The 1970s were a transitional period for the U.S. space effort. The Apollo program was coming to a close, and NASA was trying to figure out what form manned spaceflight would take. The Mariner missions expanded our knowledge of the inner planets by sending space probes to fly past (and in some cases orbit) Mars, Venus and Mercury. There were tentative plans to send a Mariner mission to visit some of the outer planets, but using chemical rocket propulsion, such a trip would take 15 years or more.
At the same time, important advances were being made in the science of gravity-assisted orbital trajectories. While the math and physics involved are pretty complicated, the basic idea is that a spacecraft can use the gravity of a nearby planet to give it a large boost in velocity as long as the spacecraft follows the proper orbit. The higher the mass of the planet, the stronger the gravitational force, and the bigger the boost. That meant that once a space probe reached Jupiter (the most massive planet in our solar system), it could use Jupiter’s gravity like a slingshot and head out to explore the more distant planets.
In 1965, an engineer named Gary Flandro noticed that in the mid-1970s, the outer planets would be aligned in such a way as to make it possible for a spacecraft to visit them all using a series of gravity-assisted boosts [source: Evans]. This particular alignment wasn't just a once-in-a-lifetime event -- it wouldn't occur again for another 176 years. It was an amazing coincidence that the technical ability to accomplish such a mission was developed a few years before the planets lined up to allow it.
Initially, the ambitious project, known as the Grand Tour, would have sent a series of probes to visit all the outer planets. In 1972, however, budget projections for the project were approaching $900 million, and NASA was planning development of the space shuttle [source: Evans]. With the immense shuttle development costs looming, the Grand Tour was cancelled and replaced with a more modest mission profile. This would be an extension of the Mariner program, referred to as the Mariner Jupiter-Saturn mission (MJS). Based on the Mariner platform and improved with knowledge gained from Pioneer 10’s 1973 fly-by of Jupiter, the new probes eventually took the name Voyager. Design was completed in 1977. Optimistic NASA engineers thought they might be able to use gravity-assisted trajectories to reach Uranus and Neptune if the initial mission to visit Jupiter and Saturn (and some of their moons) was completed successfully. The idea of the Grand Tour flickered back to life.
The final Voyager mission plan looked like this: Two spacecraft (Voyager 1 and Voyager 2) would be launched a few weeks apart. Voyager 1 would fly past Jupiter and several of Jupiter’s moons from a relatively close distance, scanning and taking photos. Voyager 2 would also fly past Jupiter, but at a more conservative distance. If all went well, both probes would be catapulted toward Saturn by Jupiter’s gravity. Voyager 1 would then investigate Saturn, specifically the rings, as well as the moon Titan. At that point, Voyager 1’s trajectory would take it out of the solar system’s ecliptic (the plane of the planets’ orbits), away from all other planets, and eventually out of the solar system itself.
Meanwhile, Voyager 2 would visit Saturn and several of Saturn’s moons. If it was still functioning properly when that was completed, it would be boosted by Saturn’s gravity to visit Uranus and Neptune before also leaving the ecliptic and exiting the solar system. This was considered a long shot, but amazingly, everything worked as planned.
Next, what kind of hardware did the Voyagers carry into space?