Move over Mars, Jupiter, Saturn – Uranus is the most intriguing planet

Every decade, NASA releases a wish list of objectives it would like to see accomplished in planetary sciences over the subsequent ten years. It compiles a report after extensive consultations with, and submissions from, a wide array of major scientific institutions. One of the objectives listed by NASA is an intended exploration of another planet. Not Mars, not Jupiter or Saturn, which seem to receive inordinate amounts of attention. The planet in mind is the oddball of our Solar system; Uranus.

Firstly, if you want to make all the usual, adolescent jokes about that name, get it over with now. Yes, we all know the infantile quips about ‘your anus’ and gas. Ok, after the juvenile japes are out of your system, let’s get busy with the important issues. Secondly, the planet’s name – correctly pronounced ‘YOOR-uh-nus’ – is an homage to the Ancient Roman god Caelus, Father Sky. Named for the equivalent Ancient Greek god Ouranos, today we have the father sky of the monotheistic religions. The other planets were named after Roman gods.

Discovered by German-born astronomer William Herschel in 1781, Uranus is classified as a gas giant, along with Jupiter, Saturn and Neptune. In fact, Uranus and Neptune form their own sub-category of ice giants. Gas giants, unlike the terrestrial Earth, do not have a rocky crust, but a gaseous exterior. While Uranus has a rocky core, it is an ice giant because its internal structure is made up of ice water, methane and ammonia. The gaseous atmosphere is incredibly dense and would likely crush any spacecraft attempting to get through to any potentially solid surface.

Axial tilt

There are many reasons why Uranus is the eccentric planet, and its unusual axial tilt is one of them. As we all know, the Earth rotates on a 23.5 degree axis. It’s what gives our planet the seasons. Uranus rotates on an axis of 98 degrees – almost rolling on its side. One pole of Uranus faces the sun, resulting in decades-long summers (such as it is), and the other hemisphere has an equally long freezing winter.

How did Uranus end up with such a pronounced axial tilt? One explanation is that a large proto-planet slammed into Uranus, knocking it on its side. And let’s bear in mind that Uranus has a radius of 25 362 kilometres. In comparison, the Earth’s radius is 6731 kilometres. Uranus has an average distance of 2.88 billion kilometres from the sun, and one Uranian year is equal to 84 Earth years.

Uranus has a remarkably cold exterior atmosphere. Its average atmospheric temperature is -195 degrees Celsius. It has the coldest temperature recorded anywhere in the Solar system; -224 degrees Celsius. Inhospitable to life, the Uranian atmosphere is mostly composed of methane, ammonia, hydrogen sulphide and water. Uranus has a bluish-green tinge, because methane absorbs light at the red end of the spectrum.

Unanswered questions and why bother?

There are numerous unanswered questions regarding Uranus. Why is its magnetic field so eccentric, tilted at 60 degrees from its rotational axis? The technical hurdles in getting a spacecraft through the dense Uranian atmosphere are enormous. Why does Uranus fail to generate internal heat, a possible reason why temperatures are so cold? And what of the moons orbiting Uranus? There are 27 of them, all travelling with the planet.

This leads to an anticipatory question – aren’t there enough serious problems on Earth, so why should we waste millions of dollars exploring other planets? This kind of question, attacking the underlying reasons for planetary science, arise whenever large space projects are proposed. Surely we should be directing our resources towards solving all of our problems here on Earth. There is no shortage of issues to solve – economic inequalities, global warming, increasingly severe weather events, wars and famines – why waste money on going into space?

This question originates from reasonable sentiments – concern for life on Earth – but is misguided. Branches of science, including planetary exploration, are not engaged in a zero sum game. We have to make decisions about funding priorities of course. The billionaires, such as Elon Musk and Jeff Bezos, want to monopolise space exploration through exerting private control. We must not allow the billionaire class to define the motivations and parameters of planetary science.

Outer space is not a playground for the rich and famous. Planetary sciences answer the deep cosmological questions about the solar system and our origins. When Sherlock Holmes contemptuously dismissed knowledge of basic science, he was not only betraying a profound ignorance of the way the natural world works. He was expressing a hostile attitude to scientific knowledge, an attitude which has contributed to our current perilous state of affairs.

Explorations of other planets are inspirational, motivating the next generation of students to get excited about solving the deep scientific questions of our age. There are not only technological benefits from space research which flow on to the general public – GPS tracking as one example. Tackling the age-old scientific problems requires a detailed understanding of the cosmos and our place in it.

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