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| Click for video animation of MESSENGER preparing to start its orbit at Mercury (2.2 MB MPEG) |
MESSENGER's orbit about Mercury is highly
elliptical (egg-shaped), 200 kilometers (124 miles) above
the surface at the lowest point and more than 15,193 kilometers
(9,420 miles) at the highest. The plane of the orbit is inclined
80° to Mercury's equator, and the low point in the orbit
is reached at a latitude of 60° North. The low-altitude orbit over the northern hemisphere allows MESSENGER to conduct a detailed investigation of the geology and composition of Mercury's giant Caloris impact basin - the planet's largest known surface feature.
About 33% of the spacecraft's propellant
is required for Mercury orbit insertion – the process
of placing the spacecraft into its primary science orbit around
Mercury. MESSENGER's thrusters must slow the spacecraft by
just over 0.81 kilometers (0.50 miles) per second. As the
spacecraft approaches Mercury, the largest thruster must be
pointed close to the forward velocity direction of the spacecraft.
The first maneuver (lasting about 14 minutes) places the spacecraft into a stable orbit; it also
sets up a much shorter cleanup maneuver two to three days
later near the orbit's lowest point.
After MESSENGER arrives in the primary
science orbit, small forces, such as solar gravity
– the gravitational attraction of the Sun - slowly change the
spacecraft's orbit. Although these small forces have little
effect on MESSENGER's 12-hour orbit period, they can increase
the spacecraft's minimum altitude, orbit inclination, and
latitude of the surface point below MESSENGER's minimum altitude.
Left uncorrected, the increase in the spacecraft's minimum
altitude would prevent satisfactory completion of certain
science goals.
To keep this minimum altitude below 500
kilometers (310 miles), propulsive maneuvers must occur in
pairs once every Mercury year - every 88 days. The first maneuver
in each pair increases the orbit period to 12 hours, 15, minutes
by speeding up the spacecraft at its closest distance from
Mercury. Two-and-a-half orbits later a maneuver at the farthest
distance from Mercury slows the spacecraft just enough to
adjust the orbit period back to 12 hours and return the minimum
altitude to 200 kilometers (124 miles). Because the sunshade
must protect the main part of the spacecraft from direct sunlight
during propulsive maneuvers, the timing of these maneuvers
is limited to a few days when Mercury is near the same point
in its orbit as it was at Mercury orbit insertion.
MESSENGER's 12-month orbit covers two Mercury solar days;
one Mercury solar day, from sunrise to sunrise, is equal to
176 Earth days. The first solar day is focused on obtaining
global map products from the different instruments, and the
second focuses on targeted science investigations.
The spacecraft orbit views below include descriptions such as "dawn-dusk" and "noon-midnight" that indicate the surface lighting conditions directly beneath the orbit near both Mercury equator crossings. The "north" direction corresponds to the direction of Mercury's axis of rotation (from Mercury's center toward the north pole). The bright surface region in Mercury's northern hemisphere marks a spot where the Mariner 10 spacecraft did not obtain images during the spacecraft's three flybys of Mercury in 1974 and 1975.
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