After nearly 20 years in flight and 177 main engine maneuvers, the Cassini spacecraft's orbit was being reshaped during 2016 to prepare for the mission's final chapter.
Running on Empty
At the time of the March 25, 2016, maneuver, the Cassini flight team’s estimate of the remaining level for one of the two propellants used to power Cassini's main engine was very close to empty. Because the exact amount of fuel remaining was uncertain, the spacecraft operations team could not be completely sure the spacecraft had enough propellant to complete the entire maneuver.
In addition to its main engine, Cassini has a system of smaller thrusters. Propellant for these thrusters is known to be more plentiful than the fuel used by the main engine. These thrusters could have been used to complete the mission. But use of the main engine for large maneuvers like this one is much preferred.
The spacecraft operations team has kept an accurate method to account for the amount of propellant used for all of Cassini's propulsive maneuvers, but like all measurements, it has a margin of error. This margin of error was greater than the amount of propellant thought to be remaining in Cassini's tanks prior to the March 25 maneuver.
However, the Orbital Trim Maneuver was successful, verifying that the engineers' accounting of propellant used during the mission has been accurate.
The maneuver, designated Orbital Trim Maneuver (OTM) 444, changed the spacecraft's velocity by about 18 mph (8 meters per second).
"Completing this large burn with the main engine allows us to proceed with the nominal tour as planned, without inserting additional cleanup maneuvers using thrusters," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory, Pasadena, California.
"Cassini has proven it can go the distance," Maize said.