The MESSENGER mission successfully completed the first of two maneuvers designed to reduce the spacecraft's orbital period about Mercury. This new trajectory will pave the way for more detailed measurements and targeted observations of the Sun's closest neighbor.

The spacecraft was 124 million kilometers (77 million miles) from Earth when the 188-second maneuver began at 3:13 p.m. EDT. Mission controllers at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., verified the start of the maneuver 6 minutes and 53 seconds later, when the first signals indicating spacecraft thruster activity reached NASA's Deep Space Network tracking station in Goldstone, Calif.

This maneuver - which adjusted the orbital period from 11 hours, 36 minutes to 9 hours, 5 minutes - was designed to deplete the remaining oxidizer of the spacecraft's propulsion system in a final firing of the large bi-propellant thruster.

A second maneuver, scheduled for the evening of April 20, will use the spacecraft's monopropellant system to complete the transition to an 8-hour orbit.

The strategy to complete this transition involves the execution by the MESSENGER flight team of carefully planned command sequences, says MESSENGER Mission Design Lead James McAdams of APL.

"The first orbit-correction maneuver consumed the remaining oxidizer, which is one of two propellants used for the higher-efficiency large thruster," he explains. Although such an "oxidizer depletion" maneuver is not uncommon, new procedures had to be developed and tested to make this MESSENGER critical event possible and safe to perform.

After Friday's maneuver, the 8-hour orbit will remain highly eccentric, with MESSENGER travelling between 278 kilometers (172 miles) and 10,314 kilometers (6,409 miles) above Mercury's surface.

Reducing the orbital period will increase from two to three the number of revolutions the spacecraft will make about the planet each day, increasing the time that the spacecraft will spend closer to the surface, says MESSENGER Mission Systems Engineer Eric Finnegan, of APL.

The additional time at lower altitude, he says, will enhance the science return. It will amplify the effectiveness of the high-energy spectrometers used to determine the composition of the planet's surface and will increase the number of altitude profiles that the laser altimeter will be able to make in the northern hemisphere of the planet, allowing for more detailed topographic maps.

Operations at this lower altitude will also enable higher-resolution imaging of Mercury's southern hemisphere.

"The MESSENGER engineering and operations teams have once again made a critical maneuver look easy," says MESSENGER Principal Investigator Sean C. Solomon, of the Carnegie Institution of Washington.

"The Science Team is now looking forward to being able to address a host of scientific questions on the composition, geological evolution, and environment of Mercury that have been raised by earlier orbital observations. With our new orbit, it feels as though we're embarking on a new mission."