Magellan: Mission Plan
Magellan orbited Venus from north to south at periapsis, so "left-looking"
radar was pointing east, and "right-looking" radar was pointing west. The
initial orbital period was 3 hours, 15 minutes, and the (elliptic) orbit
ranged from 294 km above the surface at periapsis to 8,540 km at apoapsis.
Cycle 1
Radar mapping (left-looking)
15 Sep 1990 - 14 May 1991
Cycle 1 covered about 83.7% of the surface with left-looking radar.
The spacecraft altitude varied from about 2000 km near the north pole,
to 290 km at 9.5 degrees N latitude. Imaging was performed from the
north pole to about 75 degrees south latitude. At the north pole, the
cross-track resolution was 250 m and the along-track resolution was
110 m, and at 9.5 degrees N the cross-track resolution was 101 m and
the along-track resolution was 110 m. The incidence angle ranged from
16.5 degrees at the north pole to 45.7 degrees at periapsis.
Cycle 2
Radar mapping (right-looking)
15 May 1991 - 14 Jan 1992
Cycle 2 was performed in a right-looking mode, with the purpose of filling
in the gaps from Cycle 1 and covering the south polar region. Cycle 2
covered 54.5% of the surface, but combined with Cycle 1 data accounted
for 96% coverage. Imaging was done from about 75 N to the south pole.
The incidence angle was about 25 degrees from 75 N to 55 S latitude, and
then decreased to 12.7 degrees at the south pole.
Cycle 3
Radar mapping (left-looking)
15 Jan 1992 - 13 Sep 1992
Cycle 3 was devoted mainly to acquiring images that could be combined with
images from cycle 1 to form stereo pairs. A slightly different left-looking
profile from that of Cycle 1 was used. This cycle covered from 75 N, at an
incidence angle of 13.4 degrees, to 45 S, incidence angle 14.3 degrees. The
incidence angle reached a maximum of 25.6 degrees at 15 N. Part of the
cycle was geared towards coverage of Maxwell Montes,
from 75 N to 20 N at incidence angles from 27 to 36 degrees.
About 21.3 % of the surface was
covered, bringing the cumulative coverage to 98%
Cycle 4
Gravity data acquisition
14 Sep 1992 - 23 May 1993
Gravity data were obtained by pointing the Magellan antenna towards Earth
and measuring the Doppler shift in the radio transmissions. These
measurements can be converted into spacecraft accelerations along the
line-of-sight (LOS), which can be used to estimate variations in the
gravitational field of Venus. (With the antenna pointed away from
Venus, radar imaging could not be done, so gravity measurements required
their own cycle.) Spacecraft LOS velocity was measuered to an accuracy
of 0.1 mm/sec.
Aerobraking
Circularization of orbit
24 May 1993 - 02 Aug 1993
The resolution of gravity estimates degrades rapidly with increasing
spacecraft altitude. Since Magellan was in an elliptical orbit, the
resolution of the gravity data was much higher near the equator than
at the poles. The aerobraking maneuver was used to circularize the
orbit to allow uniform high-resolution global gravity coverage.
Magellan was dipped into the Venus atmosphere, causing a loss of
orbital energy and bringing the spacecraft into a lower, more circular
orbit. The final orbit had a periapsis of 180 Km from the surface,
an apoapsis of 540 Km, and a period of 94 minutes.
Cycle 5
Gravity data acquisition
03 Aug 1993 - 29 Aug 1994
Gravity data was acquired from the newly circularized orbit in much the
same way as it was in Cycle 4. High-resolution gravity has been
measured in this way for about 95% of the planet.
Cycle 6
Windmill experiment
September 1994
The windmill experiment requires Magellan to be dipped into the Venus
atmosphere. The solar panels are tilted at an angle so that atmospheric
drag will put a torque on the craft, and measurements of the corrections
required to offset this torque give information on the upper atmosphere.
A press release describing the experiment
is available.
End of mission
Atmospheric entry
11 Oct 1994 - 14 Oct 1994
On October 11, Magellan began its descent into the Venus atmosphere. On October 12
at 10:02 UT the radio signal was lost.
The final orbit should occur on October 14, when the craft is expected to
burn up on entry. Some parts of the craft may survive to impact the surface.
Author/Curator:
Dr. David R. Williams, dave.williams@nasa.gov
NSSDCA, Mail Code 690.1
NASA Goddard Space Flight Center
Greenbelt, MD 20771
+1-301-286-1258
NASA Official: Dave Williams, david.r.williams@nasa.gov
Last Updated: 08 January 1997, DRW