The Magellan probe mapped the surface of Venus using powerful radar imaging technology and supplied important information about the planet’s surface and gravity, while ushering in a new era of planetary exploration.
Venus is often called Earth’s “sister planet” because it is similar in size, mass, and solid surface to the Earth and because it is one of the closest planets in proximity. Since 1962, the United States has received information about the planet via various spacecraft, or probes, which either passed near or orbited Venus. The Soviet Union also launched numerous spacecraft to Venus, including a few that actually landed on the surface and briefly transmitted data before burning up.
A view of Venus’s surface made by the Magellan synthetic aperture radar.
On November 1, 1980, President Jimmy Carter
Gordon H. Pettengill began working on the Venus radar mapping
Magellan’s original launch, planned for April, 1988, was canceled when the entire shuttle fleet was grounded because of the Challenger accident in early 1986. The next launch opportunity was April 28, 1989, aboard the space shuttle Atlantis, but this launch was delayed because of equipment failure.
Atlantis launched on May 4, 1989, and Magellan left the cargo bay six hours later. One hour after that, the spacecraft was out of Earth’s orbit, traveling 25,000 miles per hour. Magellan was about 15.4 feet(about 4.6 meters) long and weighed 7,612 pounds (3,460 kilograms). It was the first U.S. planetary spacecraft since 1978, and the first ever on a space shuttle.
Magellan entered orbit around Venus on August 16, 1990. There was only one scientific experiment aboard, a radar imaging system called synthetic aperture radar (SAR). Saunders hoped that SAR could reveal Venus’s landscape, which was virtually unknown because of the planet’s dense cloud layer of carbon dioxide and sulfuric acid. SAR bounced several thousand radio pulses per second off Venus’s surface. Magellan then transmitted the raw data to Earth, where computers reconstructed pictures of the landscape. The probe also relayed data on the heights and temperatures on the surface.
Magellan’s original orbit was elliptical, moving from south to north or vice versa, and it completed an orbit every three hours and fifteen minutes. The probe orbited as close as 182 miles (294 kilometers) and as far as 5,296 miles (8,543 kilometers) from the surface of Venus. The spacecraft’s radar mapped 98 percent of the entire surface of Venus by September, 1992. Repeated passes over the planet allowed scientists to look for changes and construct three-dimensional views of the surface. Never before had an interplanetary probe mapped an entire world with such fine detail and, although previous spacecraft and radar efforts yielded information about Venus’s landscape, most researchers believe that, because of the resolution of the images, the data from Magellan basically started the study of the planet anew.
An image of the Venusian surface generated by computer from Magellan data. This area is in the equatorial highlands known as Aphrodite Terra. The lowlands on the right are believed to be composed of lava flows, whereas the mountainous areas are tessera terrain. A huge rift valley is evidence of the planet’s tectonic activity.
Even though Saunders and Pettengill expected a large amount of information from Magellan, the computers at the Jet Propulsion Laboratory in California were still swamped by the quantity of data received, causing weeks of backlog. After only ten weeks of mapping, Magellan had already sent back more than a trillion bits of data. Saunders and Pettengill had hundreds of researchers working double shifts from late 1990 through most of 1991, analyzing features such as craters, tectonic fractures, and volcanoes.
Saunders, whose primary responsibility was organizing, calibrating, and archiving the data transmitted from Magellan, also analyzed data and participated in the scientific debates that arose. Some of the questions raised concern over why the radar properties of the landscape changed dramatically at a particular altitude, whether the planet was still geologically active, and why the planet looked as if the entire surface had been covered by lava only about 500 million years earlier.
