NASA aims to map the solar system's innermost planet with the Messenger spacecraft, now in orbit around Mercury.
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
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."