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Magellan probe
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The Magellan spacecraft was a space probe sent to the planet Venus, the first unmanned spacecraft to be launched by NASA since its successful Voyager 1 spacecraft to Jupiter and Saturn in 1977.
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The Magellan spacecraft was a space probe sent to the planet Venus, the first unmanned spacecraft to be launched by NASA since its successful Voyager 1 spacecraft to Jupiter and Saturn in 1977. It was also the first of three deep-space probes to be launched on the Space Shuttle (the others being the Ulysses Sun probe and the Galileo spacecraft to Jupiter) until the launching of the failed Mars Observer spacecraft on a Titan III rocket in 1992. It was also the first spacecraft to employ aerobraking techniques to lower its orbit, a technique used on the current series of orbiters around Mars that allows fuel to be conserved.
Magellan created the first (and currently the best) near-photographic quality, high resolution mapping of the planet's surface features. Prior Venus missions had created low resolution radar globes of general, continent-sized formations. Magellan, however, finally allowed detailed imaging and analysis of craters, hills, ridges, and other geologic formations, to a degree comparable to the visible-light photographic mapping of other planets. Magellan's global radar map will remain the most detailed Venus map in existence for the foreseeable future, although the planned Russian Venera-D may carry a radar that can achieve the same, if not better resolution as the radar used by Magellan.
It was named after the sixteenth-century Portuguese explorer Ferdinand Magellan. Magellan was the first planetary spacecraft to be launched by a Space Shuttle when it was carried aloft by the Orbiter Atlantis from Kennedy Space Center in Florida on May 4, 1989, on the STS-30 mission. Atlantis took Magellan into low Earth orbit, where it was released from the shuttle's cargo bay. A solid-fuel motor called the Inertial Upper Stage (IUS) then fired, sending Magellan on a 15-month cruise looping around the Sun 1-1/2 times before it arrived at its orbit around Venus on August 10, 1990. In 1994 it plunged to the surface as planned and partly vaporized; some sections are thought to have hit the planet's surface.
Mission overview
Magellan's initial orbit was highly elliptical, taking it as close as 294 kilometres (182 miles) from Venus and as far away as 8,543 km (5,296 mi). The orbit was a polar one, meaning that the spacecraft moved from south to north or vice versa during each looping pass, flying over Venus' north and south poles. Magellan completed one orbit every 3 hours, 15 minutes.
During the part of its orbit closest to Venus, Magellan's radar mapper imaged a swath of the planet's surface approximately 17 to 28 km (10 to 17 mi) wide. At the end of each orbit, the spacecraft radioed back to Earth a map of a long ribbon-like strip of the planet's surface captured on that orbit. Venus itself rotates once every 243 Earth days. As the planet rotated under the spacecraft, Magellan collected strip after strip of radar image data, eventually covering the entire globe at the end of the 243-day orbital cycle.
By the end of its first such eight-month orbital cycle between September 1990 and May 1991, Magellan had sent to Earth detailed images of 84 percent of Venus' surface. The spacecraft then conducted radar mapping on two more eight-month cycles from May 1991 to September 1992. This allowed it to capture detailed maps of 98 percent of the planet's surface. The follow-on cycles also allowed scientists to look for any changes in the surface from one year to the next. In addition, because the "look angle" of the radar was slightly different from one cycle to the next, scientists could construct three-dimensional views of Venus' surface.
During Magellan's fourth eight-month orbital cycle at Venus from September 1992 to May 1993, the spacecraft collected data on the planet's gravity field. During this cycle, Magellan did not use its radar mapper but instead transmitted a constant radio signal to Earth. If it passed over an area of Venus with higher than normal gravity, the spacecraft would slightly speed up in its orbit. This would cause the frequency of Magellan's radio signal to change very slightly due to the Doppler effect – much like the pitch of a siren changes as an ambulance passes. Thanks to the ability of radio receivers in the NASA/JPL Deep Space Network to measure frequencies extremely accurately, scientists could build up a detailed gravity map of Venus.
At the end of Magellan's fourth orbital cycle in May 1993, flight controllers lowered the spacecraft's orbit using a then-untried technique called aerobraking. This maneuver sent Magellan dipping into Venus' atmosphere once every orbit; the atmospheric drag on the spacecraft slowed down Magellan and lowered its orbit. After the aerobraking was completed between May 25 and August 3, 1993, Magellan's orbit then took it as close as 180 km (112 mi) from Venus and as far away as 541 km (336 mi). Magellan also circled Venus more quickly, completing an orbit once every 94 minutes (roughly the same amount of time its takes the Space Shuttle or the International Space Station to complete a single orbit around Earth). This new, more circularized orbit allowed Magellan to collect better gravity data in the higher northern and southern latitudes near Venus' poles.
After the end of that fifth orbital cycle in April 1994, Magellan began a sixth and final orbital cycle, collecting more gravity data and conducting radar and radio science experiments. By the end of the mission, Magellan had captured high-resolution gravity data for an estimated 95 percent of the planet's surface.
In September 1994, Magellan's orbit was lowered once more in another test called a "windmill experiment". In this test, the spacecraft's solar panels were turned to a configuration resembling the blades of a windmill, and Magellan's orbit was lowered into the thin outer reaches of Venus' dense atmosphere. Flight controllers then measured the amount of torque control required to maintain Magellan's orientation and keep it from spinning. This experiment gave scientists data on the behaviour of molecules in Venus' upper atmosphere, and lead engineers new information useful in designing spacecraft.
