MESSENGER mission controllers at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., conducted the second of two maneuvers required to reduce the spacecraft's orbital period about Mercury.

The first maneuver, completed on Monday, shortened the orbital period from 11.6 to 9.1 hours and consumed the remaining oxidizer, one of two propellants that fuel the higher-efficiency large thruster.

With this maneuver, accomplished with the spacecraft's four medium-sized thrusters, MESSENGER is now in the 8-hour orbit from which it will operate for the next year.

MESSENGER was 133 million kilometers (83 million miles) from Earth when the 4-minute maneuver began at 7:05 p.m. EDT. Mission controllers at APL verified the start of the maneuver 7 minutes and 23 seconds later, after the first signals indicating spacecraft thruster activity reached NASA's Deep Space Network tracking station in Canberra, Australia.

The shorter orbit will allow MESSENGER's science team to address new questions about Mercury's composition, geological evolution, and environment that were raised by discoveries made during the first year of orbital operations.

"For instance," says APL's Patrick Peplowski, "during the first year of orbital operations, MESSENGER's Gamma-Ray Spectrometer and X-Ray Spectrometer provided the first measurements of the abundances of many elements on Mercury's surface, including magnesium, sulfur, calcium, and potassium.

"The eight-hour orbit gives us more observing time at low altitudes, which will permit measurements of variations in surface composition on shorter spatial scales. Such information will give us new insight into the chemical and geological processes by which Mercury's crust was formed."