|Voyager at Jupiter, Credit: NASA/JPL|
The recent Alpbach summer school featured a fascinating presentation from a retired JPL employee, John Casani, who had played a major role in past exploration of the outer solar system. As his stories included tales I hadn't heard before, about topics that have greatly affected my own research, I've decided to put some of them to paper. The twin Voyager spacecraft started life in 1961 when Michael A. Minovitch discovered that the gravity of the planets could be used to slingshot spacecraft between different planetary targets. Four years later, Gary A. Flandro discovered a unique planetary alignment that would recur once every 175 years, which would be ideal fur the use of gravity assists to reach the frigid planets of the outer solar system. Such an approach would allow robotic explorers to reach Neptune much faster than a direct trajectory.
The Grand Tour mission was proposed in 1970 for a "new start" in 1971, and originally consisted of two missions: one to the gas giants and Uranus, and one to the gas giants and Neptune. However, NASA canned this concept due to concerns about its ambitious and costly nature, and JPL re-proposed a more modest mission to Jupiter and Saturn based on the Mariner series of spacecraft (Mariner Jupiter Saturn 1977). That mission became Voyager in 1978, and the idea that the spacecraft could go on to visit the ice giants after the successful completion of the gas giant studies was introduced. Casani described Uranus and Neptune as "targets of opportunity", rather than destinations which were required for NASA to consider Voyager a success, and explains why planetary mission goals often seem modest (e.g., mars rovers for 90 days) but go onto achieve great things. It also meant extended funding would be contingent on the success of the gas giant mission.
The Voyagers were launched by Titan IIIE‐Centaurs on August 20, 1977 (V2) and September 5, 1977 (V1) at a then‐year cost of $320M. The spacecraft carried six RTGs for power, half of the paid for under the “Atoms for Peace” policy. Major redesigns were required after Pioneer 11 discovered Jupiter's intense radiation belts in 1974, requiring radiation-hard components, a key feature of all Jupiter missions since. Casani recalled that fumes from facility painting 3 weeks before launch meant that several science instruments required replacement detectors.
The Voyagers went on to deliver a spectacular mission of discovery in the outer solar system, passing Jupiter in 1979, Saturn and Titan in 1981, and remain the only spa craft to ever visit the ice giants Uranus and Neptune in 1986 and 1989, respectively. Both spacecraft are now leaving our solar system, with Voyager 1 the most distant human-made object at over 120AU from the sun in February 2012, and adding 3.6AU to that tally every year. Power from those RTGs is declining, but four of the 11 science instruments are still operating. By 2020, there'll likely only be enough for one instrument, recording the passage through the heliopause and into interstellar space until around 2025.