why Uranus is the new target for space exploration

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On the night of March 13, 1781, William Herschel was looking through his telescope in his backyard on New King Street, Bath, when he noticed an unusual faint object near the star Zeta Tauri. He watched her for several nights and noticed that she moved slowly against the background stars. The astronomer first thought he had found a comet, but later correctly identified it as a distant planet. Later called Uranus, it was the first planet to be discovered since antiquity. The conquest earned Herschel membership of the Royal Society, a knighthood and enduring astronomical fame.

Since then, studies have shown that Uranus is a very strange world. While the rest of the planets in our solar system spin like tops, Uranus stands aside. And while it’s not the farthest planet from the sun, it’s the coldest in the solar system.

Uranus also passes through seasons of extraordinary magnitude. Each pole spends decades bathed in unbroken sunlight, before experiencing decades of total darkness. A human being born at sunset near the pole and disappearing into autumn darkness would have to wait 42 years to see his first spring sunrise. For good measure, Uranus is the only planet to be named after a Greek god rather than a Roman god. (Uranus was the grandfather of Zeus.)

Despite these oddities and astronomical extremes, surprisingly little effort has been made to get close to Uranus. Only one robot probe has ever visited – in 1986 – when the US spacecraft Voyager 2 passed on its grand tour of the solar system. It revealed a huge, featureless, pale blue world with an atmosphere of hydrogen, helium and methane, a rich family of moons and a powerful magnetic field. And that was it.

That slight consideration is about to change, however. Earlier this year, the US National Academy of Sciences published a report that urged NASA to launch a Uranus probe as its highest-priority main mission for the next decade. The academy publishes a report on US priorities in planetary exploration every 10 years and every decennial survey has a huge influence – meaning that NASA is now under enormous pressure to design and fund such a mission.


The ice giant Uranus. Photography: Alamy

For their part, Uranus enthusiasts are delighted. “This is tremendous news,” says planetary scientist Prof Leigh Fletcher of the University of Leicester. “There are few places in the solar system that we know less about than Uranus. The inner planets have been visited many times by spacecraft, as have Jupiter and Saturn. Even tiny, distant Pluto has been surveyed. Therefore, a mission from Uranus will fill a glaring gap in our knowledge of the processes that shaped our solar system.”

Astronomers divide the sun’s planets into three basic categories. There are Mercury, Venus, Earth and Mars, the rocky inner planets that orbit close to the sun. Then, further afield, are the gas giants Jupiter and Saturn, huge worlds made mostly of hydrogen and helium. Finally, at the edge of the solar system, is Uranus and its partner Neptune, respectively the second most distant and the most distant planet from the sun. These two are called ice giants because they are massive (though not as big as Jupiter and Saturn) and because they are made up of icy material. Fundamentally, they have high abundances of methane, water, and other ice-forming molecules in their atmospheres and interiors. (In addition to these worlds is Pluto, which was formally declassified as a planet and re-categorized as a dwarf planet by the International Astronomical Union in 2006.)

These features were always considered interesting, though not intriguing enough to warrant special attention — until astronomers began studying worlds around other stars and discovered that ice giants like Uranus and Neptune were everywhere. “It’s really intriguing: when we look at planets around other stars, we find that many of them are similar to Uranus and Neptune,” adds Fletcher. Or as Jonathan Fortney, a planetary scientist at the University of California at Santa Cruz, puts it: “Nature loves to make planets this size.”

Why ice giants are common in our galaxy is unclear. “However, there is obviously something important about planets like Uranus and Neptune,” says Fletcher. “And crucially we have two great examples of them, the most common planets in the galaxy, right here in our solar system. However, its composition, nature and origins remain a relative mystery. It’s time to fix this.”

Correcting this planetary omission will not be easy, however. Uranus orbits the Sun at an average distance of 2.8 billion kilometers; Neptune at 2.8 billion miles (4.5 billion km). The former’s relative proximity to Earth, therefore, makes it the preferred target. However, a mission from Uranus will still need help reaching its target – in the form of a gravitational assist from Jupiter. This type of maneuver has been used on other missions in our solar system and involves a spacecraft sweeping a planet that is in its path. The capsule gains energy from this close encounter and therefore can carry more instruments and fuel than would otherwise be possible. An encounter with Jupiter would therefore result in a Uranus probe that, with more fuel on board, could explore Uranus longer with a more sophisticated set of instruments.

