Messenger successfully completed an orbit-correction maneuver this evening to lower its periapsis altitude - the lowest point of Messenger's orbit about Mercury relative to the planet's surface - from 405 to 200 kilometers (251 to 124 miles).

This is the first of three planned maneuvers designed to modify the spacecraft's orbit around Mercury as science operations transition from Messenger's primary orbital mission to its extended mission.

Messenger's orbit around Mercury is highly eccentric, taking it from 200 kilometers (124 miles) above Mercury's surface to 15,200 kilometers (9,445 miles) altitude every 12 hours.

Since orbit insertion nearly one year ago, spacecraft operators at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, conducted five earlier maneuvers to counter the perturbing forces that pull Messenger away from its preferred observing geometry, including those arising from solar gravity and Mercury's slight oblateness (the flattening of its spherical shape at the planet's poles).

For this latest orbit adjustment, Messenger was 148 million kilometers (92 million miles) from Earth when the 171-second maneuver, which used all four of the medium-sized monopropellant thrusters on the deck opposite most of the science instruments, began at 8:44 p.m. EST.

APL mission controllers verified the start of the maneuver 8 minutes and 12 seconds later, when the first signals indicating spacecraft thruster activity reached NASA's Deep Space Network tracking station near Canberra, Australia.

In mid-April the team will conduct two additional maneuvers designed to reduce the period of Messenger's orbit around Mercury from 12 to eight hours.

According to Messenger Principal Investigator Sean Solomon, of the Carnegie Institution of Washington, "This reduction in orbital period will mean that Messenger spends a greater fraction of its time close to Mercur's surface than during the primary mission that is now nearing a successful completion.

"Moreover, the accomplishment of the global mapping carried out during the primary mission will free many of Messenger's instruments for a new mix of measurements to address a fresh set of scientific objectives designed to answer questions raised by the findings from orbital observations to date."

"The eight-hour orbit will provide 50% more low-altitude observation opportunities of Mercury's north polar regions, including permanently shadowed craters," explains Messenger Mission Design Lead Jim McAdams of APL.

"A one-third reduction in maximum altitude relative to the 12-hour orbit will enable higher-resolution imaging of the southern hemisphere."