MESSENGER Mission Update: First burn performed to lower extended mission altitude
The MESSENGER mission just issued a press release announcing that they have completed the first step in the two-step process of lowering the spacecraft's orbit around Mercury. This first step involved depleting all of the spacecraft's remaining oxidizer and was therefore MESSENGER's last-ever firing of its main bipropellant thruster. (A bipropellant thruster is one that requires both a fuel and an oxidizer.)
The maneuver reduced MESSENGER's orbit from its former, 12-hour orbit to one that takes just over 9 hours. It's headed toward an eventual 8-hour orbit. The 8-hour orbit has a maximum altitude at apoapsis of just over 10,000 kilometers, compared to 15,000 for the primary mission orbit. MESSENGER will perform the rocket burns necessary to achieve the 8-hour orbit on Friday, April 20.
The rest of MESSENGER's mission at Mercury will be accomplished with the aid of 16 small thrusters. The main, bipropellant thruster could achieve 660 Newtons of thrust; MESSENGER's monopropellant thrusters include four 22-Newton and twelve 4.4-Newton motors.