Magellan (spacecraft)
The Magellan spacecraft was a robotic space probe launched by NASA on May 4, 1989. Its mission objectives were to map the surface of Venus by using synthetic-aperture radar and to measure the planetary gravitational field.
The Magellan probe was the first interplanetary mission to be launched from the Space Shuttle, the first one to use the Inertial Upper Stage booster, and the first spacecraft to test aerobraking as a method for circularizing its orbit. Magellan was the fifth successful NASA mission to Venus, and it ended an eleven-year gap in U.S. interplanetary probe launches.
History
Beginning in the late 1970s, scientists advocated for a radar mapping mission to Venus. They first sought to construct a spacecraft named the Venus Orbiting Imaging Radar, but it became clear that the mission would be beyond the budget constraints during the ensuing years. The VOIR mission was canceled in 1982.A simplified radar mission proposal was recommended by the Solar System Exploration Committee, and this one was submitted and accepted as the Venus Radar Mapper program in 1983. The proposal included a limited focus and a single primary scientific instrument. In 1985, the mission was renamed Magellan, in honor of the sixteenth-century Portuguese explorer Ferdinand Magellan, known for his exploration, mapping, and circumnavigation of the Earth.
The objectives of the mission included:
- Obtain near-global radar images of the Venusian surface with a resolution equivalent to optical imaging of per line pair.
- Obtain a near-global topographic map with spatial and vertical resolution.
- Obtain near-global gravity field data with resolution and two to three milligals of accuracy.
- Develop an understanding of the geological structure of the planet, including its density distribution and dynamics.
Spacecraft design
To save costs, most of the Magellan probe was made up of flight spare parts and reused design elements from other spacecraft:
;Reuse Type Legend:
| Component | Origin |
| Attitude control computer | Galileo |
| Bus | Voyager program |
| Command and data subsystem | Galileo |
| High- and low-gain antenna | Voyager program |
| Medium-gain antenna | Mariner 9 |
| Power distribution unit | Galileo |
| Propellant tank | Space Shuttle auxiliary power unit |
| Pyrotechnic control | Galileo |
| Radio-frequency traveling-wave tube assemblies | Ulysses |
| Solid rocket motor | Space Shuttle Payload Assist Module |
| Star scanner | Inertial Upper Stage |
| Thrusters | Voyager program |
The main body of the spacecraft, a spare one from the Voyager missions, was a 10-sided aluminum bus, containing the computers, data recorders, and other subsystems. The spacecraft measured 6.4 meters tall and 4.6 meters in diameter. Overall, the spacecraft weighed 3,445 kilograms.
Attitude control and propulsion
The spacecraft's attitude control was designed to be three-axis stabilized, including during the firing of the Star 48B solid rocket motor used to place it into orbit around Venus. Prior to Magellan, all spacecraft SRM firings had involved spinning spacecraft, which made control of the SRM a much easier task. In a typical spin mode, any unwanted forces related to SRM or nozzle mis-alignments are cancelled out. In the case of Magellan, the spacecraft design did not lend itself to spinning, so the resulting propulsion system design had to accommodate the challenging control issues with the large Star 48B SRM. The Star 48B, containing 2,014 kg of solid propellant, developed a thrust of ~89 kN shortly after firing; therefore, even a 0.5% SRM alignment error could generate side forces of 445 N. Final conservative estimates of worst-case side forces resulted in the need for eight 445 N thrusters, two in each quadrant, located out on booms at the maximum radius that the Space Shuttle Orbiter Payload Bay would accommodate.The actual propulsion system design consisted of a total of 24 monopropellant hydrazine thrusters fed from a single 71cm diameter titanium tank. The tank contained 133 kg of purified hydrazine. The design also included a pyrotechnically-isolated external high pressure tank with additional helium that could be connected to the main tank prior to the critical Venus orbit insertion burn to ensure maximum thrust from the 445 N thrusters during the SRM firing. Other hardware regarding orientation of the spacecraft consists of a set of gyroscopes and a star scanner.
