After New Horizons, when will we next revisit the outer solar system?

The Grand Tour

The only spacecraft to have gone anywhere near Uranus and Neptune is Voyager 2, which captured images of the planets as it swung past both on its journey out of the solar system.

Voyager 2 was actually launched before Voyager 1, on 20 August 1977, and it’s still sending signals back to us today, almost 38 years later, as it enters interstellar space – making it one of the most distant man-made objects in the universe. But the Voyager Program shares many similarities with New Horizons – including a notably difficult gestation.

Its history begins in 1964, when Gary Flandro of NASA’s Jet Propulsion Laboratory spotted an opportunity too good to miss. An alignment of Jupiter, Saturn, Uranus, Neptune and Pluto would happen in the late 1970s, and wouldn’t recur for another 175 years.

Flandro proposed a “Grand Tour”– four probes that would use Jupiter’s gravitational well as a slingshot to propel them at a blistering pace towards the outer planets of the solar system. Two would fly by Jupiter, Saturn and Pluto, and the other two, launching a couple of years later, would pass Jupiter, Uranus and Neptune.

His proposal was made at the height of the space race between Russia and the United States, and eagerly pursued. But once the Moon landings were successfully completed, NASA’s generous funding (at one point five per cent of the entire American budget) was significantly scaled back. In 1972 the Grand Tour fell victim to these cuts.

But that wasn’t the end of Flandro’s dream. For many years, the Mariner Program had been successfully carrying out interplanetary missions resulting in the first planetary flyby (Mariner 2), the first pictures from another planet (Mariner 4), the first craft in orbit around another planet (Mariner 9) and the first gravity assist manoeuvre (Mariner 10).

Mariners 11 and 12 fell victim to the NASA budget cuts too – their missions were significantly scaled back and they were renamed the Mariner Jupiter-Saturn probes. But as work progressed, the designs for these probes were reworked into something quite different from the Mariner missions, and a decision was made to give them a name of their own. The Voyager Program was born.

Voyager

Voyager 2 was the first to launch, on a course that would see it passing Jupiter and Saturn and could be tweaked to extend the mission to Uranus and Neptune – though there was no guarantee on funding for that at the time. Voyager 1 launched soon after, along a shorter, faster trajectory that ruled out any mission extensions.

The probes achieved their targets in 1980 and 1981 respectively. Voyager 1 could have been sent towards Pluto but instead a decision was made to make a close flyby around the Saturnian moon Titan, known to possess a dense atmosphere that NASA was very keen to study. After that, it whizzed off into the great darkness of outer space. Eventually – 32 years later, on 12 September 2013 – it was announced that Voyager 1 had become the first man-made object to cross the heliopause and exit the solar system. It’s now the furthest man-made object from Earth.

But Voyager 2’s engineers had other plans. Before successfully completing its Saturn mission on 26 August 1981, they started making preparations to adjust its course to fly by the outer planets – Uranus and Neptune. But then disaster struck – its camera platform locked up, making it almost useless for scientific missions. Happily, the problem was swiftly fixed – its lubricant had been temporarily depleted by the number of photos the camera had been taking of Saturn. The probe was given the go-ahead for its extended mission.

Uranus

Voyager 2 passed Uranus on 24 January 1986, at a distance of 81,500km. From that distance, it was able to get a good look at the planet’s weird atmosphere. Due to what’s thought to have been a collision with an Earth-sized body during the early formation of the solar system, Uranus has an axial tilt of 97.77 degrees, meaning that it spins on its side with its poles where you’d normally expect to find another planet’s equator.

As a result, those poles are exposed to either continuous sunlight or complete darkness for many years at a time. Its winter and summer solstices are about 40 years apart. From this, you’d expect that the poles would be hotter than the equator – because averaged over a year they get more energy from the Sun. But strangely this isn’t the case – and even today we’re not entirely sure why. It may be related to the fact that its temperature is far lower than the other giant planets – the coldest in the solar system, at around -224C.

Unlike both Saturn and Jupiter, which are thought to have sizeable rocky cores beneath their deep layers of suffocating cloud, it’s believed that Uranus has a much smaller rocky centre, weighing a little more than half an Earth. Its atmosphere is made up mostly of molecular hydrogen and helium, but the helium hasn’t settled towards the centre like it has in the other gas giants.

Instead its central portion is thought to be comprised of a hot, dense mixture of water, ammonia and a few other volatiles, perhaps with crystals of diamond falling through it like hail. The water-ammonia mixture is sometimes referred to as an ocean, and it’s been hypothesised that at the bottom of this ocean is a layer of liquid diamond with floating solid-diamond-bergs, forced together under immense pressure. In the upper atmosphere, we’ve since learnt from looking at the planet in different wavelengths that it has titanic wind storms that roar through its skies at more than 800km/h.

Voyager 2 didn’t get to see these diamond-bergs, or the roaring storms. Instead, floating above the planet, there was nothing to see except a featureless powder-blue disc, an extensive but dark and faint ring system and ten new moons that we previously didn’t know existed.

It captured photos of the five largest moons — including Miranda, the innermost, which bears striking 20 kilometre-deep scars across its body. There’s some debate on the origin of these features, but the most commonly accepted theory is that they’re a result of intense geological activity in the moon’s past due to tidal forces from its parent planet. Another points to enormous cryovolcanic eruptions of icy magma. A third sees a period where Miranda was shattered by an impact and then reassembled itself – with denser fragments sinking below the surface. For now, all we can do is guess.

As Voyager 2 departed the Uranian system, it spun its camera around and captured one last image – the most recent to have been taken by a camera so close to the ice giant. In it, the planet appears as a fragile crescent, suspended in the darkness of space.