The Mission
When Ferdinand Magellan set sail from Spain in 1519, on his quest for a western route to India, his journey unlocked mysteries about the world around which he traveled. Now, more than 470 years later, a spacecraft bearing his name is traveling around another world, unlocking its mysteries. The new Magellan is gathering data from Venus that will be used by scientists to better understand the dynamic forces that drive planet Earth.
A carbon dioxide atmosphere hides Venus from view of Earth-based telescopes, and thick layers of swirling sulfuric acid clouds blanket the planet. Those clouds, however, are transparent to radar, which has been the only effective way to see the desolate surface.
The Magellan mission to Venus highlights years of exploring our nearest planetary neighbor. Magellan, equipped with a radar instrument built by Hughes Aircraft Company, has gathered more imaging data in significantly greater detail than the 20 other successful U.S. and Soviet Venusian missions combined.
While some of the planet's topography has been revealed, Magellan's prime concern has been revealing the condition of the surface by looking for evidence of volcanic activity and earthquakes. Knowing the condition will help scientists determine how Venus gets rid of its internal heat, and provide insight into the evolution of the planet.
The Jet Propulsion Laboratory manages the program for the National Aeronautics and Space Administration (NASA). Originally known in the 1970s as the Venus Orbiting Imaging Radar, the mission was later downscaled and called the Venus Radar Mapper and ultimately renamed Magellan.
Boeing Satellite Development Center in El Segundo, California, was responsible for the radar mapper, which was the sole scientific payload and heart of the Magellan mission. Martin Marietta Astronautics Group was responsible for the spacecraft and overall integration.
In October 2000, The Boeing Company acquired three units within Hughes Electronics Corporation: Hughes Space and Communications Company, Hughes Electron Dynamics, and Spectrolab, Inc., in addition to Hughes Electronics' interest in HRL, the company's primary research laboratory. The four are now part of Boeing's newest subsidiary, Boeing Satellite Development Center.
Magellan was the first U.S. interplanetary mission since the Pioneer Venus launches in 1978. The Pioneer Venus Orbiter and Multiprobe spacecraft, both of which were built by Hughes, rendezvoused with and sent back valuable information from the cloud-shrouded planet. Designed for an 8-month mission similar to Magellan's, the Pioneer Venus Orbiter continued to return radar images of the planet before running out of fuel and succumbing to gravity in October 1992.
The Pioneer Venus Orbiter was the first spacecraft to provide extensive radar mapping of Venus. Information gathered by the orbiter and its companion multiprobe spacecraft (which studied the atmosphere) was used to verify the "greenhouse effect" that has left the planet's surface scorched by 900 degree temperatures -- hot enough to melt lead.
In contrast to Magellan's capabilities, the resolution of the Pioneer Venus Orbiter was such that the smallest features that could be revealed were about the size of Los Angeles. The orbiter discovered large-scale formations, such as a plateau the size of Australia, in the planet's northern hemisphere.
Greater detail of the Venusian surface was provided by two Soviet spacecraft, Venera 15 and 16, that followed in 1983, and by U.S. ground-based radar in Arecibo, Puerto Rico. Findings included an immense mountain that may be the largest volcano in the solar system. Data from both the Soviet craft and ground radar were restricted, however, because of the limited amount of surface covered.
Magellan was the first interplanetary mission launched from a space shuttle. Atlantis was launched May 4, 1989, and carried Magellan into low Earth orbit. A solid-fuel rocket attached to the spacecraft propelled Magellan on a 15-month journey toward Venus. Upon its arrival there in August 1990, the spacecraft's orbital insertion motor was fired, and Magellan settled into an elliptical orbit around Venus that takes 3 hours and 9 minutes to complete. During a minimal mission time of one Venusian day, or 243 Earth days, Magellan makes 1,852 orbits.
The innovative radar sensor designed by Hughes provided a tenfold increase in resolution over previous radar images. From 1990 to 1992, Magellan unveiled more than 98 percent of the planet's surface. Details as small as 300 feet in diameter, or about the size of a major league baseball playing field, were uncovered by the radar, which also determined surface height variations to within 160 feet.
The Scientific Payload
The radar sensor viewed the surface of the planet at microwave frequencies in three modes: synthetic aperture radar (SAR), altimeter, and radiometer. Mission constraints required the spacecraft to operate in an elliptical orbit, which imposed design requirements on the sensor to accommodate changing altitude and Doppler rates. Additionally, the spacecraft's 12-foot parabolic antenna must be used for mission telecommunications and operation of the SAR.
Ideally, the radar would use a dedicated antenna of very different characteristics. The larger the antenna's dish size, or aperture, the better the resolution of the images received. The sensor operated in the SAR mode to simulate an antenna of much greater size. Each time the radar transmitted a signal, the spacecraft traveled a certain distance along its orbital path before the signal returned. By using computer processing on Earth to combine successive returns, an antenna up to one mile in diameter could be synthesized.
The radar's various electronic units, such as the stable local oscillator, pulse repetition frequency/timing, range dispersion, transmitter, output network, receiver, baseband processor, data formatter, telemetry and command, and redundant units, are contained in a single box 5 feet long, 3 feet wide, and 1 foot deep.
The 15-cubic-foot sensor weighs 340 pounds. Packed inside are 37 modules, including 177 two-sided circuit boards, 28 multilayered circuit boards, 15,000 electronic parts, and 22,000 other parts connected by a wire harness with more than 4,500 terminations.
