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.