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USA in Space, 3rd Edition Magellan: Venus Date: May 4, 1989, to October 12, 1994 Type of Mission: Uncrewed Venus probe Magellan’s primary objective was to map the surface of Venus using powerful radar imaging instruments. The probe managed to produce a detailed map of 99 percent of the planet’s surface, and also made a gravity map of Venus showing the density distribution of materials beneath the surface. The probe also contributed important information about the Venusian atmosphere. Summary of the Mission The Magellan mission was designed to produce an extremely detailed radar map of the surface features of the planet Venus, with a secondary goal of making a gravity map of the planet’s subsurface features. The project was named after the Portuguese explorer Ferdinand Magellan (1480-1521), who led the first successful attempt to circumnavigate the world by sea. Although Magellan did not survive the journey, his legacy profoundly changed humankind’s understanding of our planet. The spacecraft that bore his name had the same effect on our understanding of the planet Venus. Radar imaging (using high-frequency radio waves as “light”) is required to make images of Venus’s surface because the entire planet is perpetually enshrouded in a thick sulfuric acid cloud blanket. Unlike “normal” light waves, radar is capable of penetrating these thick clouds, reflecting off surface features back to a space probe or a ground-based radar station. The first radar images of Venus were produced from Earth-bound radar facilities at Goldstone, California, and Arecibo, Puerto Rico. The Magellan spacecraft was the culmination of a National Aeronautics and Space Administration (NASA) project begun in 1980 called the Venus Orbiting Imaging Radar Project. This project was canceled in the fall of 1981, to be resurrected later as the Venus Radar Mapper Project (VRM) with NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, given primary mission and systems development responsibilities. Tony Spear was made overall project manager, and Allan Conrad was assigned the post of mission operations systems manager, the office responsible for designing the actual hardware employed in the final spacecraft. VRM later evolved into Magellan, which was eventually headed by a new project manager, Douglas Griffith. Stephen Saunders was made project scientist, the person responsible for organizing and coordinating the gathering and analyzing of scientific data produced by the mission. Martin Marietta Corporation of Denver, Colorado, was contracted to build the actual spacecraft, while the construction of the critical radar sensor was assigned to Hughes Aircraft Company in El Segundo, California. Their task was complicated by budget constraints that demanded that the final craft be as cost-effective as possible. Early complicated designs produced during the VRM project were rejected as too costly. Spare parts from other missions were used in many critical hardware components, and existing hardware designs were employed where possible instead of creating new configurations. Magellan’s large high-gain (high-power) dish antenna, for example, was a spare left over from the Voyager project. Final costs for Magellan amounted to about $450 million. The final spacecraft was 6.4 meters tall, 3.7 meters across (high-gain antenna diameter), and weighed 3,460 kilograms. The aft section contained a network of support struts and braces supporting the four sets of attitude control rocket engines used to alter the spacecraft’s orientation in space. The attitude control system was attached to the main body of the craft consisting aft of a ten-sided instrument package belt, topped off by the boxlike forward equipment module, itself capped by the large (nearly 4 meters in diameter) high-gain dish antenna. Attached to one side of the large dish antenna was a horn-shaped antenna designed for surface altitude determinations. Projecting from opposing sides of the forward equipment module were two telescoping booms that served to deploy the nearly square solar panels away from the craft after launch and to orient them relative to the Sun for maximum efficiency. The panels were capable of producing the 1,200 watts of electrical power required to operate all on-board equipment. The craft also had a round aperture on the forward equipment module, the star tracker, that was used to orient the spacecraft relative to the star field. Magellan was originally scheduled for launch in November, 1988, from a space shuttle, using a Centaur rocket booster to achieve initial orbital characteristics. However, as a result of the Challenger shuttle accident in 1986, the launch date was moved back to September, 1989. Then the Shuttle/ Centaur program was canceled, necessitating a re- evaluation of the booster vehicle needed to send Magellan off to Venus. After considering three alternative launch date and booster configurations, NASA decided on a booster called Inertial Upper Stage (IUS) and a spring, 1989, launch opportunity. The precise window of opportunity for the launch was placed between April 27 and May 17, 1989. Magellan was eventually launched from Kennedy Space Center aboard the space shuttle Atlantis on May 4, 1989. After