Mohawk Valley Astronomical Society

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Land of the Ice Giants

by Perry Pezzolanella

Far away in the dimly lit edge of the Solar System are two giant worlds where seasons are measured in decades and a year can last nearly a century, or even longer. All the other planets have been visited by orbiters, landers, or rovers, but Uranus and Neptune have been nearly forgotten since the brief encounter in the 1980s.

Voyager 2 flew past Uranus on January 24, 1986 and Neptune on August 25, 1989, but no spacecraft has visited since. The brief encounters gave the first good close-up look at worlds that were not much more than tantalizing fuzzy bluish discs with vague cloud features. Uranus was known to be surrounded by thin, dark rings, which were discovered on March 10, 1977. The five known moons were nothing more than dim points of light. Neptune was more exciting with possible ring arcs and revealed fuzzily changing cloud features. It has one fairly large moon, Triton, which was speculated to be covered by a vast sea of liquid nitrogen. Neptune was expected to be more exciting than Uranus when Voyager 2 arrived and that turned out to be true. Uranus turned out to be a featureless, turquoise-hued world cloaked in a thick methane haze with rings as dark as charcoal that were not readily revealed in fine detail until Voyager 2 flew behind them. The moons stole the show as all of them hinted at a violent history. Ariel has smooth areas where possible water ice once flowed, but Miranda appeared to have been shattered to pieces and reassembled. It has some of the most bizarre and tortured landscape in the Solar System including a huge cliff nearly twelve miles high.

Neptune did not disappoint as it looked like a blue version of Jupiter with dark belts, white spots, and a dark spot known as the Great Dark Spot that disappeared years later. Neptune is completely surrounded by clumpy rings instead of arcs, but Triton stole the show. There were no seas of liquid nitrogen but instead there were towering geysers of nitrogen up to five miles high with plumes extending over 100 miles downwind. Triton was found to have a thin atmosphere and a huge nitrogen polar ice cap. Flow features also hinted at a violent history. Earthly telescopes, especially the Hubble Space Telescope, have revealed additional detail, but only so much can be seen from billions of miles away. A thorough mission would involve orbiters that would have years to study Uranus, Neptune, their rings and moons, instead of only a few days. These worlds have been ignored for too long. It is time to go back.

Uranus has earned the nickname, the “Boring Planet.” Thus, it has not been actively considered for an orbiter mission, unlike Neptune. A flyby mission of Uranus was considered during the last decade based on the New Horizon’s design and was referred to as New Horizons 2. It would have launched on March 19, 2008, used the same camera system, and would have encountered Uranus on October 7, 2015 at a time when its equator was in full view allowing for thorough mapping of its moons, unlike the pole-on limited view that Voyager 2 observed. Unfortunately, there was not enough money in the budget for the mission and the focus remained on Pluto.

Neptune was more aggressively pursued than Uranus for a mission since it was such an active world along with its moon, Triton. An orbiter mission was considered in the early 1990s not long after Voyager flew past it. A new class of spacecraft known as Mariner Mark V was being developed that combined the best features of the Voyagers and Cassini. Two missions based on the new design were considered: a Neptune orbiter and a Pluto flyby. The Neptune orbiter mission would have launched on July 25, 2002 with arrival on May 13, 2021. It would have deployed a probe into Neptune’s atmosphere and orbited it for at least four years coming close to Triton most of the time, but it was considered too risky given the long flight time of nearly 20 years added to the fact that the development of the Mariner Mark V was cancelled as it was deemed too expensive. Next under consideration around 2005 was a Neptune flyby mission called Argo. It was to be similar to New Horizons but designed different so that it could deploy a small probe into Neptune’s atmosphere. It never made it past the drawing board as there was no room in the budget. As the decade of the 2010s dawned, it seemed obvious there would be no mission to Uranus or Neptune in anyone’s lifetime.

Ice giants appear to be the popular type of planet that is being discovered around other stars. Uranus and Neptune are considered ice giants as both are about four times larger than Earth, but they are not giant balls of ice. They are gassy worlds with a lot of water, ammonia, and methane, which are considered ices where temperatures fall to -300ºF. Jupiter and Saturn are true gas giants where hydrogen and helium remain mainly gas. The inner planets are primarily tiny balls of rock. Uranus and Neptune helped create the Kuiper Belt that is home to hundreds of tiny icy worlds and comets. Neptune may have even captured one of these tiny, icy worlds known as Triton.

Which of the Ice Giants should be explored first? Both planets have something that is unique, but Uranus is about a billion miles closer to Earth than Neptune making it easier and quicker to reach. It is tipped over nearly 98º on its side, which makes for extreme seasons. Each pole can experience up to 42 years of continuous sunlight followed by 42 years of continuous darkness making it an excellent case-study on how seasons on planets work compared to Earth. All of the original five largest moons were barely half seen and imaged; all show signs of an active past. A Uranus mission would be like seeing the moons all over again for the first time. A superior imaging system could possibly solve why Miranda appears so badly jumbled; whether Ariel was ever active, or still is; determine the nature of the bright spots on Umbriel; detect if Titania has an underground ocean; and try to solve how the lone, four-mile high mountain formed on Oberon. Neptune has an active atmosphere unlike Uranus that is only now starting to erupt in white spots, and has plenty of storms and detail to study. Neptune’s large moon, Triton, is geologically active with nitrogen geysers and a thin atmosphere. Triton did not form around Neptune and probably came from the Kuiper Belt like Pluto. A Neptune mission would allow the perfect opportunity to compare Triton with Pluto now that Pluto has been explored up close.

Both planets have much to offer and planetary scientists would be happy to go to either one. In the end it may come down to logistics. An orbiter mission to Uranus would take 10-15 years and Neptune around 15-20 years. A gravitational boost from Jupiter or Saturn would shorten the journey by a few years. The real answer may be the new powerful rocket, the Space Launch System, which could reach Uranus and Neptune in only a few years. Engineers and scientists will be studying the possibilities of what it would take to get a spacecraft into orbit around an Ice Giant in a timely manner, what kind of spacecraft is needed, and which instruments would deliver the maximum science. Maybe someday soon the Herschel Uranus Orbiter will become reality. Or should Neptune be first? It is a tough choice, but a nice dilemma to ponder.

The tide has finally turned as many planets are being discovered around other stars that are similar to Uranus and Neptune. If we are ever to understand what kind of worlds orbit other stars, we need to start exploring and understanding both Uranus and Neptune. Will there ever be a return mission to Uranus and/or Neptune? Pluto has proven there are surprises and the Ice Giants promise to revolutionize how we understand them and their kind around other stars.