NASA's Messenger spacecraft successfully achieved orbit around Mercury at approximately 9 p.m. March 17, the space agency reported, marking the first time a spacecraft has accomplished what NASA called an "engineering and scientific milestone" at the solar system's innermost planet. The probe will continue to orbit the planet once every 12 hours for the duration of its primary mission.
For the next several weeks, APL engineers will be focused on ensuring the spacecraft's systems are all working well in Mercury's harsh thermal environment. Starting on March 23, the instruments will be turned on and checked out, and on April 4, the mission's primary science phase will begin, NASA reported. The mission's objectives include providing major-element maps of Mercury, determining local composition and mineralogy, and providing a global map with 90 percent coverage (monochrome, or black and white) and 80 percent of the planet imaged stereoscopically.
Messenger will also provide a global multi-spectral (color) map at and sample half of the northern hemisphere for topography. In addition, NASA hopes the mission will provide a multi-pole magnetic-field model and a global gravity field, determine the ratio of the solid-planet moment, identify the principal component of the radar-reflective material at Mercury's north pole and provide altitude profiles of the major neutral exospheric species and characterize the major ion species energy distributions as functions of local time, Mercury heliocentric distance and solar activity.
To accomplish these goals, the spacecraft must obtain many types of observations from different portions of its orbit around Mercury. Some major constraints must be met, including completing the observations within two Mercury solar days (equivalent to one Earth year) and keeping the spacecraft sun shade facing the Sun at all times.
The observation plan must also take into account Messenger's orbit around Mercury. The orbit is highly elliptical (egg-shaped), with the spacecraft passing 124 miles above the surface at the lowest point and more than 9,420 miles at the highest. At the outset of the orbital phase of the mission, the plane of the spacecraft's orbit is inclined 82.5 degrees to Mercury's equator, and the lowest point in the orbit is reached at a latitude of 60 degrees North, the space agency reported.
The spacecraft's orbit is elliptical rather than circular because the planet's surface radiates back heat from the Sun. At an altitude of 124 miles, the re-radiated heat from the planet alone is four times the solar intensity at Earth. "By spending only a short portion of each orbit flying this close to the planet, the temperature of the spacecraft can be better regulated," NASA documents explained.
Although a baseline plan for the entire year has been formulated, commands to execute the plan will be sent up to the spacecraft on a weekly basis. Each "command load" contains all the commands that the spacecraft will need to execute during a given week. Because each command load is different and contains many tens of thousands of commands, the mission operations engineers start each load three weeks ahead of time.
"This schedule permits the command load to be thoroughly tested and reviewed before it is sent up to the spacecraft," a NASA mission statement noted. "Because of this process, mission operations personnel at any given time will be working on several command loads, each of which is at a different stage of development."