[4] It was postponed again until October 18, 1989, because of rain-showers within 32 kilometres (20 mi; 17 nmi) of Kennedy Space Center's Shuttle Landing Facility.

[5] The primary payload, the Galileo spacecraft with its attached Inertial Upper Stage (IUS), was successfully deployed on its journey to Jupiter.

Also, the spacecraft was scheduled to make the first extended observations of the Jovian system and first direct sampling of Jupiter's atmosphere, as well as the first asteroid flybys.

There were also some minor problems with the Flash Evaporator System for cooling the orbiter, and the cryogenic oxygen manifold valve 2, which was left closed for the rest of the mission.

Demonstrators protested at launch time against the flight because the Galileo spacecraft, the mission's payload, was powered by a radioisotope thermoelectric generator.

Díaz also claimed the flight was almost aborted in orbit three times because of malfunctions, but continued because the alternative was to land Atlantis, carrying Galileo and its generator, at an airport in Senegal, which could have caused an "international incident", according to the astronaut.

Deployment occurred on schedule at 19:15 EDT on October 18, 1989, slightly more than six hours after launch, and the IUS successfully boosted Galileo toward Venus on the first leg of its six-year journey to Jupiter.

Galileo had two major components: an orbiter which examined Jupiter and its four largest moons for eight years, and a probe which descended into the Jovian atmosphere to take direct samplings before being destroyed by the gas giant's heat and pressure.

Besides the Galileo spacecraft, Atlantis' payload bay held two canisters containing the Shuttle Solar Backscatter Ultraviolet (SSBUV) experiment.

On October 22, 1989, Lucid and Baker completed the Growth Hormone Concentration and Distribution in Plants experiment by freezing samples of corn seedlings grown in orbit during the mission.