....reason that a full backup then exists. Such dual launches are generally cost effective, as a second spacecraft can be obtained for 15% of the price of the first one76. Budget constraints forced NASA to buy just one Galileo, so there was tremendous pressure to construct a highly reliable spacecraft. Additional pressure has been on the project because of changes in the launch date and booster rocket. Originally scheduled for a 1982 launch, delays in the Shuttle program and other factors caused rescheduling to 1984, then 1985 and, finally, 1986, when the grounding of the Shuttle fleet forced an indefinite postponement. At first, the Air Force's Inertial Upper Stage rocket was chosen for the booster. Later, the new "wide body" Centaur got the job. Centaur upper stages have flown on Atlas and Titan III boosters since the 1960s. The new "fat Centaur" would carry 50% more fuel than the earlier version. Other changes were made to adapt it to the Shuttle cargo bay. One JPL engineer said that it is "like Abe Lincoln's axe. The head broke and they replaced it and the handle broke and they put on a new one, but it's still Abe Lincoln's axe"77. However, NASA canceled the Shuttle version of the new Centaur in the spring of 1986 due to safety considerations, leaving Galileo without a ride to Jupiter.  By early 1987, NASA decided to go back to using the Inertial Upper Stage, but it has significantly lower lifting capability than the Centaur. As a result, the flight path has to be changed to include a Venus flyby and two Earth flybys to gain velocity by gravity assistance. Unfortunately, the total flight time to Jupiter will nearly triple to about seven years.
 Finally, computer system complexity is further increased by the number of science experiments on board and the fact that they are largely computer controlled as well. Eight of the nine instruments have microprocessors for control and data handling79. These have to communicate with the Command and Data System, itself containing six microprocessors. Attitude and Articulation Control has dual computers, and the probe also contains a dual microprocessor system. In all, Galileo contains 19 microprocessors with about 320K of semiconductor random access memory and 41K of read-only memory80. No unmanned spacecraft launched to date can approach Galileo in the power and size of its on-board computer network.
....instructions can be added by the user for specific applications. Four instructions added by the Galileo project saved over 1,500 words of code in the flight program128.