On June 21, Burt Rutan's innovative manned rocket, SpaceShipOne, touched down in the Mojave Desert after a historic trip into suborbital space. Pilot Mike Melvill flew the rocket to 328,491 feet, just barely punching out of the atmosphere. After touchdown, as the craft was being towed before a crowd of 27,000 spectators, newly crowned astronaut Melvill sat astride his spaceship holding a sign that read "SpaceShipOne, GovernmentZero."

If the flight does indeed mark the beginning of nongovernment-supported spaceflight, what comes next?

In 2002, using data collected during the three-day grounding of all aircraft in the U.S. after 9/11, scientists discovered that contrails—the wispy white streaks that trail jets—were narrowing the natural day-night temperature cycle in well-trafficked areas. Now a NASA study indicates that warmed-up nights are outpacing cooled-down days. In the U.S., for example, detailed atmospheric modeling suggests that contrails could account for a climate-warming trend—just under 0.3�C per decade—measured between 1975 and 1994. That amount may seem trivial, but