Tour and TOPS were replaced with a proposal to visit only two planets using a pair of Mariner-derived probes. Mariner Jupiter-Saturn The Mariner Jupiter-Saturn project was approved in early 1972, with an estimated cost of less than $360 million for each of two probes. The probes would be built by JPL, with the intention that they would last long enough to complete the original Grand Tour of the four giant planets, but be advertised as missions to only Jupiter and Saturn to reduce estimated total project costs. The probes were to visit Jupiter, Saturn, and Saturn's moon Titan. Titan was a valuable target, as it was the only moon known to have an atmosphere, and a flyby would gather information that would not otherwise be obtainable, including the density, composition, and temperature of the atmosphere. Two trajectories were selected. One was designated JST: its mission would take it to Jupiter, Saturn, and Titan, with the probe's trajectory designed to optimize the Titan flyby. The second was designated JSX: it would be launched on a trajectory that would preserve the option of a Grand Tour, while serving as backup for the first probe. It would arrive after JST, and if JST were successful, it could continue with the Grand Tour. If JST was unsuccessful, JSX could be diverted to perform the Titan flyby itself, which would eliminate the possibility of a Grand Tour. In March 1977, just a few months before launch, NASA held a competition to rename the project. Voyager The two spacecraft that launched retained the same mission concept. Voyager 1's course was optimized for the Titan flyby and Voyager 2 for the Grand Tour. Voyager 2 would reach Saturn nine months after Voyager 1, giving plenty of time to decide if it should proceed with the Grand Tour. Additionally, by launching Voyager 2 first, Voyager 1's launch could be re-targeted to perform the Grand Tour if Voyager 2 were lost in a launch failure. An option to skip Voyager 1's Titan flyby and proceed from Saturn to Pluto was identified, though Titan was still considered the more interesting target, especially after images from Pioneer 11 indicated a very substantial atmosphere. Though atmospheric haze obscured any images of Titan's surface, Voyager 1's flyby obtained valuable information about the moon, including data that offered compelling evidence for the existence of liquid hydrocarbon lakes on Titan's surface. With Voyager 1's mission