Even LCROSS scientists have seen nothing of a debris plume. "I'm not necessarily surprised," said LCROSS principal investigator Anthony Colaprete of NASA's Ames Research Center in Mountain View, California. In exploration, "you just never know how these things are going to go. We just have to go back with a finer-tooth comb." Colaprete's dogged optimism is grounded in enticing spectroscopic changes detected around the impact site. Determining whether it was water will take weeks or months of data combing.

Actually, Colaprete had warned his colleagues, at least, about the possibility of a no-show debris plume. "It's a very unproven and highly unpredictable science, impact cratering," he told an audience at the Lunar and Planetary Science Conference last March. Impact modelers working for the team had struggled to simulate the impact of a cylindrical--not a simpler spherical--object, and one that was hollow, not solid, like the LCROSS impactor. Plus, it smashed into a surface of unknown shape and composition. LCROSS was "the most challenging impact modeling I've ever done," said Erik Asphaug of the University of California, Santa Cruz. There were just too many unknowns for him to be entirely comfortable with his results; impact on the odd unseen boulder, for example, could have sent most of the debris into the crater wall instead of into the sky.

LCROSS scientists may yet extract a debris plume from the data, but "the spectra is where the information is" about any water, Colaprete said, referring to spectral colors in the visible, infrared, and even ultraviolet returned by the trailing LCROSS spacecraft and by the Lunar Reconnaissance Orbiter. Some of these showed intriguing blips from the impact flash and the still-warm crater. There were also spectral changes above the impact site between pre- and postimpact. "What do these little blips mean? I don't know," Colaprete said. "I'm just glad they're there. We're going to work on this feverishly." Even so, no public word about water will be forthcoming before the December meeting of the American Geophysical Union, he said.