Magellan probe

The Magellan spacecraft 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 shuttle Atlantis from Kennedy Space Center in Florida on May 4, 1989. 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 Venus on August 10, 1990. A solid-fuel motor on Magellan then fired, placing the spacecraft in orbit around Venus.

Magellan's initial orbit was highly elliptical, taking it as close as 294 kilometers (182 miles) from Venus and as far away as 8,543 kilometers (5,296 miles). 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's 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 kilometers (10 to 17 miles) 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's surface. The spacecraft then conducted radar mapping on two more eight- months 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's 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's 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 kilometers (112 miles) from Venus and as far away as 541 kilometers (336 miles). Magellan also circled Venus more quickly, completing an orbit once every 94 minutes. This new, more circularized orbit allowed Magellan to collect better gravity data in the higher northern and southern latitudes near Venus's 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 will have 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's 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 behavior of molecules in Venus's upper atmosphere, and lent engineers new information useful in designing spacecraft.

On October 11, 1994, Magellan's orbit was lowered a final time. 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.

Spacecraft design

Built partially with spare parts from other missions, the Magellan spacecraft was 4.6 meters (15.4 feet) long, topped with a 3.7-meter (12-foot) high-gain antenna. Mated to its retrorocket 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 NASA/JPL Voyager mission to the outer planets, as were Magellan's 10-sided main structure and a set of thrusters. The command data computer system, attitude control computer and power distribution units are spares from the Galileo mission to Jupiter. Magellan's medium-gain antenna is from the NASA/JPL Mariner 9 project. Martin Marietta Corp. was the prime contractor for the Magellan spacecraft, while Hughes Aircraft Co. was the prime contractor for the radar system.

Magellan was powered by two square solar panels, each measuring 2.5 meters (8.2 feet) on a side; together they supplied 1,200 watts of power. 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 is shrouded by a dense, opaque atmosphere, conventional optical cameras cannot be used to image its surface. Instead, Magellan's imaging radar uses bursts of microwave energy somewhat like a camera flash to illuminate the planet's surface.

Magellan's high-gain antenna sends out millions of pulses each second toward the planet; the antenna then collects the echoes returned to the spacecraft when the radar pulses bounce off Venus's surface. The radar pulses are 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 are 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.

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 is also planned 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 understand 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.

External links

• Magellan homepage (http://www.jpl.nasa.gov/magellan/)