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Pathways to Pluto - Part 1 of 2: The Dream Mission

by Perry Pezzolanella, MVAS

It all began with a postage stamp in the early 1990s on which the U.S. Postal Service commemorated the exploration of the planets. All of the planets were represented along with the name of the spacecraft that first encountered it, except for one planet. Pluto had the wording: Not Yet Explored. This stamp screamed for attention. Scientists, and the general public alike, were eager to see a mission fly to Pluto. It was also during the nineties that several new objects were discovered that were even farther away than Pluto. The so-called Kuiperoids, small icy-like objects, named after the Dutch scientist, Gerard Kuiper, who predicted their existence, were theorized to have played a key role in the formation of the planets. This stimulated motivation to send a mission to Pluto, but unfortunately, it was not going to be easy.

In the early days of planetary exploration, a mission to Pluto was actually in the works. If all went as planned on paper, Voyager 4 would fly past Pluto on March 9, 1986 giving humanity its first good look at this remote frozen world. Unfortunately the budget cutbacks of the 1970s and the need to fund the Space Shuttle cancelled two of the four proposed Voyager spacecraft. Voyager 1 could have flown past Pluto, but instead scientists decided to study Jupiter's moon Io and Saturn's moon Titan closely. Flying past both moons required a close approach to Jupiter, which caused Voyager 1 to gain so much speed that it arrived at Saturn too early to be redirected towards Pluto. Voyager 2 was designed to follow the Grand Tour of exploring Jupiter, Saturn, Uranus, and Neptune. Unfortunately, Neptune was in the wrong position to redirect Voyager 2 towards Pluto. The necessary boost and change in direction would require the spacecraft to pass deep within Neptune's atmosphere, which would cause it to burn up. Again, Pluto was out of reach.

The decades of the 1970s and 1980s were exciting times for Pluto. With its discovery by Clyde Tombaugh on February 18, 1930 at the Lowell Observatory in Flagstaff, Arizona, Pluto was found to be the smallest planet, estimated at 3600 miles in diameter, and inclined at an unusual angle of 17 degrees to the ecliptic plane. Even more unusual was the fact that it crossed the orbit of Neptune. For 20 years during the long, 248-year orbit, Pluto actually orbits the Sun closer than Neptune, such as was the case from 1979-99. Big discoveries began in 1976 when methane frost was discovered on Pluto's surface followed by the discovery of a large moon, Charon, on June 23, 1978 by James Christy. With these discoveries, Pluto's size shrunk to about 2400 miles in diameter. It was also found that Pluto was tipped 122 degrees on its side, which is more than Uranus! Since Pluto rotates once on its axis every 6.39 days and Charon orbits Pluto once during the same length of time, eclipses are plentiful and act as a knife-edge in determining the true sizes of both bodies along with crudely mapping the surface features.

Pluto was found to be highly contrasting with patches of bright methane frost and other exotic ices such as carbon monoxide along with dark patches of water ice and rock. Even more unusual was that Charon was uniformly gray and mostly covered with water-ice and rock and lacked methane frost. Pluto was estimated to be 1426 miles in diameter and Charon about 744 miles across. Pluto revealed a big secret while it occulted a star on June 9, 1988 when it was found to have an atmosphere. The atmosphere is composed mainly of methane and nitrogen with traces of carbon monoxide and argon. The Hubble Space Telescope, which was launched in 1990, finally confirmed the pinkish-tan mottled surface of Pluto and clearly separated Charon from Pluto's glare. The unusual color of Pluto is actually due to the ultraviolet radiation from the Sun that damages the methane ice and reddens it. To top all of this off, Voyager 2 flew past Neptune and Triton on August 24-25, 1989, revealing active geysers on Triton, a moon that is thought to be similar in characteristics to Pluto. It was obvious that Pluto could no longer be ignored; a dedicated spacecraft mission was now necessary.

A group of young scientist got together a few months before the Voyager 2 encounter with Neptune to figure out how to send a mission to Pluto. The first idea was known as Pluto 350. The goal was to send a spacecraft weighing no more than 350 kilograms (770 pounds) to Pluto in about 13-15 years. At the same time a group of Jet Propulsion Laboratory (JPL) engineers were designing a spacecraft known as Pluto Fast Flyby. It would weigh half as much and would get to Pluto twice as fast as Pluto 350. It was eventually selected over Pluto 350 and to keep costs down the mission would be launched on a Russian rocket that provided maximum boost. To sweeten the scenario, the Russians even planned to build several small atmospheric probes that would be deployed and dropped through Pluto's atmosphere. The probes would not survive the landing on Pluto's surface, but would return extremely high-resolution images of the surface prior to impact. Unfortunately the cost of the mission soared, and by 1995 it was scrapped.

In the middle of all this designing there was another mission on the drawing boards. A pair of a new class of Cassini-like spacecraft called Mariner Mark V was in the works. The second spacecraft would cost half as much as the first by using the spare components of the first spacecraft. One spacecraft was to be a Neptune orbiter with an atmospheric probe. It was to launch on July 25, 2002 and arrive at Neptune on May 13, 2021. The second was to be a Pluto flyby spacecraft, also with an atmospheric probe. It was launch on November 15, 2001 and due to arrive at Pluto on June 27, 2015. The Mariner Mark V class of spacecraft failed to materialize due to the project cost soaring over a billion dollars while NASA was adopting the "faster, better, cheaper" attitude by the mid-1990s. Expensive planetary missions were quickly becoming a thing of the past.

The increased rate of discovery of objects in the Kuiper Belt led to a resurrection of the Pluto mission as one or more of these objects could be included as a flyby along with Pluto. The Pluto-Kuiper Express was born. By the late 1990s, it was possible to build two spacecraft that could be launched as added insurance in case one of them failed to make it to Pluto. However, the cost of the mission soared yet again due to increasing design costs so one of the spacecraft was cancelled. Eventually the cost of just one spacecraft soared past one billion dollars and by September 2000 the mission was cancelled. This created a huge uproar among everyone from the science community to schoolchildren, but would Congress and NASA listen?