The Magellan mission to Venus has the primary objective of producing a very detailed radar image of the Venusian surface. During radar mapping, the Magellan spacecraft will also perform gravity experiments on Venus to detect any anomalies in the planet's gravitational field and radiometer measurements to detect thermal emissions from the Venusian surface. The Magellan mission is a scaled-back version of the VOIR (Venus Orbit Imaging Radar) mission meaning that it is subject to design constraints similar to VOIR in addition to stricter budget constraints.
The necessity to keep the development cost of the Magellan spacecraft low was a major constraint on its construction. As a result, the Magellan was built out of spare parts from the Voyager, Viking, Galileo, and Ulysses spacecraft. The one part of the spacecraft specifically designed for the mission was the radar since it is the core of the mission. Portions of the spacecraft are made from aluminum honeycomb and other materials that combine strength with low weight. Low launch weight was necessary since the launch vehicle used for Earth departure was changed from a Centaur G-Prime to an Inertial Upper Stage (IUS). This modification was necessary since NASA did not want to transport explosive liquid oxygen and hydrogen in the Shuttle. Using an IUS meant that the amount of energy available for the required ÆV was lower but, the launch vehicle weight was lower as well. The lower launch vehicle weight meant a lighter adapter could be used for attachment to the spacecraft and a lighter spring loaded mechanism could be used to detach the IUS from the spacecraft. These lighter components meant an overall reduction in launch weight. To further reduce cost and launch weight, the orbit of the Magellan spacecraft around Venus was modified from a circular mapping orbit to an elliptical one.
The radar is the most important part of mission and was specifically developed for the Magellan spacecraft. The radar design was dictated by several constraints. The Magellan has one spacecraft fixed high-gain antenna which is to be used for all radio experiments, except for altimetry, and for communication with the Earth. Since the spacecraft orbit around Venus is highly eccentric, the spacecraft is close to the planet for a certain amount of its orbital period and far away for the remainder. Restrictions are therfore imposed on when certain bands of radar frequency are used. The radar design is also restricted by data rate and volume limitations set by such things as power requirements and the availability of the Deep Space Network. Another design restriction is created since the radar is commanded from a set of stored sequences.
The Magellan spacecraft used a Type IV trajectory to travel from Earth to Venus. Using this trajectory, the spacecraft went around the sun one and a half times before arriving at Venus. The trajectory initially chosen was a Type I trajectory which would have allowed the spacecraft to arrive at Venus sooner. However the Type IV was used because the launch date would not allow a Type I to be used and the Type IV expended less fuel anyway. The ÆV for leaving shuttle orbit entring the interplanetary trajectory was approximately 4.8 km/s and the ÆV for inserting into Venus orbit was approximately 2.6 km/s.
The Magellan mission was deployed from the shuttle cargo bay on May 4, 1989. The IUS ignited its two SRMs around 11: 47 p.m. on the same day for insertion into the interplanetary trajectory. The spacecraft entered its Type IV trajectory and began the flight to Venus. Three TCMs were executed on May 21, 1989 and March 13 and July 25, 1990. During cruise, tests on varois spacecraft subsystems were performed. Magellan arrived at Venus on August 10, 1990. The SRM slowed the spacecraft down enough to insert into Venus orbit. The Magellan was then converted from interplanetary cruiser to planetry orbiter. After calibrations and checks were performed on the navigation system, the Magellen turned on its radar and began mapping the planet on August 16. A series of five Tests were performed on the spacecraft and the Magellan was put into its final radar mapping orbit. The spacecraft antenna alternates from mapping the planet to relaying data back to Earth. 1,852 data-collection and playback passes will be performed during the 243 day mapping cycle. The swath width for each pass is approximately 25 km wide by 16,000 km long. Some of the data was lost during superior conjunction, when the sun is between Earth and Venus. An extended mission past the 243 days will allow that data to be recovered.
The Magellan spacecraft is basically composed of these items: a high, medium and low gain antenna plus altimeter, forward equipment module, equipment bus, solar panels, propulsion module, solid-rocket orbit insertion motor, and an IUS adapter structure. The radar and other spacecraft subsystems were placed in the forward equipment module. The whole spacecraft was launched on an IUS and taken into LEO on the shuttle.
The Magellan was a good design because the spacecraft was built from spare parts making it a less expensive. The trajectory to Venus was optimized as were the number of available launch windows in order to save on fuel and launch costs and increase launch opportunities. Magellan's weaknesses were that (1) he antenna was fixed to the spacecraft making it necessary to turn the entire structure in order to communicate with Earth or map Venus, (2) constant turning could strain the limits of the navigation and guidance systems, and (3) star scans must be done frequently increasing the risk that Magellan will lock on to the incorrect star. However, the method of using the momentum wheels to turn the spacecraft saves an incredible amount of fuel.
Sunday, 01-Aug-2004 00:36:47 CDT
CSR/TSGC Team Web
