The Pluto Plan

Both crafts would complete a flyby of Venus first, looking for signs of volcanism. The next step was a flyby encounter with Earth before sailing through the asteroid belt and attempting encounters of two to four asteroids out of few dozen thought to be along their path. Both crafts would later have taken measurements of Jupiter’s magnetotail, and at least one of them could have flown through the jovian moons for a close encounter during a gravity assist; either Ganymede or Callisto would have been the likely targets. The Pluto probe would begin its Jupiter encounter around 2006, with the Neptune probe following within a year or two.

The Pluto encounter would have taken place around 2015, while the Neptune probe, going a hair slower as an orbiter instead of a fly-by mission, would begin its encounter in 2020. However, the Neptune probe, along with its encounters of Jupiter’s moons and a proposed Venus flyby, would have come close enough to Chiron to study it and gather a limited amount of science.

Chiron (not Charon) is a large, rocky body between the orbits of Jupiter, Saturn, and Uranus that may be a dwarf planet. It’s the largest in a class of objects known as Centaurs, which are asteroids and other small rocky bodies between the asteroid belt and the Kuiper Belt.

At 100 days before encounter in 2015, the Pluto imaging probe would be released, performing what the authors called “complete, global, high-resolution imaging coverage” of Pluto and Charon (not Chiron). By sending the probe ahead, the team would be able to use imaging from the Pluto flyby and the probe to build a complete view of Pluto and Charon. The New Horizons craft got only a partial view.

The Pluto Mariner would then fly through the Kuiper Belt, which in 1991 was only theorized. The first Kuiper Belt Object wasn’t discovered until 1992, though the region had long been believed to exist. The Kuiper Belt (called the Kuiper Disk in the paper) was at that time believed to be a region of mostly comets.