KENNEDY SPACE CENTER, FLA. — The setting sun casts purple and pink shades in the sky as the launch tower on pad 17-A, Cape Canaveral Air Force Station, rolls back to reveal the Boeing Delta II rocket with NASA’s Comet Nucleus Tour (CONTOUR) spacecraft attached. The launch is scheduled for 2:47 a.m. EDT July 3. Designed and built by The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., the 2,138-pound (970-kilogram) spacecraft will be placed into an elliptical Earth orbit until Aug. 15, when it is scheduled to fire its main engine and enter a comet-chasing orbit around the sun. The mission’s flexible four-year plan includes encounters with comets Encke (Nov. 12, 2003) and Schwassmann-Wachmann 3 (June 19, 2006), though it can add an encounter with a “new” and scientifically valuable comet from the outer solar system, should one be discovered in time for CONTOUR to fly past it. CONTOUR’s four scientific instruments will take detailed pictures and measure the chemical makeup of each comet’s nucleus — a chunk of ice and rock — while analyzing the surrounding gas and dust.