Update: On 16 October, NASA released an image of a faint ejecta plume observed by the LCROSS shepherding spacecraft. See Elusive lunar plume caught on camera after all
Weeks before NASA's LCROSS mission crashed into the moon, some scientists involved with the mission were predicting very little, if anything, would be seen from the impact – despite a well publicised observing campaign.
Others now say the $79 million mission was ill conceived and will not deliver a meaningful result even if it manages to find evidence for water on the moon.
LCROSS ended on Friday morning when a two-tonne Centaur rocket hit the floor of a perpetually shadowed crater near the lunar south pole.
Scientists hoped that dust and vapour kicked up by the impact would climb high enough to catch sunlight, allowing a satellite that trailed behind the rocket to hunt for traces of lunar water in the ejected debris. The Hubble Space Telescope and many Earth-based observers were recruited to watch for a plume of debris rising from the impact site.
But while scientists voiced disappointment when no obvious plume was spotted from any vantage point, some were not surprised.
"We had a meeting in August where we reported what we thought would be the scenario," says LCROSS team member Peter Schultz of Brown University.
He said the new estimate for the mass that would be lofted to a visible elevation was 100 to 1000 times lower than estimates that had originally informed the mission.
Schultz and his team derived their numbers from projectile experiments using a high-speed vertical gun at NASA.
Their results differed strikingly from models that assumed debris would fly outward from the impact site a 45° angle.
Instead, the team found that the fastest debris, ejected at the initial stages of crater formation, tended to depart at an angle closer to 30° – more of a sideways spray than an upward trajectory. "That's not going to see sunlight," Schulz told New Scientist.
Another potential problem is that the 10 metre-long rocket was expected to produce a crater only 20 to 30 m in diameter. That crater size is small enough for the shape and orientation of the rocket to have played a role in how the debris was ejected, confounding expectations.
"Under those circumstances, the usual scaling relations may not have applied as well as we thought, even though we tried to take that into account," says LCROSS team member Don Korycansky of the University of California, Santa Cruz.
So was it a waste of time for professional and amateur astronomers who tried to observe what may have been, in principle, unobservable? Probably not, says Schultz, because the risk of missing something valuable, however unlikely, outweighed the cost of looking.
But some question whether the LCROSS mission made any sense at all.
"LCROSS was not a sound strategy to pursue if your objective was to answer the question, is there water ice on the moon? And if so, where is it and what is its state?" says Paul Spudis of the Lunar and Planetary Institute in Houston, Texas.
If LCROSS is unsuccessful in finding water, says Spudis, it will not say anything definitive about the moon because it could simply mean that scientists were unlucky in hitting a dry patch.
On the other hand, a positive detection of water would not provide any information about the extent and distribution of ice on the moon's surface, which Spudis says is the point of looking in the first place. "That tells me the fundamental rationale behind the mission was flawed."
A better, though admittedly more expensive, alternative would be a series of missions culminating in a rover, he says. The rover could move from sunlit to dark regions near the poles in order to compare lunar environments and characterise any ice found. "Instead, [NASA] came up with a PR stunt, and it kind of backfired," he says.
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