by Richard A. Kerr
Image of the LCROSS target crater before impact (top), and the last visible-light photo from the spacecraft (bottom). Although intended to produce a visible plume of ejecta, the impact has so far not yielded anything quite so spectacular. (Photos: NASA)
I didn't see anything. Did you? After the 2-ton upper stage of an Atlas rocket slammed into the lunar surface at 7:31 EDT this morning, no one at NASA admitted to spotting the expected spray of dirt and debris rising into the sunlight over the moon's Cabeus crater, as depicted so dramatically in the LCROSS mission overview video. The impactor most certainly plowed into the dark, frigid shadow inside Cabeus as planned, and the nine instruments on the trailing LCROSS spacecraft returned all the planned data. But no flash was reported at the moment of impact, and no debris could be seen. The science team's only report was "confirm crater in mid IR [infrared]," an apparent sighting of the impactor's hole in the ground. LCROSS itself then hit the moon, with mission control reporting a "loss of signal."
If the rocket impact failed to throw sufficient debris out of the lunar shadows to be detectable, it would not come as a complete surprise. Calculating just how the impact would excavate a crater in the lunar soil and rock "was the most challenging impact modeling I've ever done," Erik Asphaug of the University of California, Santa Cruz, said last spring. If high-rising ejecta were a no-show, there will be little chance of detecting minerals that could have been hydrated by subsurface ice. On the bright side, any water that turned to vapor would have expanded well beyond any solid ejecta and surely have risen into the sun, where some of LCROSS's instruments could have seen it. One can hope.