On the night of 13 March 1781, William Herschel was peering through his telescope in his back garden in New King Street, Bath, when he noticed an unusual faint object near the star Zeta Taurus. He watches it for several nights and notices that it moves slowly against the stars. The astronomer first thought he had discovered a comet, but later correctly identified it as a distant planet. Subsequently named Uranus, it was the first planet discovered by antiquity. The achievement won Herschel membership of the Royal Society, a knighthood and lasting 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 lies on its side. And although it is not the farthest planet from the sun, it is the coldest in the solar system.
Uranus also endures seasons of extraordinary magnitude. Each pole spends decades bathed in continuous sunlight before experiencing decades of total darkness. A human being who was born at sunset near the pole, disappearing into autumn darkness, would have to wait 42 years to see his first spring sunrise. For good measure, Uranus is the only planet named after a Greek rather than a Roman god. (Uranus was the grandfather of Zeus.)
Despite these astronomical oddities and extremes, surprisingly little effort has been made to approach Uranus. Only one robotic probe has ever visited it – in 1986 – when the US spacecraft Voyager 2 passed by on its grand tour of the solar system. It revealed a massive, 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 that.
However, such a lighthearted attitude is about to change. Earlier this year, the US National Academy of Sciences released a report that called for NASA to launch a Uranus probe as its top priority mission for the next decade. The academy publishes a report on US priorities in planetary exploration every 10 years, and each decadal survey carries a huge impact – meaning NASA is now under enormous pressure to design and fund such a mission.
The ice giant Uranus. Photo: Alamy
For their part, Uranus enthusiasts are delighted. “This is great news,” said planetary scientist Prof Lee Fletcher of the University of Leicester. “There are few places left in the Solar System that we know less about than Uranus. The inner planets have been visited many times by probes, as have Jupiter and Saturn. Even small, distant Pluto has been explored. So a mission to Uranus would fill a glaring gap in our knowledge of the processes that shaped our solar system.
Astronomers divide the solar planets into three main categories. There’s Mercury, Venus, Earth, and Mars, the rocky inner planets that orbit close to the sun. Then, further out there are the gas giants Jupiter and Saturn, huge worlds made mostly of hydrogen and helium. Finally, at the edge of the solar system, there is Uranus and its partner Neptune, the second-farthest and most distant planets from the sun, respectively. These two are called ice giants because they are massive (though not as large as Jupiter and Saturn) and because they are formed of icy material. Most importantly, they have a large abundance of methane, water and other ice-forming molecules in their atmospheres and interiors. (Beyond these worlds lies Pluto, which was officially declassified as a planet and recategorized as a dwarf planet by the International Astronomical Union in 2006.)
These features were always considered interesting, though not intriguing enough to merit 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, Santa Cruz, puts it: “Nature loves to make planets this size.”
It is not clear why ice giants are widespread in our galaxy. “However, there is clearly something important about planets like Uranus and Neptune,” says Fletcher. “And most importantly, we have two great examples of them, the most common planets in the galaxy, right here in our solar system. Yet their composition, nature and origin remain a relative mystery. It’s time to fix that.”
However, correcting this planetary omission will not be easy. Uranus orbits the sun at an average distance of 1.8 billion miles (2.8 billion km); Neptune at 2.8 billion miles (4.5 billion km). Therefore, the former’s relative proximity to Earth makes it a preferred target. Nevertheless, the Uranus mission will still need help to reach its goal – 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 moving low over a planet that lies in its path. The capsule gains energy from this close encounter and so can carry more instruments and fuel than would otherwise be possible. A Jupiter rendezvous would therefore lead to a Uranus probe that, with more fuel on board, could explore Uranus for a longer time with a more sophisticated set of instruments.
The launch of the rocket carrying the Voyager 2 probe from Cape Canaveral, Florida, on August 20, 1977, 16 days before the launch of its twin, Voyager 1. Photo: NASA
But time is short. Celestial mechanics dictate that a mission to Uranus would need to be launched in 2031 or 2032 to reach Jupiter in time to use it for gravitational assistance. That leaves NASA with a decade to design the probe, raise the $4bn (£3.3bn) or more it will take to build it, complete its construction and then launch it on its 13-year travel. Given these pressures and the widespread interest in the study of the ice giants, the project will almost certainly involve the participation of other organizations, such as the European Space Agency.
Most mission ideas envision a main craft orbiting Uranus, exploring the planet, occasionally swinging by some of its moons and rings for a closer look. A companion probe could also be launched into Uranus’ atmosphere to study its composition.
In addition to finding evidence that could explain why ice giants are common around other stars, the mission will aim to solve many other mysteries surrounding Uranus. Why is the planet so cold, and why is its spin axis tilted sideways, almost in the plane of its orbit around the sun, a phenomenon that means its north and south poles lie where most other planets have their equators?
“One theory is that a very large object – perhaps a huge asteroid – hit Uranus sometime in the past and knocked it off,” says physicist Prof Patrick Irwin of the University of Oxford. “Such an event would also explain another strange feature of 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 internal heat and radiate more energy into space than they collect from the sun. But not Uranus. All of its internal heat appears to have disappeared, possibly because the massive impact turned it inside out, so that its hot internal contents were exposed and its heat was quickly radiated into space. We need research to solve problems like these.
It remains to be seen whether NASA can act quickly enough to build and launch a mission as complex as the one required to explore Uranus. Many astronomers are nervous about the tight schedule – although there is one ray of light. In 2011, the National Academy of Sciences published its final decade-long review and called on NASA to focus over the next 10 years first on a mission to Mars to begin collecting rocks to return to Earth, and second on a probe to be sent to Europa, Jupiter’s icy moon, to see if it could harbor life. A decade later, the US robotic rover Perseverance has already begun the previous task, while NASA’s Europa Clipper is due to launch in 2024. “This shows that these deadlines can be met, and that gives us hope,” says Fletcher.
It remains to be seen how plans for the Uranus mission will play out – although there is an intriguing coda to the story of choosing the planet as the primary destination. As a minor candidate for a lead mission within the next 10 years, the National Academy of Sciences has recommended that a spacecraft be sent to Enceladus. This small moon of Saturn behaves in an amazing way. It ejects organic-rich jets of water into space, making it an ideal candidate for sampling in order to search for microbes or other primitive life forms that may exist on the small moon.
The mission to Enceladus shares many of the complexities that plague plans for any mission to a distant gas or ice giant. However, it is also remarkably united with the proposed probe to Uranus. Enceladus was discovered in 1789 – by no less a figure than William Herschel.
German-born British astronomer William Herschel. Photo: Stock Montage/Getty Images
Herschel’s discovery is remembered
A memorial stone will be unveiled next month in the garden at 19 New King Street, Bath, to mark the exact spot where William Herschel set up his home-made telescope and first glimpsed Uranus. The event – on August 25 – will be a key part of commemorations that began this weekend to mark the 200th anniversary of the German-born astronomer’s death.
Herschel died on August 25, 1822, and the new exhibition 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 his sister Caroline, a professional astronomer in her own right, a prolific discoverer of comets and…
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