On St. Patrick's Day, for the first time a small NASA spacecraft called Messenger will enter into Mercury's orbit, circling at times as close as 125 miles from the planet's surface. And by coincidence, a few days before that will be the best time all year for people on Earth to see Mercury with the naked eye. Barely bigger than our moon but much more distant, Mercury is not easy to see without a telescope. An odd pairing with giant Jupiter will make it easier to spot starting Sunday — probably the best opportunity for a year.
NASA's Mariner probe flew by Mercury in 1974 and 1975, and Messenger has whizzed by it in 2008 and 2009. But this is the first time a spacecraft will attempt the tricky maneuver of entering Mercury's orbit, circling it for a year. To do that, Messenger, overbudget at $446 million, will have to thwart the enormous pull of the sun. The overall heat on the sunny side of Mercury will melt parts of some of Messenger's instruments, by design, acting as a heat buffer for the more sensitive parts of the equipment. Then the melted parts will refreeze when the spacecraft hits a cooler zone, said Messenger system engineering chief Eric Finnegan.