Under partly cloudy, cold skies, the GRACE twins lifted off on a Russian Rockot launch vehicle. Riding over 1,500,000 newtons (approximately 350,000 pounds) of thrust, the rocket headed northward over the Arctic Ocean and Alaska, then south across the Pacific Ocean and Antarctica before heading north again over Africa and Europe. At 85 minutes, 38 seconds into the mission — or 2:47 a.m. Pacific time — the satellites separated from the launch vehicle's third stage above Africa into a polar orbit 500 kilometers (311 miles) above Earth. Ground controllers successfully acquired the spacecraft's signal from the German Space Operations Center's ground tracking station in Weilheim, Germany at 2:49 a.m. Pacific time. After separation, the leading satellite pulled away from the trailing satellite at a relative speed of about 0.5 meters (1.6 feet) per second. Over the course of the next four days, the satellites were spaced 220 kilometers (137 miles) apart — a little more than the distance between Los Angeles and San Diego.
As they race around the globe 16 times a day, the satellites will sense minute variations in Earth's surface mass below and corresponding variations in Earth's gravitational pull. Regions of slightly stronger gravity will affect the lead satellite first, pulling it slightly away from the trailing satellite. By measuring the constantly changing distance between the two satellites using an extremely sensitive microwave ranging system and combining that data with precise positioning measurements from Global Positioning System instruments, scientists will be able to construct a precise Earth gravity map. During the next two and a half weeks, basic satellite operations will be established. During a subsequent three-week commissioning phase, GRACE's science instruments and supporting systems will be powered up, evaluated and calibrated. The performance of the GRACE system for measuring Earth gravity will then be validated over the following six months. The mission then enters its observational phase, during which routine operational data products will be made available to scientists.
Video produced by the University of Texas Center for Space Research.