March 14, 1996
Operating on a sequence of computer commands sent to the spacecraft a week ago, Galileo turned yesterday morning to the correct orientation for the rocket firing and then increased its spin rate to 10.5 rpm yesterday afternoon to stabilize the spacecraft during today's burn. The rocket firing started at 12:01 p.m. PST today, and stopped at 12:25 p.m. Tomorrow the craft will spin down and return to its normal orientation, with its antenna pointed toward Earth.
The rocket engine, part of a propulsion system built by Messerschmitt-Bolkow-Blohm and furnished by the German government as a partner in the Galileo project, delivers 400 newtons or about 88 pounds of thrust.
Acting on the 1-1/2-ton spacecraft for about 24 minutes, this force increased Galileo's speed in its orbit around Jupiter by about 377 meters per second or 842 miles per hour, nearly doubling its speed at the outer end of the orbit.
Now Galileo is aimed for an 844-kilometer (524-mile) encounter with the satellite Ganymede on June 27. Equally important, Galileo will not pass so close to Jupiter as it would have otherwise, resulting in much less radiation exposure from the planet's trapped radiation belts. It passed through the intense inner radiation zone last December of necessity to accomplish the probe mission, and had no problems then, but the spacecraft was not designed for multiple passes through that hazardous region.
Today the spacecraft is 19.3 million kilometers (12 million miles) from Jupiter, just past the farthest point in its orbit. Starting Saturday Galileo will begin to fall, and pick up speed, back toward Jupiter. Earth and the spacecraft are now less than 840 million kilometers (about 520 million miles) apart, as Jupiter and Earth approach each other in their solar orbits. As a result, Galileo's radio messages now take only 46 minutes, 19 seconds to get here.