Magellan (spacecraft) - astronomy.

Magellan (spacecraft) - astronomy. I INTRODUCTION Magellan (spacecraft), United States space probe that was launched in 1989 and mapped most of the surface of the planet Venus. The Magellan Venus radar mapper provided the first detailed global map of Venus's hidden surface. Magellan was built mostly from spare parts from other spacecraft. It carried a large dish-shaped radar and radio antenna. Magellan left Earth on May 4, 1989, and reached an orbit around Venus on August 10, 1990. Its goal was to map 70 percent of Venus's surface in great detail. By the time Magellan's main mission ended in September 1992, it had mapped 98 percent of Venus. More than 20 spacecraft visited Venus before Magellan, but Magellan was the first to make detailed maps of almost the entire planet. Magellan burned up in Venus's atmosphere in September 1994. II SPACECRAFT The Magellan spacecraft was 6.4 m (21 ft) tall and had a mass of 3452 kg (7610 Earth lb) when it began its trip to Venus. In Venus orbit, the spacecraft jettisoned its solid rocket motor to become 4.7 m (15.4 ft) tall and have a mass of 1306 kg (2880 lb). Magellan measured 9.5 m (31 ft) wide, including its twin 2.5-m (8.2-ft) solar arrays. These arrays converted sunlight into electricity to power Magellan's systems (see Solar Energy). The National Aeronautics and Space Administration (NASA) saved money on Magellan by constructing 70 percent of the spacecraft from spare parts from other programs. Magellan had four radio antennas. The most important of these was the dish-shaped high-gain antenna, a spare from the Voyager program. This antenna was 3.7 m (12.1 ft) wide and doubled as Magellan's mapping radar and main communications link to scientists and controllers on the earth. During each orbit, Magellan pointed the high-gain antenna at Venus to bounce radar pulses off the surface, then turned to point the antenna toward Earth and transmit its data. The medium-gain antenna was a spare from the Mariner 9 Mars orbiter. The altimeter antenna measured the heights and depths of Venus's mountains and valleys. Controllers communicated with Magellan through the small low-gain antenna while the spacecraft was close to the earth. The antennas were mounted on the forward equipment module, which contained radar and radio electronics and three spinning momentum wheels. The wheels steered Magellan without rocket power, saving fuel. The ten-sided bus, a compartment for computers, was another spare part from the Voyager program. The bus contained computers originally built for the Galileo Jupiter orbiter. White thermal blankets covered the bus and equipment module, shielding delicate systems from the intense sunlight at Venus. The propulsion module carried 109 kg (239 lb) of fuel in spare space shuttle tanks, and included four clusters of six thrusters each. The thrusters were Voyager spares. The Inertial Upper Stage (IUS), a solid-fueled rocket weighing 14,800 kg (32,600 lb), pushed Magellan out of Earth orbit and on course for Venus. The IUS was discarded after its fuel was gone, along with the IUS adapter that linked the rocket to Magellan. When Magellan arrived at Venus, a 2146-kg (4731-lb) Star 48B rocket motor pushed it into the proper mapping orbit. The rocket was then discarded. The Star 48B was built originally to launch communication satellites into orbit around the earth. III MISSION Magellan lifted off aboard the space shuttle Atlantis on May 4, 1989, becoming the first interplanetary probe to launch from a space shuttle. Magellan and the IUS rocket bounced out of Atlantis's payload bay on springs, and the IUS fired, giving Magellan enough speed to escape the earth's strong gravitational pull. Magellan then coasted for 15 months to reach Venus. Normally, spacecraft reach Venus in four months, but NASA decided to send Magellan on a longer path because of a cosmic traffic jam--the Galileo orbiter needed to use the four-month path to Venus. Magellan reached Venus on August 10, 1990. The Star 48B rocket placed Magellan into an elliptical mapping orbit ranging from 289 to 8030 km (180 to 4990 mi) above the planet's surface. The orbit took Magellan over Venus's poles about seven times each Earth day. Venus slowly rotated under Magellan, so the spacecraft flew over all points on the surface over the course of one Venus day (243 Earth days). Routine mapping began on September 15, 1990. During each orbit, Magellan's radar mapped a strip 17,000 km (10,600 mi) long and 20 to 25 km (12 to 15 mi) wide. Magellan stopped mapping on September 13, 1992, after imaging 98 percent of the surface. Details as small as 100 m (350 ft) wide were visible in Magellan's radar pictures. Until mid-1993 scientists used Magellan's orbital motion to plot Venus's gravitational field, which would help them determine how the planet's mass was distributed. Controllers then used the spacecraft's thrusters to lower its orbit to graze Venus's thin upper atmosphere. Each time Magellan passed through the atmosphere it slowed, lowering its orbit's high point. This procedure, called aerobraking, saves rocket fuel by allowing friction with the atmosphere--instead of thrusters--to slow the craft. Engineers used Magellan's aerobraking data to help design the Mars Global Surveyor 96 spacecraft. Controllers carried out one final aerobraking experiment in October 1994 by turning Magellan's solar arrays so that the spacecraft behaved like a propeller. Communications were lost on October 12, 1994, and the probe burned up in Venus's atmosphere a few days later. The momentum wheels probably survived to hit Venus's surface. Magellan collected more data than all other space probes launched to explore the solar system combined. Before Magellan, scientists knew almost nothing about Venus's surface. Magellan's maps show that although Venus is almost the same size as Earth, its geology has little in common with Earth geology. Venus has many volcanoes, but they occur in small areas all over the planet instead of being concentrated along boundaries of tectonic plates like Earth's volcanoes. Lava flows cover much of Venus, creating gently rolling hills over almost all of the planet's surface. Magellan's discoveries permit scientists to better understand our own planet's crust by comparing and contrasting Earth and Venus. Contributed By: David S. F. Portree Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.