On October 11, 1994, Magellan's orbit was lowered a final time and radio contact was lost the next day. Within two days after that maneuver, the spacecraft became caught in the atmosphere and plunged to the surface. Although much of Magellan was vaporized, some sections are thought to have hit the planet's surface intact.
Imaging cycles
From its arrival in August, 1990 until its demise in 1994, the Magellan spacecraft's primary mission was divided into "imaging cycles," each lasting 243 days total (the time it takes Venus to complete a single rotation on its axis). During each of the early cycles, the probe would complete a total of 7.3 orbits for each Earth day, imaging strips approximately 17 to 28 km. (11 to 17 mi.) wide and 70,000 km. (43,486 mi.) long. It took a total of 1,800 strips to cover the entire planet, which were then combined into a single mosaic image.
The first images of Venus were received on August 16, 1990, and routine mapping operations began on September 15, 1990. The first mapping cycle (Cycle 1) was completed successfully on May 15, 1991, mapping 84% of the Venusian surface.
Cycle 2 began immediately afterwards and lasted until January 15, 1992. In each cycle, the probe was inclined at a different "look angle", producing stereoscopic data which enabled scientists to compile a three-dimensional map of the surface—a technique known as interferometric synthetic aperture radar.
Cycle 3 was due to finish on September 14, 1992, but was terminated a day early due to problems with onboard equipment. In total, radar coverage of 98% of the surface of Venus was obtained, with 22% of the images in stereo. Magellan produced surface images of unprecedented clarity and coverage, which are still unsurpassed.
Cycles 4, 5 and 6 were devoted to collecting gravimetric data, for which Magellan was aerobraked to its lowest possible stable orbit, with a periapsis or closest approach of 180 kilometers (112 mi). At the end of Cycle 6 its orbit was reduced further, entering the outer reaches of the atmosphere. After carrying out a few final experiments, Magellan successfully completed its mission on October 11, 1994, and was de-orbited to burn up in Venus's atmosphere.
Spacecraft design
Built largely from spare parts from both the Voyager and Galileo missions, the Magellan spacecraft was 4.6 meters (15.4 feet) long, topped with a 3.7 m (12 ft) parabolic antenna. Mated to its retrorocket (which was jettisoned after orbital insertion) and fully tanked with propellants, the spacecraft weighed a total of 3,460 kilograms (7,612 pounds) at launch.
The high-gain antenna, used for both communication and radar imaging, was a spare from the Voyager Program to the outer planets, as were Magellan's 10-sided main bus section and a set of thrusters. On board computer systems, and power distribution units are spares from the Galileo mission to Jupiter. and its medium-gain antenna is from the NASA/JPL Mariner 9 project. Martin Marietta Astronautics (Now Lockheed Martin) was the prime contractor for the Magellan spacecraft, while Hughes Space & Communications was the prime contractor for the radar system.
Magellan was powered by two square solar panels, each measuring 2.5 m (8.2 ft) on a side; together they supplied 1,200 watts of power (100 watt per mē). Over the course of the mission the solar panels gradually degraded, as expected; by the end of the mission in the fall of 1994 it was necessary to manage power usage carefully to keep the spacecraft operating.
Because Venus was shrouded by a dense, opaque atmosphere, conventional optical cameras could not be used to image its surface. Instead, Magellan's imaging radar used bursts of microwave energy somewhat like a camera flash to illuminate the planet's surface.
Magellan's high-gain antenna sent out millions of pulses each second toward the planet; the antenna then collected the echoes returned to the spacecraft when the radar pulses bounce off Venus' surface. Because the radar pulses were not sent directly downward but rather at a slight angle to the side of the spacecraft, the radar is thus sometimes called "side-looking radar". In addition, special processing techniques were used on the radar data to result in higher resolution as if the radar had a larger antenna, or "aperture"; the technique is thus often called "synthetic aperture radar", or SAR. Magellan's maps had an ultimate resolution of about 120m; the published maps are oversampled at 75m.
Synthetic aperture radar was first used by NASA on JPL's Seasat oceanographic satellite in 1978; it was later developed more extensively on the Spaceborne Imaging Radar (SIR) missions on the space shuttle in 1981, 1984 and 1994. An imaging radar was also used as part of the NASA/JPL Cassini mission to Saturn in 1997 to map the surface of the ringed planet's major moon Titan.
Besides its use in imaging, Magellan's radar system was also used to collect altimetry data showing the elevations of various surface features. In this mode, pulses were sent directly downward and Magellan measured the time it took a radar pulse to reach Venus and return in order to determine the distance between the spacecraft and the planet.
Mission results
Study of the Magellan high-resolution global images is providing evidence to better understand Venusian geology and the role of impacts, volcanism, and tectonism in the formation of Venusian surface structures. The surface of Venus is mostly covered by volcanic materials. Volcanic surface features, such as vast lava plains, fields of small lava domes, and large shield volcanoes are common. There are few impact craters on Venus, suggesting that the surface is, in general, geologically young - less than 800 million years old. The presence of lava channels over 6,000 kilometers long suggests river-like flows of extremely low-viscosity lava that probably erupted at a high rate. Large pancake-shaped volcanic domes suggest the presence of a type of lava produced by extensive evolution of crustal rocks.
The typical signs of terrestrial plate tectonics - continental drift and basin floor spreading - are not in evidence on Venus. The planet's tectonics is dominated by a system of global rift zones and numerous broad, low domical structures called coronae, produced by the upwelling and subsidence of magma from the mantle.
Although Venus has a dense atmosphere, the surface reveals no evidence of substantial wind erosion, and only evidence of limited wind transport of dust and sand. This contrasts with Mars, where there is a thin atmosphere, but substantial evidence of wind erosion and transport of dust and sand.
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