Because of the highly eccentric orbit of the spacecraft, the distance from Magellan to the planet's surface during the pass varied from approximately 1,300 miles above the north pole to 155 miles at periapsis and back out again. Processing radar imagery at such rapidly varying altitudes and at speeds accelerating to nearly 19,000 mph would be analogous to taking a series of pictures with a camera as you zoomed by your subject in a race car. Like the camera, the radar had to be continuously repointed and refocused.
Mission performance constraints required a prestored three-day look-ahead operation of the radar. A computer software program written by Hughes called Radar Mapping Sequencing Software used a model of the radar, a model of the planet, and navigation data to develop the required pointing and focusing commands. The radar was operated through commands generated on the ground and loaded into the spacecraft's memory three times a week for execution during mapping passes. Accurate models and navigational information were needed to operate this way because real-time command of the radar was not feasible.
One of the greatest challenges to the Hughes team, which included Space and Communications specialists and senior analysts from Radar Systems, was designing a computer software program capable of handling the nearly 950 commands needed to optimize radar performance during each pass. Hughes incorporated NASA algorithms to compensate for dramatic orbital variations.
The spacecraft's 12-foot parabolic antenna looked downward and to the side of Magellan's orbital path, and the SAR illuminated a 15-mile-wide swath on the planet's surface. Each 10,000-mile-long swath overlapped, accommodating Venus' rotation velocity at the equator of about 4 mph, or the speed of a brisk walk. By comparison, the Earth's rotational velocity at the equator is more than 1,000 mph.
The side-look angles ranged from 14 degrees to 52 degrees depending on the spacecraft's altitude and provided a more pronounced image of surface features than that available from directly overhead. The sensor's electronics measured the strength of the returning 2.385 gHz SAR signal, which traveled at the speed of light; determined how long it took each signal to complete its round trip; and computed the distance to the ground target. Up to 1,800 megabits of data were acquired during each mapping pass.
The sensor also monitored changes in the signal's frequency pitch, or Doppler shift, resulting from motion of the spacecraft relative to the ground target. In the Doppler effect, wavelengths of radar return were shortened when Magellan approached the target feature and lengthened when it left the target. This effect determined the target's location in reference to Magellan's flight path.
A two-dimensional image of the Venusian surface has been constructed by computers on Earth by combining the intensity of radar returns, the Doppler shift, and the distance data.
In addition, the parabolic antenna was used between SAR signal returns as a radiometer to passively collect data on brightness or emissivity (the thermal radiation of the surface).
Mounted to one side of the parabolic antenna is a 5-foot-long, pyramid-shaped altimeter antenna built by Hughes that remains pointed vertically down at the surface. The altimeter measured the height of geological features to within approximately 160 feet.
The data generated by the three modes of the radar sensor were stored on two tape recorders on board the spacecraft for transmission back to Earth at a high rate of 268.8 kilobits per second (kbps) during the two 57-minute playback segments of Magellan's orbit. In contrast, the Pioneer Venus Orbiter transmitted at 1.2 kbps. The SAR data were processed digitally by computer to generate high-resolution maps of near-photographic quality of the Venusian surface, the altimeter data yielded a global topography map of the planet, and the radiometer data were used to study the chemical composition of Venus.
Discoveries
Magellan's penetrating radar sensor eye has given scientists graphic proof of the effects of plate tectonics, volcanism, and impact cratering on Venus.
Data from Magellan indicate that for the most part, plate movement is not evident and surface heights are less extreme than those of Earth. However, two steep Venusian trenches rival the great oceanic trenches on Earth, causing scientists to suspect that Venus, like Earth, remains geologically active. Wind "streaks" on the surface offer clues to the planet's global wind patterns.
Magellan also revealed that lava flows cover much of the surface of the planet, indicating that Venus' volcanic activity probably is an ongoing phenomenon. Also, the surface is dotted with at least 100,000 "shield" volcanoes -- broad volcanic cones with gentle slopes constructed of successive nonviscous, mostly basaltic lava flows and measuring less than 15 kilometers in diameter, as well as hundreds of larger volcanoes. Giant calderas -- depressions usually found at the peaks of volcanoes -- are more expansive than those found on Earth. Magellan also uncovered unique circular "coronae," which may result from deep-seated interior processes that uplift and deform the surface. Surface features on Venus appear to be much more closely linked to interior processes than those of Earth. In addition, scientists have counted more than 900 impact craters from the images transmitted by Magellan.
Scientists also use the spacecraft's radio communication link to try to gauge Venus' gravity field. Variations in the spacecraft's orbital speed are linked to the thickness of layers comprising the planet's interior, suggesting geological processes that may be occurring. The spacecraft began a global gravity survey in September 1992. In May 1993, the Jet Propulsion Laboratory began an 80-day process of circularizing Magellan's orbit to allow the spacecraft to conduct high-resolution gravity studies at higher and lower latitudes.
Magellan completed three mapping cycles -- each one taking a Venusian day, or 243 Earth days -- from 1990 to 1992. Magellan mapped approximately 37 minutes per orbit for the first cycle, with the incidence angle ranging from 14 degrees to 45 degrees and the parabolic antenna looking off to the left. The second cycle, much of which was done with an incidence angle of about 25 degrees and looking off to the right of the ground track, was devoted mostly to collecting constant incidence angle images and images of the south pole of Venus. In the third cycle, despite being plagued by spacecraft downlink transmitter problems that lowered the data rate, Magellan relayed left-looking images taken at incidence angles several degrees different from the data in the first cycle. Scientists combined these images with images from the first cycle to get three-dimensional stereo images of roughly 25 percent of the planet's surface. The radar was powered off in January 1993.
It will take scientists years to fully interpret the barrage of data assembled over Magellan's voyage of discovery. Once they do, the remarkable store of information Magellan has already provided will be enhanced, as will our understanding of the Earth's evolution.
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