a flawless launch from the shuttle and IUS booster burn, Magellan spent the first part of its fifteen-month journey establishing a spiraling orbit around the Sun. It circled the Sun one and one-half times before finally achieving an elliptical, nearly polar orbit (north-south) around Venus on August 10, 1990. This orbit carried the craft about 200 kilometers from Venus’s surface at its closest approach (periapsis), and about 8,500 kilometers from the surface at its farthest point (apoapsis), on the planet’s opposite side. One complete orbit took about three and a quarter hours. Radar mapping was done each time the probe dipped down to its periapsis, producing high-resolution images of the surface of Venus and transmitting them back to Earth. Magellan incorporated a sophisticated synthetic aperture radar (SAR) located in the large Voyager-type dish antenna that revealed objects as small as 120 meters across, one tenth the size previously detectable. To fulfill its mapping mission Magellan would transmit a radar signal with its SAR on close approach, illuminating a 16-to-28-kilometer-wide, 16,200-kilometer-long swath across the planet’s surface. The reflected signal was then stored on tape and periodically played back to Earth through the high-gain antenna located within the large dish mounted on the forward end of the craft. Data from Magellan were beamed back to JPL’s Deep Space Network with receiving stations in Spain, Australia, and the Mojave Desert. However, all did not go according to plan, particularly during the initial phases of the radar mapping mission. Twice before mapping began, ground controllers at JPL thought they had lost the craft when the main computer malfunctioned and the high-gain antenna refused to point toward Earth. After thirteen hours of concentrated effort, the JPL team managed to communicate with Magellan through a smaller antenna, and the mission was saved. Three more times over the next several months, however, Magellan went silent (known as loss of signal events) but was revived each time by the resourceful improvisation of the JPL mission control scientists. In September, after mapping had begun, one of Magellan’s two tape recorders failed. Luckily, the remaining recorder performed flawlessly for the rest of the mission. The loss of signal events were eventually traced to a minor “bug” in the flight software. With this bug corrected, Magellan never again refused to transmit its valuable data. For planning purposes, the Magellan mission was divided into mission cycles, one cycle being equal to the very slow rotational period of Venus on its axis, 243 Earth days. At the end of each mission cycle the JPL mission controllers assessed mapping progress by contrast to planned objectives and time tables. Magellan performed far more efficiently than anticipated. For example, at the end of the first mission cycle cumulative planetary coverage was 84 percent; at the end of the second cycle coverage had jumped to 98 percent. By the end of the third cycle 99 percent of the planet had been covered, an area three times as large as Earth’s combined continental land masses. Originally, Magellan was expected to cover 70 percent of Venus’s surface at most. On September 15, 1992, Magellan initiated a fourth 243-day mission cycle, this time to perform a global gravity survey. Gravity data help scientists to determine the thickness of subsurface layers on a planet, and to determine relative densities of materials in the planet’s interior. For example, low-density areas may suggest hot rising magma—or a deep valley. By the end of mission cycle four, Magellan had generated a gravity map of most of Venus, but the most reliable data were collected when the craft was closest to the planet’s surface, within 30° or 40° latitude of the equator. Mission scientists, delighted at having a still functioning spacecraft after four cycles, decided to dedicate a fifth cycle to recovering high-quality gravity data from the polar regions as well. To provide better gravity data from Venus’s polar regions, JPL mission specialists had to devise a way to alter Magellan’s orbit so that it would swoop in closer to the polar regions than its elliptical orbit would allow. This was accomplished in a particularly imaginative fashion, using a technique first demonstrated in Arthur C. Clarke’s science fiction novel and film, 2010. This technique, called aerobraking, involves using frictional drag with the atmosphere to slow the momentum of a spacecraft so that it assumes a nearly circular orbit. In May, 1993, Magellan became the first nonfictional spacecraft to use aerobraking to alter its orbital characteristics. Mission cycle six, Magellan’s last, was used to obtain important information about Venus’s atmosphere. With its attitude control fuel nearly exhausted, its useful life as a planetary space probe was nearing its end. However, the low circular orbit acquired during cycle five left the craft flying within the planet’s atmosphere, allowing measurements of atmospheric density and other parameters. To measure density, Magellan was oriented so that its extended solar panels acted like the blades of a windmill. The force (expended by attitude control rockets) required to keep the craft from rotating provided