[meteorite-list] MESSENGER Settles into Eight-Hour Orbit Around Mercury, Poised for New Discoveries

[Ron Baalke]

MESSENGER Mission News
April 20, 2012

MESSENGER Settles into Eight-Hour Orbit Around Mercury, Poised for New Discoveries

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 today's 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."

An animation of the maneuvers that guided MESSENGER into its new orbit is available online at 
http://messenger.jhuapl.edu/the_mission/movies/OCM7and8_transition_to_8hour_orbit.mp4 .

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MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and
Ranging) is a NASA-sponsored scientific investigation of the planet
Mercury and the first space mission designed to orbit the planet closest
to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and
after flybys of Earth, Venus, and Mercury will start a yearlong study of
its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie
Institution of Washington, leads the mission as Principal Investigator.
The Johns Hopkins University Applied Physics Laboratory built and operates
the MESSENGER spacecraft and manages this Discovery-class mission for NASA.