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

“We have completed initial reconnaissance of the solar system,” said Alan Stern, the chief scientist of the New Horizons mission, shortly after the probe swung past Pluto – the most distant large object in our solar system. It’s a hell of a quote – combining triumph with a stark reminder that this is just the beginning of the next phase of our civilisation.

It means that we’ve now visited every world in the Sun’s sphere of influence. I use the term “world” to avoid arguments over the exact definition of a “planet”, but all nine of the objects that most people consider to constitute the solar system have now been visited at least once by a probe. How do we now best focus our efforts to deepen that knowledge?

We know a remarkable amount about Mars — our nearest interplanetary neighbour. There are two rovers on its surface (Opportunity and Curiosity), beaming signals back to Earth via three spacecraft (Mars Odyssey, Mars Express and the Mars Reconnaissance Orbiter) surveying the Red Planet from orbit, all connected up through an interplanetary internet. We also know a good amount about the Sun (SOHO and SDO), Mercury (MESSENGER), Venus (Venus Express), Jupiter (Galileo and the soon-to-arrive Juno) and Saturn (Cassini), thanks to the probes we’ve sent to orbit each one.

But scientists studying Uranus, Neptune and the other smaller planetoids beyond, like Pluto, Eris, Makemake and Haumea, have to make do with the data gathered during brief flybys and the scraps of information obtained from long-distance telescopes. Here’s what we know about them all, and what we’d love to find out.

New Horizons

You’ve probably been closely following the New Horizons mission, but what’s not so well known is how long scientists have been clamouring for data on Pluto.

New Horizons began as a letter-writing campaign from an alliance of scientists called the ‘Pluto Underground’ in 1989, aiming to pressure NASA into visiting what was then still considered the last planet in the solar system. Several low-cost, high-risk concepts were put together to satisfy budget requirements, but there was little appetite at the time for these kind of missions – NASA’s missions were either cheap and cheerful or big and impressive.

Image: Wikipedia

In 1991, things changed when the US Postal Service released a set of stamps bearing the image of each planet in the solar system and the name of a spacecraft sent to it. Pluto’s stamp was an embarrassment – depicting a featureless sphere and the words ‘Not Yet Explored’. It was a call to arms for engineers at NASA’s Jet Propulsion Laboratory, who swiftly begun putting together more feasible proposals.

After the discovery of the Kuiper Belt in the late ’90s, one project in particular – the Pluto Fast Flyby mission – gained traction by adding a mission goal to visit the belt, rebranding itself as the Pluto Kuiper Express. The budget was expanded as a result, but the entire project was almost scuppered in 2000 when its cancellation was ordered by Edward J Weiler, then NASA’s science mission director.

There was an immediate outcry among much of the scientific community, and several internal NASA divisions. After extensive lobbying, a new class of mission was created – one that would fit between NASA’s low-budget Discovery Program and its high-end Flagship Program. In November 2011, most of the team from Pluto Kuiper Express were recruited into a mission called New Horizons, which was then officially selected for funding.

But just as it was finally looking like a Pluto probe would launch, disaster struck once again. NASA’s new administrator, Sean O’Keefe, was not supportive of the project and refused to include it in NASA’s 2003 budget – effectively cancelling it. Again, there was an immediate outcry, which prompted the scientific community to push the project to the top of the Planetary Science Decadal Survey – a highly influential “wish list” compiled by the US National Research Council of the projects considered the highest priority by scientists. O’Keefe and Weiler backed down, with the latter saying it was a result that the “administration was not going to fight”. The funding was finally secured.

Launch

New Horizons  launched from Cape Canaveral on 19 January 2006 (when Pluto was still considered a planet) and reached its destination on 14 July 2015. You might think that’s a remarkably brief travel time, considering how long it took the Voyagers to cover the same distance, and you’d be right — New Horizons hit a top speed of an astonishing 58,536km/h. That’s not even the fastest spacecraft we’ve built, though. The honour goes to Helios 2 – one of a pair of probes sent to the Sun in the late ’70s. During its closest approach to our star, it swung through the Sun’s gravity well at a whopping 252,792km/h.

For New Horizons, radio signals take approximately four hours to reach the craft from Earth, meaning that almost all of its operations need to be pre-programmed. As well as taking some holiday snaps, New Horizons will map the surface composition of Pluto and its moon Charon, analyse their atmospheres, and gather as much data as possible on the smaller moons in the Plutonian system.

It’ll take about 16 months for the probe to send back all the data that it’s collected on Pluto and its moons, because it can only transmit at 2kb/s. As a result, most of the major scientific discoveries from our brief brush with Pluto will trickle out over the coming months. Meanwhile, New Horizons’ momentum will carry it further, past a trio of interesting objects in the icy Kuiper Belt that marks the edge of our solar system. The spacecraft is healthy and has plenty of fuel, so by 2016 the New Horizons team will have submitted a request for the mission (and its funding) to be extended. The odds are good that it’ll be approved, so it’s likely we’ll get our first sight of one of those objects in 2019.

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.

Neptune

After three and a half more years of whizzing through deep space, Voyager 2 arrived at the final stop of its grand tour – Neptune – on 25 August 1989.

