Cassini maneuver sets stage for Saturn arrival
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: June 17, 2004
The Saturn-bound Cassini spacecraft's main engine fired for 38 seconds Wednesday, slowing the vehicle by about 8 mph and putting it on course for Saturn orbit insertion the night of June 30, project officials said today. This was the final planned course correction prior to the make-or-break 96.4-minute orbit insertion rocket firing that will cap a seven-year voyage to the ringed planet.
"This should be our final approach maneuver," deputy program manager Earl Maize said in a statement. "It's on to Saturn."
Wednesday's rocket firing, known as Trajectory Correction Maneuver 21, or TCM-21, was needed to ensure Cassini crosses the plane of Saturn's rings in the right position.
The $3.3 billion Cassini is targeted to cross the ring plane in a broad gap between the F and G rings, a region thought to be empty of orbital debris. To be on the safe side, the spacecraft will make the crossing, once during approach and again on the other side of the planet, with its 13-foot-wide high-gain dish antenna facing forward to act as a shield.
On Saturday, Cassini's flight computers will begin executing a 41-day sequence of commands known as tour sequence 2, which includes the all-important Saturn Orbit Insertion maneuver. On June 22, the computers will begin executing the SOI "critical sequence," a complex set of instructions that include fault protection routines that will allow the spacecraft to sidestep problems that might otherwise cut off the orbit insertion burn.
The critical sequence begins with an eight-day "quiet period" before critical SOI commanding begins on June 29. Around 9 p.m. EDT the next day - about 90 minutes before the SOI rocket firing - the spacecraft will stop transmitting telemetry and begin sending a radio carrier signal to Earth using one of its low-gain antennas. Twenty minutes later, Cassini will turn to its protective attitude for the first ring plane crossing at 10:11 p.m.
The spacecraft then will turn to point its main engine in the direction of travel. The SOI rocket firing should begin at 10:36 p.m. and end around 12:12 a.m. on July 1.
The latest telemetry from Cassini shows the spacecraft is in good health and on target for SOI. Engineers are still analyzing the results of TCM-21 and another minor pre-SOI course correction could be carried out if necessary. But as of this writing, Cassini appears solidly on course.