The Grand Tour of Voyager An historic event occurred on August 25, 2012, and no one was aware of what had even happened. As Voyager 1 slipped out of the Sun’s sphere of influence and into interstellar space, even the researchers studying data the probe was sending back to Earth were unsure a momentous event had occurred: The Dawn of the Interstellar Age. Not until April of 2013 would researchers at the Jet Propulsion Laboratory (JPL) in Pasadena, California prove that their longest working probe had finally left the Sun’s harbor. The probes are now venturing into deep space, opening yet another chapter in the decades spanning Voyager Mission. Launched in 1977, the Voyager 1 and Voyager 2 space probes are the longest serving machines operated by NASA. Through these machines humanity was able to truly see the outer solar system for the first time and discover more than we ever dreamed we would. The Voyager flybys provided humanity with most of the information we know now about the outer solar system. The Voyager Mission laid the foundation of our knowledge about the Gas Giants, their moons, and interstellar space. The “Grand Tour”, as the Voyager Mission became known, began in the mid-1960s as a theorized trajectory path created by graduate student Gary Flandro. “I was looking for ways to reach the outer solar system efficiently, making practical missions possible.” (Swift) Flandro was exploring the possibility of gravity assisted flight paths into the outer solar system at the request of his supervisor. Flandro quickly discovered in his research that all four of the outer planets would be in a rare alignment in the late 1970s that favored gravity assisted trajectories. The next occurrence of this alignment wouldn’t happen for another 175 years. After sharing his results with his superiors, everyone was in agreement that a well-timed opportunity had been presented to them. The benefits of gravity assist, when a probe uses the gravity of a large celestial body such as a moon or planet to accelerate, was too much to overlook. By using gravity assist, spacecraft could reach the outer limits of the solar system years before fuel propulsion spacecraft. After quickly putting together a plan, the prospect was presented to the upper echelons in NASA as well as congress. The initial plan was gigantic in scope. Four probes would be sent, each equipped with cutting edge technology that generated an expected cost much larger than the government was willing to pay. The plan wasn’t shot down, however. As mission manager Harris Schurmeier recalls, “They told us, ‘If you guys can come up with something less grandiose, we’ll consider it.’” (Bell). After a revision, they had dropped the expected $900 million cost by more than two thirds, to $250 million. For NASA, having a plan that saved money was critical to getting ambitious, large-scale projects like Voyager through congress. (Butrica) This new plan was approved. Officially known as Mariner Jupiter Saturn ’77 (MSJ-77), the new project had been drastically scaled back. Only two probes would be built, with substantially fewer instruments than their first conceptualizations. Jupiter and Saturn remained the targets of observation for the flybys, but Uranus and Neptune had to be left out, cutting the Grand Tour into “a less ambitious mission, a mini-Grand Tour.” (Lakdawalla) The planners of the project weren’t completely crestfallen about the loss in scope. Because of newly emerging technology, researchers would be able to send commands to the probes to extend the mission if possible, brilliant foresight that allowed Voyager 2 to continue to Uranus and Neptune, and to allow both probes to continue functioning as they traveled towards interstellar space. Voyager 2 was launched first, on August 20, 1977, followed by Voyager 1 on September 5, due to the different trajectories each probe was planned to undertake. By the time the probes had passed the asteroid belt, Voyager 1 had achieved a lead over its twin, and has remained further away from Earth ever since. Traveling close to 35,000 miles an hour, Voyager 1 descended upon the Jovian system on March 5, 1979. Scientists from around the world gathered at JPL to watch this momentous occasion in person and became ecstatic as high resolution photos began to trickle in. Pictures of distant Io started to raise questions. What were the colored splotches on its surface? The questions quickly turned to panic as incoming photos became terribly smeared, making them worthless. A behemoth, Jupiter contains more mass than every other object in the solar system combined, excluding the Sun. Because of its size, Jupiter emits harsh radiation that can affect the computer components of the probes sent to it. As Voyager 1 approached, this intense field of radiation was able