a measure of the density of atoms and molecules in the atmosphere. On October 12, 1994, Magellan’s radio transmissions to Earth fell silent. Shortly thereafter, the spacecraft, which had orbited Venus for more than four years, was destroyed as it plunged through the atmosphere toward the surface. Knowledge Gained The Magellan mission to Venus provided humankind with the most detailed surface map of a planet in the entire solar system. The Venus revealed by Magellan shows a tortured surface, 85 percent of which is covered by volcanic lava flows that form the planet’s vast plains. Much of the rest of the surface consists of mountainous highlands crisscrossed by fold belts and rift valleys in complicated patterns. Magellan also revealed for the first time the full extent of impact cratering on the surface. Impact craters are fairly evenly distributed over the entire planet and are much more abundant than on Earth. Magellan also revealed the planet-wide extent of cratering on Venus from rocky meteoroids originating elsewhere in the solar system. It showed that impact craters less than about 30 kilometers in diameter are rare on Venus, the result of diminished momentum of small bodies encountering the thick carbon dioxide atmosphere. Using crater densities recorded by Magellan (number of craters in a given area) and comparing them to densities on the Earth and Moon, the age of the present crust of Venus was calculated at about 500 million years. Another important Magellan accomplishment was the construction of a high-resolution, planet-wide gravity map of Venus. In 1978 the Pioneer orbiter made a gravity map of Venus, but the Magellan map shows superior surface coverage and resolution of fine details. A comparison of the radar-produced topographic map with the gravity map shows an extremely close relationship between topography and gravity; high topographic features (domes, volcanic peaks) show high gravity, whereas low topographic areas (valleys) are associated with low gravity. This close topographic/gravity association is generally not observed on Earth. Finally, Magellan contributed important information about Venus’s atmosphere. Windmill experiments showed expected atmospheric densities at altitudes from 180 to 172 kilometers above the surface where atomic oxygen dominates the atmosphere, but at 160 to 150 kilometers above Venus atmospheric density was about one-half previously estimated values. Context Magellan was the twenty-second space probe to be sent to Venus, making Venus the target of more probes than any other planet. The first interplanetary probe, Mariner 2 (Mariner 1 was destroyed on launch), was launched toward Venus in 1962 and made infrared temperature determinations as it flew by the planet. In 1967 the United States launched Mariner 5, and the Soviet Union launched Venera 4, the first spacecraft to drop a probe into the Venusian atmosphere. They confirmed the high temperature readings (minus 30 degrees centigrade at the cloud tops; 477 degrees at the surface) of Mariner 2 and established the atmospheric composition as carbon dioxide-rich and extremely thick (914,000 kilograms per square meter surface pressure; ninety times that of Earth). Other notable U.S. probes included Mariner 10, which produced the first close-up, high-resolution pictures of the cloud formations on Venus in 1972. Pioneer Venus missions 1 and 2, launched about three months apart in 1978, released four “hard-landers” (Pioneer 2; probes designed to fall through the atmosphere and crash on the surface), and made the first gravity map of Venus (Pioneer 1 orbiter). Pioneer also produced radar-altimetry maps that allowed the first delineation of Venus’s global topographic provinces. A few months before Magellan reached Venus in 1990, the Galileo spacecraft transmitted detailed images of cloud patterns to Earth as it flew by Venus on its way to a December, 1995, rendezvous with Jupiter. The Soviet Union launched the most spacecraft toward Venus, fifteen in all. Beginning with Venera 5, these craft included orbiters, atmospheric probes, and soft landers capable of operating on the surface. Notable among the soft landers were Veneras 9, 10, 13, and 14, all of which carried cameras that produced the first images of the surface features. The Venera 15 and 16 orbiters were the first probes to produce radar images of Venus. The last Soviet probes were Vegas 1 and 2, which launched balloons to assess wind patterns in the atmosphere, and landers to assess surface conditions. The scientific legacy of Magellan is immense. In addition to discovering many new surface features on Venus, Magellan revolutionized scientists’ understanding of how the planet has evolved over time. Correlations of topographic features with gravity data show that, unlike Earth, Venus does not have lithospheric plates, thick slabs of rigid crust plus mantle material that slide horizontally over a weak, partially molten layer in the mantle. The sliding motion of these plates on Earth (a process called plate tectonics) accounts for most of the Earth’s mountain belts, volcanic chains, ocean basins, and earthquake activity. On Venus the high surface temperatures produce ductile behavior (easily deformable) at very shallow depths in the crust, precluding