Mission Overview
================
The Magellan spacecraft was launched from the Kennedy Space
Center on 4 May 1989. The spacecraft was deployed from the
Shuttle cargo bay after the Shuttle achieved parking orbit.
Magellan, using an inertial upper stage rocket, was then placed
into a Type IV transfer orbit to Venus where it carried out
radar mapping and gravity studies starting in August 1990. The
Mission has been described in many papers including two special
issues of the Journal of Geophysical Research [VRMPP1983;
SAUNDERSETAL1990; JGRMGN1992]. The radar system is also
described in [JOHNSON1990].

Magellan was powered by single degree of freedom, sun-tracking,
solar panels. The spacecraft was 3-axis stabilized by reaction
wheels using gyros and a star sensor for attitude reference.
The spacecraft carried a solid rocket motor for Venus orbit
insertion. A small hydrazine system was used for trajectory
corrections and certain attitude control functions. Earth
communication with the Deep Space Network (DSN) was by means of
S- and X-band channels. The high-gain antenna also functioned
as the SAR mapping antenna during orbital operations.

The interplanetary cruise phase lasted until 10 August 1990.
During the cruise phase there were small trajectory correction
maneuvers to ensure proper approach geometry. Using the solid
rocket motor, the spacecraft was placed into an elliptical orbit
around the planet, with a periapsis latitude of approximately 10
degrees north, a periapsis altitude of 295 km, a period of 3.263
hours, and an apoapsis altitude of approximately 7762 km.

After orbit insertion, the radar system acquired test data.
Then, unexpectedly, the signal from the spacecraft was lost
twice. Following an intense recovery process, commands were sent
to avoid further communication interruptions, and the spacecraft
resumed mapping operations on 15 September 1990.

Each mapping cycle lasted 243 days, which was the time required
for Venus to make one rotation under the spacecraft orbit. The
first mapping cycle ended on 15 May 1991. Typical activities
during a single mapping pass on Cycle 1 were as follows. As the
spacecraft neared periapsis, it was oriented so the high-gain
antenna pointed slightly to the side of the ground track. At a
true anomaly of -59 degrees, the radar was commanded on. The
radar continued to take data to a true anomaly of 80 degrees and
then the radar was commanded off. On the next pass the swath
started at -80 degrees and went to 59 degrees. Alternating
north and south swaths were repeated throughout Cycle 1.

The range of latitudes covered by the synthetic aperture radar
(SAR) during Cycle 1 was 67 degrees S to 90 degrees N. The
range of SAR incidence angles was from just under 20 to just
over 40 degrees. The SAR data were taken at a data rate of 750
kilobits/second and were stored in the spacecraft tape recorder.
Altimeter and radiometer data were also taken when SAR data were
acquired. The altimeter data were taken using a small fan beam
antenna at a data rate of 30 kb/s. As the spacecraft moved away
from the planet toward apoapsis, the spacecraft reoriented the
high-gain antenna towards Earth and the stored radar data were
transmitted to DSN stations. This data taking- and
transmitting-cycle was repeated for every orbit. By 15 May
1991, the planet had been completely mapped except for the area
near the South Pole and a few regions which had been missed
because of temporary equipment failures.

Cycle 2 observations focused on filling the gaps in Cycle 1
coverage (including the south pole area), acquiring SAR data at
a constant incidence angle (25 degrees), and conducting a suite
of ad hoc experiments, including high resolution imaging and
radar stereo. To observe the south pole the spacecraft was
rotated 180 degrees about its nadir-pointing axis so as to
conduct right-looking SAR observations. Gaps in the Cycle 1
coverage were filled by rotating the spacecraft back to its
initial left-looking direction. The orbit plane was adjusted
slightly at the beginning of Cycle 2 so that altimetry tracks
would be offset by about 10 km at the equator, bisecting the
orbit-to-orbit offset of altimetry tracks in Cycle 1. The
spacecraft was rotated 90 deg about the HGA boresight on orbits
3716-3719 to obtain SAR and radiometry data with VV
polarization. Radio occultation measurements were made on
orbits 3212-3214.

