Magellan was launched on May 4, 1989, at 18:46:59 UTC by the National Aeronautics and Space Administration from KSC Launch Complex 39B at the Kennedy Space Center in Florida, aboard Space Shuttle Atlantis during mission STS-30. Once in orbit, an Inertial Upper Stage booster, deployed from the shuttle and launched on May 5, 1989 01:06:00 UTC, sending the spacecraft into a Type IV, heliocentric orbit where it would circle the Sun 1.5 times, before reaching Venus 15 months later on August 10, 1990.
Originally, Magellan had been scheduled for launch in 1988 with a trajectory lasting six months. However, due to the Space Shuttle Challenger disaster in 1986, several missions, including Galileo and Magellan, were deferred until the shuttle flights resumed September 1988. Intended to be launched with a new, liquid fueled, Centaur-G shuttle deploy-able upper-stage booster, subsequently canceled after the Challenger disaster, Magellan had to be modified to attach to a less powerful solid-fueled, Inertial Upper Stage. The next best opportunity for launch would occur in October 1989.
Further complicating the launch however, was the upcoming Galileo mission to Jupiter, which included a flyby of Venus. Intended for launch in 1986, the pressures to ensure a launch for Galileo in 1989, mixed with a short launch-window necessitating a mid-October launch, resulted in replanning the Magellan mission. Weary of rapid shuttle launches, the decision was made to launch Magellan in May, and into an orbit that would require 1 year and 3 months before encountering Venus.