Neptune’s deep blue atmosphere was imaged in full as Voyager 2 swooped just 3,000 miles above its north pole. Such a close encounter, its closest with another planet since leaving Earth, meant that the craft could get a great view of its gaseous body, its rings and its 14 moons.

The reason for such a close swipe past Neptune was because the team wanted to swing past the larger of the only two moons known to orbit the planet at this point – Triton. This manoeuvre was complex, as Triton was located behind and below Neptune at the time of the flyby, meaning that the craft had to make a perilously close pass over the top of the planet, but it still managed to get a good glimpse of the methane-rich upper atmosphere.

Methane absorbs the red light from the Sun, but scatters blue wavelengths back out to space – making the planet appear blue to observers. Hence it’s named  after the Roman god of the sea,  but only after much astronomical infighting over credit for the discovery that almost saw a French astronomer named Le Verrier name it after himself.

Neptune is similar in composition to Uranus. It’s slightly smaller, but a little denser and it’s thought to have the same icy, rocky centre. However, its weather is very different – unlike the hazy, featureless view of Uranus from space, the atmosphere of Neptune has active and visible weather patterns. At the time of Voyager 2’s flyby, the Southern hemisphere was marked by a “Great Dark Spot”, a little like Jupiter’s “Great Red Spot”.

The spot has since disappeared, but is thought to have been an anticyclonic hole in the methane cloud deck – with cirrus clouds above it comprising of crystals of frozen methane. Since then, other holes have appeared and disappeared, most recently including an almost identical one in the northern hemisphere that has remained visible for several years.

The flyby of Triton, the only one of Neptune’s moons to be spheroidal, yielded yet more surprises for the Voyager 2 team. It’s one of the few moons in the solar system known to be geologically active – its southern pole is covered with geysers that erupt for a year, spewing nitrogen 8km into its weak atmosphere, and volcanoes that flow with lavas of water and ammonia.

Closer to its equator in the western hemisphere, the moon features a unique surface known as “cantaloupe terrain”, due to its resemblance to melon skin. This terrain consists of water ice that’s formed into depressions between 30km and 40km in diameter. It’s unlikely that they’re impact craters, as they’re evenly distributed and all of similar size. Instead, it’s thought that they formed due to diapirism – the same process that forms salt domes on Earth and can be seen in lava lamps.

Almost nothing is known about the moon’s northern pole. It was in shadow during Voyager 2’s flyby and nearly everything we know of Triton comes from this single, brief encounter.

After the success of Voyager 2’s farewell photo of Uranus, a similar shot was scheduled for Neptune as it sped off on its own course out of the solar system on 28 August 1989 – now 48 degrees below the solar plane as a result of its Triton encounter. The result was perhaps even more beautiful – a thin crescent of Neptune with Triton hanging below it, like a mother and her child. The blue colour has disappeared, as the light is no longer being scattered back by the atmosphere – merely reflecting off the top. It remains the last, closest photo taken of the Neptune system.

Future projects

A return visit to Neptune is pretty unlikely for the time being. In 2005, a proposal was picked up by NASA for a Neptune Orbiter, launched in 2016, to visit the planet and answer many of the questions that were raised by Voyager 2’s brief flyby. It would have included a pair of atmospheric probes, and study the planet’s rings, atmospheric weather and climate, and its moons. However, since 2008 the mission has been removed from official consideration and other than a tentative suggestion for another flyby spacecraft called Argo, there are no current plans to return to Neptune.

Pluto? Well, no doubt New Horizons’ flyby will raise far more questions than answers too, but unless something is discovered that totally changes the way we see the universe, it’ll likely be many decades before we return.

A mission to Uranus, however, may be more likely in the medium term. In 2011, the latest Planetary Science Decadal Survey passed over Neptune in favour of a Uranus orbiter, launching between 2020 and 2023, which would take 13 years to reach the inner ice giant. It ranked this mission third in the priorities’ list behind a Mars sample return mission and Jupiter/Europa orbiter. However, the most recent NASA budget proposals have put flagship-class planetary missions, other than Mars Science Laboratory and its Curiosity rover, on hold – leaving the plans on ice until the budgetary situation improves.

The only other possibility may be a European mission called Uranus Pathfinder – submitted to the European Space Agency in December 2010. It’s listed as “medium-class” – capping its budget at €470 million. Unfortunately, the current estimated costs are between $1.5 billion and $2.7 billion, so unless there’s a way to dramatically slash those costs, or split them between several countries, then we won’t be going back to Uranus any time soon.

Ask anyone who studies the ice giants and they’ll tell you that there are many reasons to return to them. For starters, we’d like to find out more about the peculiarities spotted by Voyager 2 – Uranus’ odd axial tilt and diamond glaciers, and Neptune’s dark spots and melon-like moons. Unless funding for space research is increased, then those questions will be left hanging in the inky darkness, like the planets themselves.

So scroll up and have another look at those photos taken as Voyager 2 departed Uranus and Neptune. It’s going to be some time until we see anything like them again.

Images from NASA unless otherwise stated. Lead image from L Cacada used under Creative Commons.