the formation of strong rigid plates. In terms of technological advances, Magellan demonstrated the feasibility of aerobraking for the first time, a technique that would be used to control the orbit of the Mars Global Surveyor, which was launched in 1996. Magellan also demonstrated the feasibility and economy of building a very successful spacecraft out of spare parts from other missions, although this was not without its problems. For example, the Voyager-type antenna dish was not an ideal configuration for radar work and had to be jury-rigged to adjust itself three thousand times during each mapping pass. Nevertheless, the spacecraft broke records for the amount of data received, exceeding all previous U.S. missions combined. To illustrate the amount of data Magellan collected, a magnetic data tape from just one mapping cycle would stretch halfway around Earth. Bibliography Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew Chaikin, eds. The New Solar System. 4th ed. New York: Sky Publishing, 1999. This is a comprehensive, engaging account of all solar system objects from planets to comets, asteroids, and meteorites. Richly illustrated with color and monochrome photographs, drawings, graphs, and charts, this is the perfect book to start anyone on an exploration of the solar system. Information on Venus is from pre-Magellan probes but is nevertheless very timely. Has an appendix with planetary characteristics, glossary, and suggestions for further reading. Comprehensive planetary maps are also included. Suitable for general audiences. Cattermole, Peter, and Patrick Moore. Atlas of Venus. New York: Cambridge University Press, 1997. This is a complete, up-to-date atlas of the planet shrouded in mysterious clouds. This colorful collection of maps and pictures draws heavily from the Magellan spacecraft’s radar imaging, some of it published here for the first time. There are twenty color plates, forty-eight black and white photos, and twenty-two line diagrams. Chapman, Clark R. Planets of Rock and Ice: From Mercury to the Moons of Saturn. New York: Charles Scribner’s Sons, 1982. This book, by one of the premier planetary scientists in the world, is a serious but sometimes whimsical overview of the solid (as opposed to gaseous) planets and planetoids of the solar system. This is a particularly good book for the study of comparative planetology, showing how the planets evolved and what their characteristics tell us about the origin of the solar system. Includes a center section with monochrome photographs. Suitable for general audiences. Grinspoon, David Harry. Venus Revealed: A New Look Below the Clouds of Our Mysterious Twin Planet. Cambridge, Mass.: Perseus Publishing, 1998. This is a somewhat romantic look at the lore of Venus and the historical attempts to study it. Grinspoon draws on data from U.S. and Soviet Venusian missions, as well as conventional astronomical observations (and folklore) to present for lay readers the most detailed picture of Venus available. Hunten, D. M., et al., eds. Venus. Tucson: University of Arizona Press, 1983. This collection of essays by Soviet and U.S. authors is a comprehensive compendium of data on all missions to Venus up to that time. Has many maps, charts, and photographs illustrating important aspects of the geological and geophysical aspects of Venus. A history of Venus studies is also included. Most articles contain technically challenging material. This book is aimed at professional specialists, but should be accessible for college-level audiences. National Aeronautics and Space Administration. Jet Propulsion Laboratory. Magellan: Revealing the Face of Venus. JPL 400-494 3/93. Washington, D.C.: Government Printing Office, 1993. This is a twenty-five-page booklet produced by NASA to publicize the Magellan mission. It contains full color diagrams and pictures of the spacecraft itself, colorized radar images including a global view of Venus, altimetry map showing principal topographic regions of Venus, and monochrome images illustrating important geological surface features. The text is packed with interesting facts, including a concise rundown on mission history, spacecraft dimensions and specifications, descriptions of mission objectives and results, facts and figures about the planet Venus, and the procedure for naming newly discovered features on its surface. It also contains a complete list of the principal scientific investigators on the Magellan project. Saunders, R. Stephen. “The Surface of Venus: Razor-Sharp Images of the Earth’s Near Twin Reveal a Mix of Familiar and Perplexing Geologic Features.” Scientific American 263 (1990). This article by the Magellan project scientist at JPL presents a wide array of excellent radar images of surface features on Venus. Images include impact craters, mountains, faulted plains, and volcanoes, all shown in exquisite detail. Also shows a picture of the Magellan probe and a drawing illustrating how it obtained its images. The text gives a synopsis of the Magellan mission and explanations of the illustrated surface features. This article is easily accessible in the serials section of most libraries. It is intended for general audiences. John L. Berkley |
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