The principal objective of Cycle 3 was to perform radar stereo
mapping of the Venusian surface. About 30 percent of the Cycle
1 coverage was remapped in this cycle with a different,
left-looking incidence angle on the surface. Gravity data were
collected over Artemis Chasma. In addition, high resolution
altimetry data were collected by pointing the high gain antenna
straight down during orbits 4919 to 4921. Transmission of
acquired radar data to Earth became nearly impossible after
spacecraft equipment failures late in Cycle 3, and the radar was
not used for science purposes after that.

Cycle 4 was used for full (360 degree) longitudinal collection
of gravity data because of favorable planetary and spacecraft
geometry. The cycle was extended by about ten days to
compensate for passage of the radio ray through the Venus
atmosphere during the first ten days. To improve sensitivity to
gravity features, orbit periapsis was lowered on orbit 5752.
Radio occultation measurements were made on orbits 6369, 6370,
6471, and 6472.

The aerobraking phase of the mission was designed to change the
Magellan orbit from eccentric to nearly circular. This was
accomplished by dropping periapsis to less than 150 km above the
surface and using atmospheric drag to reduce the energy in the
orbit. Aerobraking ended on 3 August 1993, and periapsis was
boosted above the atmosphere leaving the spacecraft in an orbit
that was 540 km above the surface at apoapsis and 197 km above
the surface at periapsis. The orbit period was 94 minutes. The
spacecraft remained on its medium-gain antenna in this orbit
until Cycle 5 began officially on 16 August 1993.

During Cycles 5 and 6 the orbit was low and approximately
circular. The emphasis was on collecting high-resolution
gravity data. Two bistatic surface scattering experiments were
conducted, one on 6 October 1993 (orbits 9331, 9335, and 9336) and
the second on 9 November 1993 (orbits 9846-9848).


Mission Phases
==============
Mission phases were defined for significant spacecraft activity
periods. During orbital operations a 'cycle' was approximately
the time required for Venus to rotate once under the spacecraft
(about 243 days). But there were orbit adjustments and other
activities that made some mapping cycles not strictly contiguous
and slightly longer or shorter than the rotation period.


PRELAUNCH
---------
The prelaunch phase extended from delivery of the spacecraft
to Kennedy Space Center until the start of the launch
countdown.

Spacecraft Id : MGN
Target Name : VENUS
Mission Phase Start Time : 1988-09-01
Mission Phase Stop Time : 1989-05-04
Spacecraft Operations Type : ORBITER


LAUNCH
------
The launch phase extended from the start of launch countdown
until completion of the injection into the Earth-Venus
trajectory.

Spacecraft Id : MGN
Target Name : VENUS
Mission Phase Start Time : 1989-05-04
Mission Phase Stop Time : 1989-05-04
Spacecraft Operations Type : ORBITER


CRUISE
------
The cruise phase extended from injection into the Earth-Venus
trajectory until 10 days before Venus orbit insertion.

Spacecraft Id : MGN
Target Name : VENUS
Mission Phase Start Time : 1989-05-04
Mission Phase Stop Time : 1990-08-01
Spacecraft Operations Type : ORBITER


ORBIT INSERTION
---------------
The Venus orbit insertion phase extended from 10 days before
Venus orbit insertion until burnout of the solid rocket
injection motor.

Spacecraft Id : MGN
Target Name : VENUS
Mission Phase Start Time : 1990-08-01
Mission Phase Stop Time : 1990-08-10
Spacecraft Operations Type : ORBITER


ORBIT CHECKOUT
--------------
The orbit trim and checkout phase extended from burnout of the
solid rocket injection motor until the beginning of radar
mapping.

