The exploration of Pluto began with the arrival of the New Horizons probe in July 2015, though proposals for such a mission had been studied for many decades. There are no plans as yet for a follow-up mission, though follow-up concepts have been studied. Early mission proposals Pluto, discovered by Clyde Tombaugh in 1930, is an interesting target for planetary exploration, but Pluto presents significant challenges for exploration because of its small mass and great distance from Earth. In 1964, Gary Flandro of the Jet Propulsion Laboratory proposed a mission called Grand Tour, taking advantage of the fact that an alignment of planets in the late 1970s would enable a single spacecraft to visit all of the outer planets, including Pluto, by using gravity assists. Due to the cost, the mission was not funded, although the planetary alignment was used by the two probes of the Voyager program, launched in 1977. One of many possibilities for the Voyager 1 spacecraft after its flyby of Saturn in 1980 was to use Saturn as a slingshot towards Pluto for a flyby as early as March 1986. However, scientists decided that a flyby of Titan during the Saturn encounter would be a more important scientific objective, making a subsequent flyby of Pluto impossible, because the close approach to Titan put it on a trajectory that slingshotted it out of the plane of the solar system. Because no mission to Pluto was planned by any space agency at the time, it would be left unexplored by interplanetary spacecraft for years to come. After the success of the Voyager program, planetary scientists looked to Pluto as the destination for a subsequent mission. In the 1980s, Robert Farquhar proposed that a Pluto mission could be added to the proposed Solar Probe mission, which at the time was proposed to fly by Jupiter on the was to its solar encounter. The mission was tagged "Fire and Ice." However, the proposal was not accepted. The Pluto Underground, Pluto 350 and Mariner Mark II In May 1989, a group of scientists and engineers, including Alan Stern and Fran Bagenal, formed an alliance called the "Pluto Underground". It was named in homage of the Mars Underground, another group of scientists that successfully lobbied for the restart of missions to Mars, following the lack of such since the Viking program. The group started a letter writing campaign which aimed to bring