But time is tight. Celestial mechanics dictates that a mission from Uranus will have to launch in 2031 or 2032 to reach Jupiter at the right time to explore it for a gravity assist. That leaves NASA with a decade to design the spacecraft, raise the $4 billion (£3.3 billion) or more that will be needed to build it, complete its construction and launch it on its 13-year journey. Given these pressures and the widespread interest in exploring the ice giants, the project will almost certainly involve the participation of other organizations such as the European Space Agency.

Most ideas for the mission envision a main ship that would orbit Uranus, inspect the planet while occasionally swinging close to some of its moons and rings for close inspection. A companion probe could also be launched into Uranus’ atmosphere to study its composition.

In addition to finding evidence that might explain why ice giants are common around other stars, the mission would aim to solve many other mysteries about Uranus. Why is the planet so cold and why is its axis of rotation tilted to the side, almost in the plane of its orbit around the sun, a phenomenon that means its north and south poles are where most other planets have their equators?

“One theory is that a very large object – a huge asteroid perhaps – hit Uranus at some point in the past and knocked it down,” says physicist Prof Patrick Irwin of the University of Oxford. “Such an event would also explain another strange feature about Uranus: it appears to have almost no internal heat left over from its formation – making it the coldest planet in the solar system.

“Jupiter, Saturn and even Neptune still have some internal heat and they emit more energy into space than they collect from the sun. But not Uranus. All of its internal heat seems to have disappeared, possibly because that huge impact turned it inside out, so that its hot internal contents ran out and its heat quickly radiated out into space. We need a probe to solve problems like these.”

It remains to be seen whether NASA can move quickly enough to build and launch a mission as complex as the one needed to study Uranus. Many astronomers are nervous about the tight schedule – although there is a ray of light. In 2011, the National Academy of Sciences published its latest decennial survey and urged NASA to focus the next 10 years on a mission to Mars to begin collecting rocks to return to Earth, and secondly, on a probe that would be sent to Earth. to Europa, Jupiter’s icy moon, to see if it can harbor life. A decade later, the American robot rover Perseverance has already started the previous task, while NASA’s Europa Clipper is due to launch in 2024. “It shows that these deadlines can be met and that gives us hope”, says Fletcher.

It remains to be seen how the plans for the Uranus mission pan out – although there is an intriguing coda in the story of the planet’s selection as a prime destination. As its secondary candidate for a major mission over the next 10 years, the National Academy of Sciences has recommended that a spacecraft be sent to Enceladus. This tiny moon of Saturn behaves surprisingly. It launches plumes of organic-rich water into space, making it an ideal candidate for sampling in order to look for microbes or other early life forms that might exist on the tiny moon.

A mission to Enceladus shares many of the complexities that plague plans for any mission to a distant ice or gas giant. However, it also remarkably joins the proposed probe to Uranus. Enceladus was discovered in 1789 – by none other than William Herschel.

A portrait of William Herschel.

German-born British astronomer William Herschel. Photography: Stock Montage/Getty Images

Herschel’s Discovery Remembered

A commemorative stone will be unveiled next month in the garden at 19 New King Street, Bath, to mark the exact spot where William Herschel rested his homemade telescope and glimpsed Uranus for the first time. The event – ​​on August 25 – will be a key part of the commemorations, which began this weekend, to mark the 200th anniversary of the death of the German-born astronomer.

Herschel died on August 25, 1822, and the new exhibit at his home – now the Herschel Museum of Astronomy – will include his observation book in which he wrote notes from the night he first observed Uranus. Also on display will be a catalog of stars recorded by her sister Caroline, a professional astronomer in her own right, a prolific discoverer of comets and the first woman to earn a salary as a scientist.

William Herschel was also a talented musician and could play the oboe, violin, harpsichord and organ and composed numerous concertos and symphonies. Its scientific achievements include the first detection of infrared radiation; the observation that Mars’ polar caps vary seasonally; and the subsequent discovery of previously unseen moons around Saturn.

However, the discovery of Uranus remains its main claim to fame. Interestingly, Herschel originally wanted the name of his patron, King George III, which would make him Georgium Sidus, or Star of George. However, the nickname was not very popular outside of England and eventually the name Uranus was agreed upon – although it did not receive full international acceptance until the mid-19th century.

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