Spacecraft Id : MGN
Target Name : VENUS
Mission Phase Start Time : 1990-08-10
Mission Phase Stop Time : 1990-09-15
Spacecraft Operations Type : ORBITER


MAPPING CYCLE 1
---------------
The first mapping cycle extended from completion of the orbit
trim and checkout phase until completion of one cycle of radar
mapping (approximately 243 days). Mapping orbits included in
the first cycle were 373 through 2165. Orbits 2159-2171 were
used for an interferometry test, and orbits 2172-2175 were used
to conduct an orbit trim maneuver (OTM).

Spacecraft Id : MGN
Target Name : VENUS
Mission Phase Start Time : 1990-09-15
Mission Phase Stop Time : 1991-05-15
Spacecraft Operations Type : ORBITER


MAPPING CYCLE 2
---------------
The second mapping cycle extended from completion of the first
mapping cycle through an additional cycle of mapping.
Acquisition of 'right-looking' SAR data was emphasized.
Orbits included in the second cycle were 2176 through 3976.
Radio occultation measurements were first carried out
on orbits 3212-3214. A period of battery reconditioning
followed completion of Cycle 2.

Spacecraft Id : MGN
Target Name : VENUS
Mission Phase Start Time : 1991-05-16
Mission Phase Stop Time : 1992-01-17
Spacecraft Operations Type : ORBITER


MAPPING CYCLE 3
---------------
The third mapping cycle extended from completion of battery
reconditioning through an additional cycle of mapping
(approximately 243 days). Acquisition of 'stereo' SAR data
was emphasized. Orbits included in the third cycle were
4031 through 5747.

Spacecraft Id : MGN
Target Name : VENUS
Mission Phase Start Time : 1992-01-24
Mission Phase Stop Time : 1992-09-14
Spacecraft Operations Type : ORBITER


MAPPING CYCLE 4
---------------
The fourth mapping cycle extended from completion of the third
mapping cycle through an additional cycle of mapping.
Acquisition of radio tracking data for gravity studies was
emphasized. Radio occultation measurements were carried out
on orbits 6369, 6370, 6471, and 6472. Because of poor
observing geometry for gravity data collection at the
beginning of the cycle, this cycle was extended 10 days beyond
the nominal 243 days. Orbits included within the fourth cycle
were 5748 through 7626. Periapsis was lowered on orbit 5752
to improve sensitivity to gravity features in Cycle 4.

Spacecraft Id : MGN
Target Name : VENUS
Mission Phase Start Time : 1992-09-14
Mission Phase Stop Time : 1993-05-25
Spacecraft Operations Type : ORBITER


AEROBRAKING
-----------
The aerobraking phase extended from completion of the fourth
mapping cycle through achievement of a near-circular orbit.
Circularization was achieved more quickly than expected; the
first gravity data collection in the circular orbit was not
scheduled until 11 days later. Orbits included within the
aerobraking phase were 7627 through 8392.

Spacecraft Id : MGN
Target Name : VENUS
Mission Phase Start Time : 1993-05-26
Mission Phase Stop Time : 1993-08-05
Spacecraft Operations Type : ORBITER


MAPPING CYCLE 5
---------------
The fifth mapping cycle extended from completion of the
aerobraking phase through an additional cycle of mapping
(approximately 243 days). Acquisition of radio tracking data
for gravity studies was emphasized. The first orbit in the
fifth cycle was orbit 8393, and the last was orbit 12248.

Spacecraft Id : MGN
Target Name : VENUS
Mission Phase Start Time : 1993-08-16
Mission Phase Stop Time : 1994-04-15
Spacecraft Operations Type : ORBITER


MAPPING CYCLE 6
---------------
The sixth mapping cycle extended from completion of the fifth
mapping cycle through an additional cycle of mapping
(approximately 180 days). Acquisition of radio tracking data
for gravity studies was emphasized. The first orbit in the
sixth cycle was orbit 12249, and the last was orbit 15032.
The sixth cycle ended when radio contact was lost as the
spacecraft entered the atmosphere and was destroyed in a
'terminal windmill' experiment.

Spacecraft Id : MGN
Target Name : VENUS
Mission Phase Start Time : 1994-04-16
Mission Phase Stop Time : 1994-10-12
Spacecraft Operations Type : ORBITER

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