Rosetta, the Comet, and the Science of Surprise

Comet 67P looks little like the illustration created by the European Space Agency before Rosetta's arrival. Reality is far stranger. (Credit: ESA–C. Carreau/Rosetta-Navcam) There is a cliche you hear all the time when scientists describe their experiments: "We expect the unexpected," or its jokier cousin, "If we knew what we were doing it wouldn't be called research." (That second one is often, but dubiously, attributed to Albert Einstein.) But like many cliches, this one is built on a foundation of truth--as the comet explorations by the Rosetta spacecraft and Philae lander keep reminding us. The latest shocks come from the huge batch of science results released last week, but the Rosetta mission has been a series of surprises going all the way back to its origins. And with another 11 months of exploring to go (the nominal mission runs to December 31) , it is safe to say that the surprises are far from over. Comet 67P/Gerisimayev-Churisamenko is not what we expected, the landing was not what we expected, and even the spacecraft itself is not what its designers intended. You can read a nice summary of the brand new Rosetta results here, but those specific findings only begin to capture the story. Sorry, No Nuclear-Powered Comet Delivery Some of the surprises surrounding the mission have been ones of human capriciousness. Rosetta's origins go back--way back--to a May, 1985 meeting of the European Space Agency which laid out the agency's "Horizon 2000" plan. A key component was a wildly ambitious plan to visit a comet, collect samples, and bring them back to Earth. The probe was intended to be nuclear powered--essential for carrying out its complicated goals--and developed jointly with NASA. The comet nucleus sample return mission was to be built on an architecture similar to that of NASA's planned Cassini mission, along with a related U.S. mission called the Comet Rendezvous/Asteroid Flyby (CRAF). Things didn't work out that way, of course. During early-1990s budget cuts, NASA was forced to cancel CRAF and pull out of the Rosetta project, transforming it into a smaller, solar-powered mission that would no longer bring back samples. Version 2 of Rosetta included two landers, one of them a joint French-U.S. design. Further NASA cutbacks led to the cancellation of the second lander, leaving the final Rosetta-Philae mission that actually flew. Human circumstance intruded another way: Rosetta was supposed to visit a different, slightly smaller comet named Comet 46P/Wirtanen, but the failure of a previous Ariane rocket delayed the launch, forcing the ESA to pick a new target: Comet67P/Gerisimayev-Churisamenko (often called just Comet 67P, to save keystrokes and public embarrassment when spoken aloud).