Robot Explorers: US Unmanned Space Missions

Robot Explorers: US Unmanned Space Missions: page 16

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Magellan spacecraft under construction.

Magellan was launched on May 4, 1989 aboard the space shuttle Atlantis. It went into orbit around Venus on August 10, 1990. Magellan's solid rocket motor placed it into a near-polar elliptical orbit around the planet. Its mission was to map the surface of Venus with imaging radar. The basic scientific instrument was a synthetic aperture radar, which could look through the thick layer of clouds that continuously shields the surface of Venus.

During the first mapping cycle, more than 80% of Venus' surface was mapped. The spacecraft completed one orbit every three hours, passing as close to the planet as 183 miles, and as far away as 5,000 miles. Thanks to the extended mission which ran through September of 1992, the mapping coverage was increased to 98% of the planet, with a resolution of approximately 100 meters. In addition to radar mapping, Magellan compiled a high-resolution, comprehensive gravity field map for 95% of the planet.


Visible light image of Venus showing clouds.	Precision radio tracking of the spacecraft allowed scientists to measure Venus' gravitational field to show the planet's internal mass distribution and the forces which have created the surface features. Magellan's data gave us insight into the global geology of Venus, which is the planet most like Earth in our solar system. Venus resembles Earth in size, physical composition, and density more closely than any other planet. There are many differences as well, such as Venus' retrograde rotation, from west to east. The variety of terrain formations we are able to study from the images returned by Magellan has helped scientists understand the processes that formed this intriguing landscape. Hopefully, by comparison, these continuing studies will help to shed light on our own planet's evolutionary processes.	Radar image of Venus showing surface features.


Magellan has shown us a landscape dominated by volcanic features, faults and impact craters. The radar image below shows a region about 160 miles across. The large circular structure near the center of the image is a volcanic feature, approximately 120 miles in diameter. Just north is one of the flat-topped volcanic constructs known as 'pancake' domes for their shape and flap-jack appearance. This pancake dome is about 21 miles in diameter and is thought to have formed by the eruption of an extremely viscous lava. Complex fracture patterns like the one in the upper right of the image are often observed in association with coronae and various volcanic features. They are thought to form because magma beneath the surface follows pre-existing fracture patterns. When eruptions or other movements of the magma occur, the magma drains from the fractures and the overlying surface rock collapses.
Vocanic features or alien depiction of muppet Miss Piggy?

	Venus' thick atmosphere is about 96% carbon dioxide; its clouds contain sulfuric acid. The atmosphere acts like a greenhouse, permitting solar radiation to reach the surface, but trapping the heat that would ordinarily be radiated back into space. The result is an average surface temperature that is over ten times as hot as Earth's. Between the boiling hot temperatures on the surface of the planet (900º F), and the atmospheric pressure which is about 90 times Earth's, Venus would not be a nice place to visit! The greenhouse effect on Venus works as follows. A certain amount of sunlight is able to penetrate the thick clouds (the yellow waves). This sunlight heats the planet's surface. The hot surface radiates heat in the form of infrared radiation (the red waves). The infrared radiation cannot pass through the carbon dioxide clouds, but instead is reflected back toward the planet. This process continued until the temperature reached an equilibrium at a scorching 900º F.

The Magellan spacecraft ended its successful mission when it was commanded to plunge into the planet's dense atmosphere in October of 1994. Magellan gathered scientific data on the planet's upper atmosphere, including aerodynamic interactions with it during the spacecraft's final descent, by orienting its wing-like solar panels in opposite directions, like a windmill. The data returned from this experiment allowed scientists to better understand basic gas-surface interactions and to gain additional aerodynamic and atmospheric data on Venus for future mission designs. The craft burnt up in the atmosphere three days after this final experiment.

For more information on Magellan visit http://www2.jpl.nasa.gov/magellan/

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	Copyright ©2004 Colleen Gino	Images courtesy JPL and NASA.