Hermosillo: Sonora 05: November: 2010
Comet and Spacecraft: A Fleeting Meeting
For all their visual flair,comets are extraordinarily mundane things — clusters of rock and ice left over from the primordial days of the solar system. There is a vast belt of the ancient objects beyond the orbits of Neptune andPluto and every now and then one of them gets knocked onto an irregular trajectory, falling in toward the sun and soaring back out again in orbits that can play out over decades or centuries. When oneof these rogues whizzes by, it is visiting not just from the farthest reaches of the solar system, but from the furthest reaches of time — a 4.5 billion-year-old scrap of the solar system's originalraw material. For that reason, scientists have always been interested in getting as close to comets as they possibly can.
Thursday's flyby, executed by NASA's Deep Impact spacecraft — an SUV-sizevehicle launched from Earth in January 2005 — is not the first time a probe has encountered a comet. In fact it's the sixth. Other comets, including Borrelly, Tempel 1, Wild 2 and the true cometarysuperstar — Halley — have all been approached by ships. But Deep Impact is special, and not just because of the extraordinarily crisp images it has returned.
The ship was not originally intended to visitHartley 2 at all. Instead, its prime target was Tempel 1, which it reached in July 2005 after its launch. Once there, it fired what amounted to a cosmic cannonball into the comet's core, then analyzedthe plume of ice and other debris that flew up into space. When the last stream of data had been transmitted home, the mission was done, but the spacecraft was still fit. Mission directors thusdecided to point its prow another way and send it off for an encounter with comet Boethin in 2008. Nice plan, but by 2007, Boethin was nowhere to be found — scientists speculated that it disintegrated —...