space Telescope, scientists have taken more than a dozen images of Pluto as it begins its 124-year trip to its farthest point from the sun. The final results they coaxed from the data yield the most detailed views yet of the planet.
Just as ground-based telescopes gave astronomers their first glimpse of the broad surface features of Mars early in this century, "Hubble has brought Pluto from a fuzzy distant dot of light to a world which we can begin to map and watch for surface changes," says Alan Stern of the Southwest Research Institute's office in Boulder, Colo. Dr. Stern led the team that used the orbiting telescope to study the planet.
Stern and his colleagues - Marc Buie of the Lowell Observatory in Flagstaff, Ariz., and Laurence Trafton of the University of Texas at Austin - found sharp contrasts between light and dark areas across Pluto's surface.
Although some variations may be basins or craters on the surface, most are thought to be the result of different types of frosts that wander across the planet as its seasons and distance from the sun change. Pluto's atmosphere, largely carbon monoxide, methane, and nitrogen, shifts from a gas during its closest approach to the sun to largely ice during its farthest swing.
At Pluto's current position, scientists estimate its surface temperature at about minus 380 degrees F. over the frosty areas and a relatively toasty minus 350 degrees F. over the darker areas.
Pluto is a puzzle to astronomers. The planet warms and loses some of its atmosphere into space as it approaches the sun, like a comet. But at two-thirds the size of Earth's moon, it is too large to be a comet. Some researchers hold that it could be the lone survivor of a class of objects known as ice dwarfs that may have populated the early solar system and whose distant relatives are found in the Kupier Belt, a disk of icy leftovers from the solar system's birth.
The Hubble images "are very tantalizing," says Robert Staehle of the National Aeronautics and Space Administration's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. "The contrast variations on Pluto's surface are more pronounced than on almost any other body in the solar system imaged at the same resolution."
Dr. Staehle heads the team planning NASA's Pluto Express mission. If Congress approves it, a pair of probes will be launched in 2002 or 2003 to Pluto and its moon Charon and beyond. They would reach Pluto in 2010 or 2011.
The mission, the last in a series of projects that formed NASA's 1990s strategy for space science, started as a "1 percent chance of going anywhere" idea, Staehle explains.
As he tells it, the US Postal Service unwittingly threw down the gauntlet. The Post Office had commissioned a set of 10 stamps honoring US solar-system exploration. When it unveiled the set at JPL, each stamp had a picture of a planet and the space probe that explored it. All that accompanied Pluto's image was the inscription, "Not yet explored."
"That afternoon I had a corner conversation with a trajectory engineer," he recalls. "She came back a day or so later and said, 'We can do this.'"
Pluto Express's position as clean-up batter in NASA's strategic plan is less a reflection of its priority than of its technical challenge, says Wesley Huntress, NASA's associate administrator for space science. "Pluto Express is the last in line because of the technological development required. We wanted to do the most important science we could with a small, light, mostly autonomous spacecraft launched with the biggest rocket we could find."
If Congress approves the project, JPL would build two 100-kilogram (220-pound) spacecraft for launch within a few months of each other. Only about 7 kilograms of that weight represent the entire science package, Staehle says.
By contrast, the video camera alone for the Cassini craft to Saturn, the last of what NASA Administrator Daniel Goldin has referred to as "Battle Star Galacticas," weighs 28 kilograms.
The twin Pluto Express crafts would map the planet's surface features to a resolution of 1 kilometer, map the distribution of its surface frosts, and determine the composition of its atmosphere using small "kamikaze" probes similar the one used on the Galileo mission to Jupiter.
Two spacecraft are needed, Dr. Huntress says, because at the speeds they will be traveling and with Pluto's "day" of 6.4 earth days, one craft would pass the planet too quickly to gather data from both hemispheres. In addition, the economies of scale for building the two spacecraft would help keep the total mission cost close to $400 million - a bargain by Galileo standards. And with time of the essence as Pluto heads deeper into space, two spacecraft mean much of the mission can be salvaged if one craft fails.
"We hope to begin cutting hardware in 1999," Huntress says.