At 8:45 p.m. EDT, Messenger - having pointed its largest thruster very close to the direction of travel - will fire that thruster for nearly 14 minutes, with other thrusters firing for an additional minute, slowing the spacecraft by 862 meters per second (1,929 miles per hour) and consuming 31% of the propellant that the spacecraft carried at launch.

Less than 9.5% of the usable propellant at the start of the mission will remain after completing the orbit insertion maneuver, but the spacecraft will still have plenty of propellant for future orbit correction maneuvers.

The orbit insertion will place the spacecraft into an initial orbit about Mercury that has a 200 kilometer (124 mile) minimum altitude and a period of 12 hours. At the time of orbit insertion, Messenger will be 46.14 million kilometers (28.67 million miles) from the Sun and 155.06 million kilometers (96.35 million miles) from Earth.

"The journey since launch, more than six and a half years ago, has been a long one," says Messenger Principal Investigator Sean Solomon, of the Carnegie Institution of Washington. "But we have rounded the last turn, and the finish line for the mission's cruise phase is in sight. The team is ready for orbital operations to begin."

Engineers recently tested the arrayed-antenna configuration that will be used during the Mercury orbit insertion. During the maneuver, Messenger's orientation will be optimized to support the burn, not to support communications with the team on the ground. As a result, the signal home will be weaker than usual.

To boost the signal, communications engineers will use four antennas at the Goldstone Deep Space Communications Complex - one 70-meter dish and three 34-meter dishes.

"This arrangement is not typical for a maneuver, so we wanted to do a few dry runs before orbit insertion," says Messenger Communications Engineer Dipak Srinivasan, of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. "We are still analyzing the data, but everything went as expected."

Since the last deep-space maneuver (DSM) almost a year and a half ago, the primary focus of the team has been on preparing for the orbit insertion maneuver and for orbital operations.

Detailed plans have been developed and vetted through an extensive series of meetings ranging from internal peer reviews of each subsystem to formal reviews with external experts assessing overall readiness. Three of the major reviews were dedicated specifically to the activities associated with the MOI maneuver itself.

In addition to taking advantage of planned DSMs to practice aspects of the orbit insertion maneuver, the team has conducted a number of flight tests to characterize key subsystem behavior and to confirm the proper operation of various spacecraft components.

Three full-team rehearsals using the hardware simulator have been conducted to practice all activities to be followed during the upcoming maneuver. The first of these exercises mimicked a nominal orbit insertion, and the following two presented anomalies for the team to recognize, analyze, and address.

"Although we feel that the preparations to date - and those scheduled for the next month - have been well thought-out, that the decisions made to define the specific activities were sound, and that the level of review and rehearsal has been more than adequate, we recognize the extraordinary complexity and unique nature of this endeavor," says APL's Peter Bedini, Messenger's project manager.

"But at this point, four weeks out, we are well positioned for success. The spacecraft is healthy, continues to operate nominally, and is on course to be at the right place at the right time at 8:45 P.M. ET on the evening of March 17."

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