While William Herschel officially discovered Uranus in 1781, he wasn’t the first to observe it. Others thought it was a star and Herschel himself called it a ‘comet’ before deciding that it was, in fact, a planet – and the first one discovered by telescope.
Although Uranus can be seen from Earth with the naked eye, it’s so dim and has such a slow orbit compared to the other known planets that it didn’t register as one. It just looks like a faint pinpoint of greenish or bluish light.
Uranus’s acceptance as a new planet overturned beliefs that had been held for millennia about the size of our Solar System, and kicked off a flurry of planetary discovery. But despite Uranus’s significance, we haven’t spent much time visiting the planet. A flyby by Voyager 2 in 1986 marks the only time we’ve explored it. Because of this, we simply don’t know a great deal about Uranus. Until telescope observations in the past few decades, we thought of it as a rather bland planet: dark, cold, slow and with few interesting features.
Uranus is the third-largest planet by radius and the fourth-largest by mass. It’s about 3 billion kilometres (1.86 billion miles) from the Sun, which means that it receives 0.0025 per cent of the sunlight that the Earth gets. Uranus is a gas giant, along with Jupiter, Saturn and Neptune, with the latter planet sometimes being referred to as its twin. It is the least massive of the four, but still more than 14 times more massive than Earth.
The tilt has impacted just about everything about Uranus. It has extreme seasons and weather fluctuations since each hemisphere experiences either full Sun or deep space. Its magnetosphere is tilted and asymmetric. The ring system is also on its ‘side’, and comes close to rivalling Saturn’s in complexity. The moon system is less massive than any other gas giant’s system. So, in many ways, Uranus is unique.
Uranus inside and out
The term ‘gas giant’ implies that Uranus is solely composed of gases, but studies indicate that it actually has a core of silicate rock, encased in ices and topped with a gaseous layer. The core must be very small, since Uranus is the second-least dense planet. It likely takes up only 20 per cent of the planet’s radius. The ice mantle surrounding the core is fluid, with volatiles like methane, ammonia and water. In fact, this electrically conducive fluid is often called an ammonia-water ocean. The outer layer is mostly helium and hydrogen.
The atmosphere contains three layers: the thermosphere, the stratosphere and the troposphere. The lowest layer, the troposphere, is the most interesting and is rich in volatile ices like methane and ammonia. It has four cloud layers: methane, hydrogen sulphide and ammonia, ammonium hydrosulphide, and water clouds at the upper limit. We’ve only observed the top two layers, along with a hazy layer above them. The stratosphere sits between the troposphere and the outermost layer, the thermosphere. Uranus tends to look light bluish or greenish in colour, and it has faint darker bands. The overall colour is due to the way that methane absorbs visible and near-infrared light.
Until Voyager 2 explored Uranus’s atmosphere, we didn’t know much about its features. The probe found a bright polar cap at the south pole, as well as a lighter band called a collar. There were darker bands in the southern hemisphere and about ten lighter clouds around the middle latitudes. The timing of Voyager 2’s arrival meant that it could not fully observe the northern hemisphere. In the Nineties, Hubble and ground-based telescopes like the Keck Observatory began to see more atmospheric features on Uranus. They spotted many more clouds in the northern hemisphere, which are brighter and at a higher elevation than the ones in the southern hemisphere. They also observed in 2007 that the southern collar had nearly disappeared, while one in the north had grown.
Discovering the rings
Dr James Elliot, an astrophysicist with the KAO team, was an expert in stellar occultation – learning about a planet by measuring the light blocked when it passed between the Earth and a distant star. While working with data recording during an observation of Uranus, Dr Elliot noticed that the light between the planet and the distant star went dim for a few minutes before Uranus appeared. This was the first evidence of the rings, confirmed by Voyager 2 about a decade later.
Exploring Uranus
With just one probe visiting the planet, most of our Uranian exploration has been done via telescope.
NASA’s Voyager 2 probe encountered Uranus in January 1986, as part of its Planetary Grand Tour mission. The spacecraft passed within 81,500 kilometres (50,600 miles) of the planet’s cloud tops and made numerous discoveries. Voyager 2 studied the Uranian atmosphere and magnetosphere, revealing that the latter was asymmetric. It provided a number of high-resolution images of the planet and its rings, as well as its five major moons. The probe also discovered ten additional moons and two new rings. The Hubble Space Telescope revealed two further rings and two more satellites in the mid-2000s. Ground-based telescopes, such as the Keck Observatory, have continued to reveal more about Uranus, but there are no current plans to send another spacecraft to explore it up close.
The trajectory of Voyager
One of the obstacles to exploring the outer planets was how to get there, without needing so much fuel that the mission would be too costly. In the Seventies, NASA began planning a Planetary Grand Tour to take advantage of the outer planets’ rare alignment. In 1974, Mariner 10 was the first to use a new technique called gravity assist, or slingshot, to reach Mercury. Spacecraft can use a planet’s movements and gravity to change course or speed to save both fuel and time. After reaching Jupiter, both probes followed a circular trajectory, using each planet’s gravity to travel on to the next. Voyager 2 reached Neptune, then continued on to deep space, while Voyager 1 performed a flyby of Saturn’s moon Titan and was sent out of the plane of the ecliptic and into deep space.