The Magellan mission to Venus, originally started in 1977, is managed by the Jet Propulsion Laboratory. Its primary objective is to collect high-resolution radar imagery of the surface of the planet Venus. The spaceprobe's synthetic aperture radar (SAR) can resolve features down to 120 meters. Magellan also has a radar altimeter and a thermal emission radiometer. Magellan has three secondary missions. Ground controllers will study the spacecraft's radio downlink to characterize Venus's gravity field, to study the Solar corona (when the probe is near opposition), and to refine data on Venus's atmosphere (through limb sounding).
Mission Constraints
The Magellan mission has orbit and hardware constraints. The probe (see Figure 1) was launched aboard the space shuttle Atlantis on May 4, 1989. If it had not been launched between April 28 and May 23 of 1989, there would have been a 25-month delay until the next planetary launch window. To perform its mapping mission, the probe had to enter a near-polar orbit of Venus. Earth occultation required the probe to record the mapping data on tape, and then transmit it to Earth when the orbit brought it out from "behind" Venus.
Hardware constraints greatly affected mission operations. Trans-Venusan-injection was originally to be performed by a Shuttle Centaur (liquid fuel) upper stage. After the Challenger was destroyed, shuttle rules required Magellan to use an IUS (solid fuel) upper stage. The lower-energy IUS required a Type IV Earth-Venus transfer (see Figure 2), which took fifteen months and covered a heliocentric angle of 540¡. Using one antenna for the SAR and the communications downlink meant the probe had to be constantly re-oriented for mapping and data transmission. On Earth, NASA's Deep Space Network had to be modified to receive the high downlink data rate (269 kb/s), and time had to be scheduled to provide constant coverage of Magellan.
Earth-Venus Transfer
The Magellan mission started with the launch of the space shuttle Atlantis into a 160 nmi-high orbit inclined 28.9¡. At 6 hrs. 18 min. (orbit five), the shuttle had aligned Magellan, which was deployed using springs. The shuttle then moved away from the probe. Solar panels deployed after ten minutes, and the IUS first stage fired 50 minutes later. The burn provided 185kN thrust for 2 min. 40 s. After first stage burnout, the IUS computer activated the probe's batteries, reoriented the spacecraft for the second stage burn, and then dropped off the first stage. The IUS second stage burn provided 76.5kN thrust for 1 min. 45s, after which the second stage separated from the probe. A six-second midcourse correction burn on May 21 added about 3 m/s to the probe's velocity. Magellan's three reaction wheels controlled the spacecraft's attitude during cruise, counteracting gravity and solar radiation perturbations. The 0.9-N thrusters were periodically used to unload the reaction wheels. Magellan travelled 1.5 revolutions around the Sun (Type IV transfer orbit) before reaching Venus. Venus orbit insertion required the probe to slow down by about 2625 m/s.
Venus Operations
There are seven segments to the Magellan mission: six data-gathering cycles and an aerobraking maneuver. Each cycle lasts eight months, and each mapping pass (one per orbit) lasts 37 min. (see Figures 3 and 4). Cycle 1 (Sept. 15, 1990 - May 17, 1991) provided radar mapping data (SAR, altimeter, radiometer). Cycle 2 (May 1991 - Jan. 15, 1992) performed more radar mapping, with the ground track shifted 10km to interleave with Cycle 1 data. Cycle 3 (Jan. 1992 - Sept. 1992) mapped the south pole, and filled in other regions missed during earlier cycles. Cycle 4 (Sept. 14, 1993 - May 1993) was used to gather low-latitude gravity field data. This data came from analyzing Doppler shifts in an unmodulated Magellan downlink. Useful data could only be collected near the periapsis of Magellan's elliptical orbit, which was at low latitude (10¡N).
The "Transition Experiment" lasted from May 25, 1993, to August 5, 1993. This was the aerobrake maneuver, used to lower Magellan's apoapsis. A lower, more circular orbit was needed to characterize the planet's gravity field at all latitudes. The spacecraft, however, could not carry enough fuel (900kg) to attain this orbit. By lowering Magellan into Venus's atmosphere, controllers effected a 1221 m/s change in velocity. Only 31.6 kg of fuel were used to control attitude and periapsis altitude. The maneuver changed the orbit from 8468x172 km altitude to 541x197 km altitude. Cycles 5 and 6 could then be used to gather high-resolution gravity data over all latitudes. Two cycles are required because of increased Earth occultation in the lower orbit.
Spacecraft operations are scheduled to finish in December 1994. Data archiving could be completed by February 1995, at which time the project would shut down. One final set of atmospheric data might be gained by dropping Magellan into Venus's atmosphere and studying it as it burned up.
Magellan Hardware
At the front of the spacecraft are the radio antennas: the 3.7-meter high gain antenna (HGA), the altimeter horn antenna, and the low- and medium-gain antennas. Magellan transmits X-band mapping data and S-band engineering data simultaneously. The HGA is used for mapping and Earth communications. The medium-gain antenna was used primarily during the interplanetary cruise. Behind the antennas is the Forward Equipment Module, which contains the radar electronics, telecommunication system, attitude reference equipment (including attitude gyros), three reaction wheels, and NiCd batteries. Next is the Bus Module, housing the command, data, and data storage systems, as well as the power system and attitude control computers. At the rear of the probe is the Thiokol Star 48B orbit insertion stage (solid rocket motor). Magellan also has two 12.6m2 solar arrays, which rotate about an axis perpendicular to the spacecraft. Attitude control is performed by three orthogonal reaction wheels and three sets of hydrazine thrusters. Twelve 0.9-N thrusters control roll and trim, and desaturate the reaction wheels. Four 22-N thrusters control roll during Venus orbit insertion. Eight 445-N thrusters are used for propulsive maneuvers.
Commentary
The Magellan mission has three obvious weaknesses. First is the shared HGA. The probe must first point the antenna at the planet's surface to gather the radar data, and then must change attitude to transmit the data to Earth. This and orbit constraints required the use of tape recorders to hold the data until each mapping pass was finished. One tape recorder became unreliable early in the mission, leaving only the backup available for most of the mission. The third weakness was in the probe's orbit about Venus. The eccentric orbit, a necessity of the low-energy Earth-Venus transfer, required constant changes in the SAR look angle during the mapping passes. As the spacecraft's altitude increased, the look angle had to decrease to keep the signal-to-noise ratio high enough (points closer to nadir reflect brighter). These weaknesses are important, but were largely overcome through redundant systems and (complicated) satellite operations.
Magellan has several strengths. It provided as many as six types of data, including the best imagery and greatest coverage of any mission to Venus. The data will generate questions and answers for years to come. Seventy percent of the spacecraft's mass came from Voyager, Galileo, and Ulysses spare parts (the other 30% was the radar equipment and solar arrays). The mission was (is) flexible, but has definite goals leading toward completion. The proof of the aerobrake maneuver is important for future planetary missions. Aerobraking demonstrated its indispensability by reducing fuel consumption by a factor of thirty during the orbit circularization. The Magellan mission has shown how a well-planned mission can overcome budget and operational constraints to return a wealth of data.
Sunday, 01-Aug-2004 00:36:47 CDT
CSR/TSGC Team Web