Magellan began a gravity survey in September, 1992, and by May 25, 1993, had already collected gravity measurements from almost 70 percent of the surface. A new experiment, involving a technique called aerobraking, began May 25. This technique used the planet’s atmosphere to slow or steer the spacecraft. Aerobraking was a new maneuver to change a planetary spacecraft’s orbit that had never been used before, but it proved to be highly successful. The experiment took seventy-two days, gathered valuable atmospheric data at different altitudes and solar times, and provided engineering data about aerobraking and aeroheating. In its smaller orbit, Magellan passed as close as 112 miles (180 kilometers) and as far as 336 miles (541 kilometers) from Venus’s surface. The probe completed an orbit of the planet every ninety-four minutes and collected better gravity data near the north and south poles. By the end of the mission, Magellan had transmitted high-resolution gravity data for almost 95 percent of the planet’s surface.
While the radar images provided information on the shape, roughness, and reflectivity of Venus’s landscape, the gravity measurements provided information on the mass below the visible features. This unique and invaluable information about Venus’s geophysical foundation helped researchers better understand the movements in the interior of the planet. The information indicated that Venus had no evidence of plate tectonics (the movement of pieces of the planet’s crust), a process present on Earth that causes earthquakes and volcanic eruptions.
In September, 1994, another experiment involving an orbit-change test, called a “windmill experiment,” began. The solar panels of the spacecraft, which resemble the blades of a windmill, were changed so that the orbit was lowered into the upper part of the atmosphere. This experiment provided data on the molecules in Venus’s upper atmosphere and proved helpful in designing future spacecraft.
On October 12, 1994, Magellan’s mission ended when the spacecraft was sent into Venus’s surface, the first time an operating spacecraft was intentionally crashed. All of Magellan’s significant mission goals were accomplished and funding was nearly depleted. The last signals received indicated that Magellan had encountered denser layers of gas and finally crashed into the surface.
Scientists knew that Venus’s surface temperature was about 900 Fahrenheit (470 Celsius), had no water, and that the surface pressure was ninety times that of Earth. Even with this knowledge, however, theories remained that Venus was similar enough to Earth to provide insight into the Earth’s geological processes. Among the more important results from the data collected from Magellan was that Venus is not as much like the Earth geologically as was originally believed. The planet’s geological processes are very different from the Earth’s, which dramatically changed scientists’ views of Venus. Although the original view was that Venus’s and Earth’s processes were similar, and therefore the geological history of the planets would be similar, the data from Magellan’s mission suggested that researchers had to consider other possible histories of Venus. The amount of data from Magellan’s radar and gravity mapping activities was enough to analyze for decades. Geologists know that Venus underwent a dramatic volcanic event 300 to 500 million years ago that totally resurfaced the entire planet with lava flows. What this event means for Earth’s future is a matter of scientific debate.
Additionally, the aerobraking and windmill experiments provided valuable information used in later spacecraft designs and missions. Since aerobraking was shown to be safe, the technique was used on future missions to Venus and Mars, saving weight and reducing the complexity of the spacecraft.
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Beatty, J. Kelly. “Working Magellan’s Magic.” Sky and Telescope (August, 1993): 16-20. Discusses questions raised by the data. Includes an extremely helpful chronology. -
Grinspoon, David Harry. Venus Revealed: A New Look Below the Clouds of Our Mysterious Twin Planet. Reading, Mass.: Addison-Wesley, 1997. Discusses historical views of, and beliefs about, Venus. Also discusses the impact of Magellan data on scientific knowledge and theories. Photographs and index. -
Kargel, Jeffrey S. “The Rivers of Venus.” Sky and Telescope 94 (August, 1997): 32-37. New ideas about volcanic behavior, suggesting that Venus originally may have had an atmosphere more similar to Earth’s. -
Kerr, Richard A. “Craters Suggest How Venus Lost Her Youth.” Science 284 (May 7, 1999): 889. Brief article about new reviews of radar images that indicate volcanoes may have been active longer than originally thought. -
Portree, David S. F. “A Planetary Comeback: Magellan Heads for Venus.” Astronomy 17 (September, 1989): 38-42. Provides background on the launch of Magellan. Includes photographs. -
Robinson, Cordula. “Magellan Reveals Venus.” Astronomy 23 (February, 1995): 32-41. Lengthy article on how new information changed beliefs about the geology of Venus and its similarity to Earth.
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