In 1964, a rare planetary alignment occurred that would allow a single spacecraft to visit Jupiter, Saturn, Uranus, and Neptune, a configuration that happens only once every 175 years. This astronomical coincidence was the catalyst for the Planetary Grand Tour, a visionary concept developed by aerospace engineer Gary Flandro at the Jet Propulsion Laboratory. The initial plan was audacious, proposing multiple pairs of probes to explore the outer solar system, but political and budgetary realities soon intervened. By December 1971, the full Grand Tour was canceled as funding was redirected to the Space Shuttle program, leaving the dream of a complete planetary tour in limbo. However, the spirit of exploration did not die. In 1972, a scaled-down mission was proposed, utilizing spacecraft derived from the Mariner series, initially designated as Mariner 11 and Mariner 12. This mission, later renamed the Voyager program, was designed to take advantage of the same gravity-assist technique that had been successfully demonstrated by Mariner 10, allowing the probes to achieve significant velocity changes by maneuvering through intermediate planets. The mission was to send two identical spacecraft to compare the characteristics of Jupiter and Saturn, and potentially Uranus and Neptune, collecting data on their atmospheres, magnetic fields, and ring systems. The probes were launched in August and September 1977, with Voyager 2 launching first on the 20th of August 1977, followed by Voyager 1 on the 5th of September 1977. The trajectories were carefully chosen to maximize the scientific return, with Voyager 1 taking a shorter, faster path to Saturn to ensure a close flyby of its moon Titan, while Voyager 2 followed a trajectory that allowed it to visit all four gas and ice giants. The mission was a triumph of engineering and planning, as the probes successfully completed their primary objectives, with Voyager 1 completing its flyby of Saturn in 1980 and Voyager 2 completing its flyby of Neptune in 1989, making it the only spacecraft to have visited Uranus and Neptune.
Crossing The Heliosphere
The Voyager probes did not stop at the outer planets; they continued their journey into the unknown, becoming the first human-made objects to enter interstellar space. In December 2004, Voyager 1 crossed the termination shock, where the solar wind slows to subsonic speed, and entered the heliosheath, a region where the solar wind is compressed and made turbulent due to interactions with the interstellar medium. This crossing marked a significant milestone in the exploration of the solar system, as it was the first time a spacecraft had entered this region. Voyager 2 followed suit, crossing the termination shock in August 2007, about 10 AU closer to the Sun than Voyager 1, indicating that the solar system is asymmetrical. The probes continued to monitor conditions in the outer expanses of the solar system, with Voyager 1 reporting in 2010 that the outward velocity of the solar wind had dropped to zero, suggesting it was nearing interstellar space. In June 2012, scientists at NASA reported that Voyager 1 was very close to entering interstellar space, which was indicated by a sharp rise in high-energy particles from outside the solar system. On the 25th of August 2012, Voyager 1 crossed the heliopause, the boundary where the sun's influence begins to decrease and interstellar space can be detected, making it the first spacecraft to enter interstellar space. Voyager 2 crossed the heliopause on the 5th of November 2018, becoming the second spacecraft to enter interstellar space. The probes have continued to provide valuable data on the interstellar medium, including the discovery of a region of magnetic bubbles and no indication of an expected shift in the solar magnetic field. The Voyager probes have also detected a significant unexpected increase in density in the space beyond the solar system, as reported in October 2020, implying that the density gradient is a large-scale feature of the very local interstellar medium in the general direction of the heliospheric nose. The probes are expected to continue operating and collecting science data through at least 2026, with the capability to communicate until 2036, in the absence of additional failures.