Communications
For communications, the spacecraft included a lightweight graphite/aluminum, 3.7-meter high-gain antenna left over from the Voyager Program and a medium-gain antenna spare from the Mariner 9 mission. A low-gain antenna attached to the high-gain antenna was also included for contingencies. When communicating with the Deep Space Network, the spacecraft was able to simultaneously receive commands at 1.2 kilobits/second in the S-band and transmit data at 268.8 kilobits/second in the X-band.Power
Magellan was powered by two square solar arrays, each measuring 2.5 meters across. Together, the arrays supplied 1,200 watts of power at the beginning of the mission. However, over the course of the mission the solar arrays gradually degraded due to frequent, extreme temperature changes. To power the spacecraft while occulted from the Sun, twin 30 amp-hour, 26-cell, nickel-cadmium batteries were included. The batteries recharged as the spacecraft received direct sunlight.Computers and data processing
The computing system on the spacecraft was partially modified equipment from the Galileo. There were two ATAC-16 computers forming one redundant system, located in the attitude-control subsystem, and four RCA 1802 microprocessors, as two redundant systems, to control the command and data subsystem. The CDS was able to store commands for up to three days, and also to autonomously control the spacecraft if problems were to arise while mission operators were not in contact with the spacecraft.For storing the commands and recorded data, the spacecraft also included two multitrack digital tape recorders, able to store up to 225 megabytes of data until contact with the Earth was restored and the tapes were played back.
Scientific instruments
Thick and opaque, the atmosphere of Venus required a method beyond optical survey, to map the surface of the planet. The resolution of conventional radar depends entirely on the size of the antenna, which is greatly restricted by costs, physical constraints by launch vehicles and the complexity of maneuvering a large apparatus to provide high resolution data. Magellan addressed this problem by using a method known as synthetic aperture, where a large antenna is imitated by processing the information gathered by ground computers.The Magellan high-gain parabolic antenna, oriented 28°–78° to the right or left of nadir, emitted thousands of microwave pulses per second that passed through the clouds and to the surface of Venus, illuminating a swath of land. The Radar System then recorded the brightness of each pulse as it reflected back off the side surfaces of rocks, cliffs, volcanoes and other geologic features, as a form of backscatter. To increase the imaging resolution, Magellan recorded a series of data bursts for a particular location during multiple instances called, "looks". Each "look" slightly overlapped the previous, returning slightly different information for the same location, as the spacecraft moved in orbit. After transmitting the data back to Earth, Doppler modeling was used to take the overlapping "looks" and combine them into a continuous, high resolution image of the surface.
Radar System (RDRS)
The Radar System functioned in three modes: synthetic aperture radar, altimetry, and radiometry. The instrument cycled through the three modes while observing the surface geology, topography, and temperature of Venus using the 3.7-meter parabolic, high-gain antenna and a small fan-beam antenna, located just to the side.– In the Synthetic Aperture Radar mode, the instrument transmitted several thousand long-wave, 12.6-centimeter microwave pulses every second through the high-gain antenna, while measuring the doppler shift of each hitting the surface.
– In Altimetry mode, the instrument interleaved pulses with SAR, and operating similarly with the altimetric antenna, recording information regarding the elevation of the surface on Venus.
– In Radiometry mode, the high-gain antenna was used to record microwave radiothermal emissions from Venus. This data was used to characterize the surface temperature.
The data was collected at 750 kilobits/second to the tape recorder and later transmitted to Earth = 85 Foto to be processed into usable images, by the Radar Data Processing Subsystem, a collection of ground computers operated by JPL. The principal investigator for this instrument was Gordon Pettengill from MIT.
Experiments
Magellan returned data to perform three so called experiments:- Synthetic Aperture Radar , already covered above while discussion the RDRS instrument;
- Gravimetry, consisting on detailed measurements of the Venus gravitational field, with the principal investigator being Georges Balmino from Centre National d'Etudes Spatiales;
- Magellan Radio Science Occultation Experiment , consisting on measurements of the atmospheric density and radio occultation data on the atmospheric profile. The principal investigator was Jon M. Jenkins from NASA Ames Research Center.