to bypass the probe’s shielding, slightly altering the clocks between the camera and the main computer, putting them out of sync. The computer began moving the camera before it had closed its shutter, ruining pictures that scientists and researchers had been waiting years to see. The problem was discovered quickly, and the probe was able to resume its mission before leaving Jupiter and its moons behind. The first moon to fall under scrutiny was Io. Craterless, covered in a palette of orange, white, black, and red, the mystery of this tidally abused moon was answered by accident when Voyager 1 had already passed by and had taken photos for navigation, not discovery. After changing the color saturation of the navigation photos at a lab back on Earth to view the dim stars past the moon to determine the probe’s position along its trajectory, Linda Morabito saw a strange crescent off Io’s surface. When Morabito showed her finding to the leaders of the project and explained that they were looking at plumes of material ejecting from the surface of Io, the other scientists were skeptical. Additional data proved conclusive, and Io became the most geologically active body in the solar system, surpassing Earth. Vicious tidal forces from Jupiter and moons Europa and Ganymede place Io under immense stress, creating volcanism not seen anywhere else in the solar system. These same tidal forces affect Europa, cracking its icy surface to allow slushy water to seep over the surface, erasing craters and mountains and valleys, creating a surprisingly fresh and young surface. The Jovian system proved to be very interesting for the Voyager team as they discovered the unexpected personalities and vastly different characteristics of each moon. Before the flybys, it was widely believed that all moons in the solar system would be much like Earth’s; an ancient, cratered rock with no atmosphere and no significant mysteries. Io shattered that perspective, and Europa cemented the idea that many things in the outer solar system could not be predicted accurately; humanity simply did not know enough about these distant worlds. Even Ganymede and Callisto, appearing more like how a moon was thought to be, harbor evidence of subterranean oceans. The bar had been set for Voyager as it picked up speed from Jupiter’s massive gravity, and eyes turned towards Saturn. On November 12, 1980, Voyager 1 descended upon the ringed planet with a very specific goal that needed to be accomplished: the planet’s largest moon, Titan, needed to be accurately photographed and measured. If the probe failed to obtain enough data, Voyager 2’s trajectory would be altered to finish the measurements. The spacecraft would lose the window needed to make it to Uranus and Neptune, effectively ending the Grand Tour. Obtaining information on Titan was so crucial to the Voyager team at JPL they were prepared to lose the rest of the Solar System. Voyager 1 did not disappoint. A world larger than Mercury, Titan surprised the researchers back on Earth with an atmosphere of methane and ethane almost 50% thicker than Earth’s. Orange clouds covered the surface, making it impossible for the probes to snap pictures of the surface, likely to have rivers, lakes, and oceans of liquid methane because of Titan’s frigid temperatures that are only ninety degrees above absolute zero. After measuring the solar system’s second largest moon, Voyager 1 continued through Saturn’s miniature solar system, collecting readings on Saturn itself, with wind and storm belts raging across its atmosphere, and shocking scientists and mathematicians with braided rings. Having gathered enough information successfully during the short flyby, Voyager 1 opened the path for its twin to continue to Uranus, a planet completely unknown to astronomers. The probe’s own Grand Tour was over, however. While possible for Voyager 1 to intercept Pluto at one point, Titan had been deemed too important to observe at a distance. After the flyby of Saturn, the probe was launched upward (relative to the disk or plane of the solar system) at ten miles per second. The only purpose for Voyager 1 now was to serve as a practice dummy for critical fixes Voyager 2 might need and to measure space as it approached the end of the sun’s sphere of influence. The wait time between the Jupiter and Saturn encounters had only been a year and a half; Uranus would be a much longer wait. Almost five years passed between Voyager 2’s August 26, 1981 flyby of Saturn and the January 24, 1986 passing of Uranus. This giant planet proved no less interesting than the two that proceeded it. The turquoise planet is unique almost immediately: it rolls as it orbits the sun instead of spinning. Many researchers were curious to see how this affected the magnetosphere the planet emits.