centaurg

This is the Centaur stage that was designed for use with the space shuttle. It was scheduled to launch the Galileo probe to Jupiter. In fact, in the upper left quadrant, the sign behind the viewing stand has a picture of Jupiter and Saturn. There were many risks involving having a liquid fueled rocket in the shuttle's cargo bay: in the case of an abort, the explosive liquid fuel of the Centaur would have to be jettisoned. Moreover, the increasing weight of the shuttle and the Centaur stage meant that the shuttle main engines would have to operate at 109% for part of the ascent phase. The increase in risk was significant, but engineers and astronauts had signed off on it. However, following the space shuttle Challenger explosion on January 28, 1986, the program was cancelled.

Here's a short video about choosing the boosters for the Galileo mission. It includes some musings on Congress' role in the process: vimeo.com/87712113

Actually, this is the short G version which was intended for GSO missions. The the G' (G Prime) was the long version intended for the really heavy payloads going to GSO or interplanetary trajectories. There was several Gs in production at the time of the program's cancellation in 1986. Some had been bought by the DoD and one had been bought by NASA for the Venus Radar Mapping mission (later renamed to Magellan which launched on a IUS in May 1989).

Model of the Centaur-G and the Galileo probe it was originally slated to propel to Jupiter after boosted into orbit aboard a Space Shuttle. After the explosion of Challenger (OV-099)in 1986, the risk of using the liquid fueled Centaur-G to boost payloads from the Shuttle's low Earth orbit were deemed to be too great, and Centaur-G was never used. However, a variant, the Centaur-T, would be used to boost payloads launched into orbit by the Titan IV rocket (itself used to launch payloads originally intended for the Shuttle prior to the Challenger's loss). See: en.wikipedia.org/wiki/Centaur_(rocket_stage)