Using the Hubble Space Telescope and ground-based observatories, "we're starting to get enough information to point to a particular place on the surface and detect differences," says Marc Buie, an astronomer at the Lowell Observatory in Flagstaff, Ariz., where Pluto was discovered. For example, observations have pinpointed a spot on the surface unusually rich in carbon-monoxide ice.
This could be a carbon-monoxide deposit bared by an impact crater, he says, or the result of cryovulcanism - the frosty equivalent of volcanic activity on Earth.
"What the heck is going on at the surface? There's no way of knowing without a mission to the planet," he says.
As currently envisioned, the Pluto-Kuiper Belt mission would launch in 2006 and fly by Pluto and Charon before 2020. Timing is important, because planetary alignments will be most favorable during this period for using Jupiter's gravity to sling the spacecraft toward Pluto.
The timing also is urgent because Pluto is heading for the solar system's outskirts. The planet's orbit traces an oval around the sun, and Pluto has just finished its closest approach. Astronomers estimate that by 2020, much of the planet's atmosphere will have frozen and fallen to the surface, preventing scientists from getting a handle on Pluto's tenuous envelope of gas and how it interacts with the solar wind, a stream of charged particles constantly flowing from the sun.
"This is a very ambitious mission," acknowledges Colleen Hartman, director of NASA's solar-system exploration division.
For all the scientific interest, the mission's finances may be as ephemeral as Pluto's atmosphere. The project is funded only through the end of the current $30 million mission-design study, a condition that Dr. Hartman says "is very unusual for a planetary mission."
Until November 2000, CalTech's Jet Propulsion Laboratory (JPL) in Pasadena spearheaded the project. But cost estimates ballooned to nearly $1 billion, threatening to become one of the Battlestar Galacticas that former NASA administrator Daniel Goldin fought to ground in favor of smaller, more frequent missions.