groups of two probes each, with one group visiting Jupiter, Saturn, and Pluto and the other Jupiter, Uranus, and Neptune. The spacecraft would be designed with redundant systems to ensure survival through the entire tour. By 1972 the mission was scaled back and replaced with two Mariner program-derived spacecraft, the Mariner Jupiter-Saturn probes. To keep apparent lifetime program costs low, the mission would include only flybys of Jupiter and Saturn, but keep the Grand Tour option open. As the program progressed, the name was changed to Voyager. The primary mission of Voyager 1 was to explore Jupiter, Saturn, and Saturn's moon, Titan. Voyager 2 was also to explore Jupiter and Saturn, but on a trajectory that would have the option of continuing on to Uranus and Neptune, or being redirected to Titan as a backup for Voyager 1. Upon successful completion of Voyager 1's objectives, Voyager 2 would get a mission extension to send the probe on towards Uranus and Neptune. Spacecraft design Constructed by the Jet Propulsion Laboratory (JPL), Voyager 2 included 16 hydrazine thrusters, three-axis stabilization, gyroscopes and celestial referencing instruments (Sun sensor/Canopus Star Tracker) to maintain pointing of the high-gain antenna toward Earth. Collectively these instruments are part of the Attitude and Articulation Control Subsystem (AACS) along with redundant units of most instruments and 8 backup thrusters. The spacecraft also included 11 scientific instruments to study celestial objects as it traveled through space. Communications Built with the intent for eventual interstellar travel, Voyager 2 included a large, parabolic, high-gain antenna (see diagram) to transceive data via the Deep Space Network on the Earth. Communications are conducted over the S-band (about 13 cm wavelength) and X-band (about 3.6 cm wavelength) providing data rates as high as 115.2 kilobits per second at the distance of Jupiter, and then ever-decreasing as distance increases, because of the inverse-square law. When the spacecraft is unable to communicate with Earth, the Digital Tape Recorder (DTR) can record about 64 megabytes of data for transmission at another time. Power Voyager 2 is equipped with three Multihundred-Watt radioisotope thermoelectric generators (MHW RTG). Each RTG includes 24 pressed plutonium oxide spheres, and provided enough heat to generate approximately 157 W of electrical power at launch. Collectively, the RTGs supplied the spacecraft with 470 watts at launch (halving every 87.7 years). They were predicted to allow operations to continue until at least 2020 and have already done