It's 3:57 p.m. EST, and as I am typing this a European-built spacecraft named Rosetta is swooping past Earth at nearly 8 miles per second.In my mind's eye I can see it cruising 3,293 miles above the Pacific Ocean, give or take a mile, and zipping eastward over the Antarctic Peninsula.
I wave goodbye, because Rosetta isn't coming back for a while. In exactly two years, it'll make a third and final flyby of Earth, giving the spacecraft the last gravitational kick it needs to reach a periodic comet called Churyumov-Gerasimenko in mid-2014. Basically, Rosetta is using our planet as a zero-sum slingshot — it speeds up (a lot) and Earth slows down (very, very, very, very little).
These "gravity-assist" flybys have become pretty routine for interplanetary craft; they're a great way to gain velocity without the need for bulky rockets. And ordinarily Rosetta's dash past Earth wouldn't warrant much notice. (For example, the visiting craft's snapshots of the Moon and Earth are nice but nothing special.)But this time the spacecraft was unknowingly "discovered" on November 7th by astronomers in Arizona scanning the skies for Earth-threatening asteroids. They dutifully reported the 20th-magnitude blip in their images to the Minor Planet Center here in Cambridge, Massachusetts, and the next day the MPC announced that the newfound object, now designated 2007 VN84, would have a close brush with Earth … right about now!
An observant Russian skygazer named Denis Denisenko was the first to point out that 2007 VN84 was, in fact, Rosetta. The connection had been missed apparently because no one from the European Space Agency had bothered to update the MPC as to Rosetta's recent whereabouts. And so on November 9th the Cambridge clearinghouse issued an Editorial Notice to declare that "The minor planet 2007 VN84 does not exist and the designation is to be retired."