It's going to be at least another two decades before any commercial models are built, but researchers are at work designing the Generation IV nuclear reactors. Unlike the generation II and III models now in use that use water to cool and control the fission (preventing runaway reactions, subsequent meltdowns and the environmental apocalypse that would result), the leading contender for cooling material for the Gen IV reactors is molten sodium. Not sodium chloride (plain, unreactive table salt), but sodium metal.

Once upon a time, the only fielding statistic listed on the back of baseball cards was fielding percentage, a simple calculation of the number of assists and putouts a player records divided by total chances.

Here's a mildly reassuring fact from today's AAAS news briefing on nuclear forensics: There are no known cases of a finished nuclear weapon being stolen or sold on the black market. But raw nuclear materials are a different story. In the past fifteen years, more than 1,300 cases of nuclear trafficking have been registered. Anita Nilsson of the International Atomic Energy Agency, a member of today's panel, said that most of these cases were "innocent," but some are anything but. The 400 grams of weapons-ready plutonium seized at the Munich airport in 1994?

Day 1,464 of the Mars rovers' 90-day mission to Mars (for those of you keeping track), and Steve Squires, the head of science operations for the Spirit and Opportunity rovers is getting us up to date on their latest findings. Most important: serendipity in action. The Spirit rover's right front wheel has broken, so engineers turn the rover around, drive it in reverse, and drag the wheel behind the rover. As it slogs across the planet, it carves a trench. And my, what a trench it carves.

The annual American Association for the Advancement of Science conference covers arguably the greatest variety of subjects of any science conference in the world. This year's gathering, held in St. Louis, Missouri, hosted symposia on everything from astrobiology to veterinary ethics. And although it's impossible for one reporter to cover more than a small fraction of the 200-plus scientific sessions held over five days, here are a few highlights of the most exciting research happening now.