Knocking on Pluto’s door

When I was in primary school my parents gave me a book called All About Famous Science Expeditions. I devoured the tales of komodo dragons, King Tut’s tomb and the first deep-sea dives. But two stories hypnotised me.

One described the first ascent of Alaska’s 6,194-metre Mount McKinley, the other an expedition that braved polar bears and ship-crushing ice to discover Canadian Arctic islands unknown even to the Inuit. Forget the science. I wanted to know what lay over the horizon. And the colder, more distant and more exotic the horizon, the better.  

So I was always going to be a sucker for the voyage to Pluto.

If all goes to plan, on 14 July at 11:50am UTC, a NASA spacecraft called New Horizons will pass within 10,000 kilometres of it. This is the last great voyage of discovery we’re likely to see in the next 20 years – “the closing of the first era in planetary reconnaissance”, in the words of Alan Stern, the mission’s principal investigator.

 

Pluto has always been a place of mystique. Its discovery was the culmination of a 25-year hunt that remains one of the great legends of astronomy. It began in 1905 when American astronomer Percival Lowell predicted a far-distant planet based on irregularities in the orbits of Neptune and Uranus. It ended in 1930 when a young astronomer named Clyde Tombaugh picked it out of the starry background. Working at Lowell’s observatory, he used a device called a “blink comparator” to compare images taken several days apart to spot the one tiny dot that had moved.

Pluto’s elusiveness added to its mystery. For years it was too small and far away to be more than a dot in most telescopes. Pluto’s name fuels the mystique. It was suggested by an 11-year-old British girl named Venetia Burney who noted that Pluto, the Roman god of the underworld, lived in a dark, gloomy place and had the unusual ability to disappear, like the elusive planet. Tombaugh liked the idea, and Pluto it became.

Over the years astronomers did manage to fill in some vital statistics. They pinned down its diameter to the now-accepted figure of 2,390 kilometres (roughly two-thirds of our Moon) and its surface gravity to about 6% that of Earth. They learnt that its eccentric, elliptical path sees it occasionally wandering into Neptune’s orbit, raising the question of whether Pluto might be an escaped moon – Neptune has 13 – that periodically returns to Neptune’s neighbourhood. 

They found that Pluto takes a surprisingly slow 6.39 Earth days to complete a rotation – and that when the Sun does rise, it does so in the west, because Pluto spins clockwise, which is the opposite to the Earth and most other bodies in the Solar System.

In 2006, radio astronomers measuring thermal emissions from Pluto concluded its average temperature is -230°C, cold enough to freeze every gas in the Earth’s atmosphere solid. It is so far from the Sun that even at midday the light is 1,100 times dimmer than on Earth – slightly brighter than one of our moonlit nights. 

Cold, dark and distant. Those are the three words that have always described Pluto. 

Only in the 1980s were telescopes able to make a rudimentary map of Pluto’s surface. Even today, the best Hubble Space Telescope images have resolutions measured in hundreds of kilometres. On that scale, Manhattan wouldn’t be visible on a map of the United States. Hubble’s best photos of Pluto’s surface show little more than fuzzy blobs. New Horizons is poised to change that. Its best photos could have resolutions as fine as 70 metres, at this scale you could identify buildings on Manhattan. “It’s going to be an improvement of something like a factor of 5,000,” says planetary scientist William McKinnon, a co-investigator on the mission.

It’s not clear who first dreamed of going to Pluto. Science fiction writers started writing about the voyage almost as soon as the planet was discovered, but most of the stories were absurd. Then a little more than 50 years ago, a NASA engineer named Gary Flandro realised the mission was possible. Once every 175 years or so the planets align. Flandro calculated it would be possible to use Jupiter’s gravity to boost a probe on a “grand tour” of the outer Solar System. Jupiter orbits the Sun at almost 50,000 kilometres per hour. A carefully aimed space probe briefly caught in Jupiter’s gravity would receive a huge kick of speed – somewhat like grabbing hold of a speeding bus – before being flung towards the outer planets.

NASA took note. In 1977 two Voyager spacecraft were launched. Over the next 12 years they provided the first close-up views of Jupiter, Saturn, Uranus and Neptune. The Voyager probes looped around Jupiter and were duly catapulted towards the more distant planets. 

Pluto could have been added to the list back then but during the flyby of Saturn there was a choice between swinging close to Titan, or visiting Pluto. 

Titan won, leaving Pluto alone and unexamined. It took another 30 years for NASA to launch New Horizons to finally visit Pluto – again using the Jupiter slingshot method, although this time without adding other planets to the itinerary. The 14,000 kilometre per hour speed boost the manoeuvre garnered meant New Horizons will arrive at Pluto almost four years faster than it would have otherwise. 

As it happened, a few months after New Horizons’ January 2006 launch, Pluto was exiled from the pantheon of planets into the netherworld of “dwarf planets” – a category created to encompass Pluto, Ceres (the Solar System’s largest asteroid) and a growing array of Pluto-sized objects that are being discovered in the outer edges of the Solar System. At its 2006 General Assembly, the International Astronomical Union altered its definition of a planet: a body now has to have swept its orbit clear of all other large objects. As some of the newly discovered objects near Pluto cross its path, Pluto was demoted. One consequence is that fewer people are following New Horizons’ progress. Back in the days when every schoolchild knew of Pluto as the ninth and most mysterious planet, the imminent arrival of a spacecraft would have drawn much more attention. 

When everyone knew of Pluto as the ninth and most mysterious planet, the arrival of a spacecraft would have drawn much more attention.

But if we’d visited with Voyager in the 1980s, we almost certainly wouldn’t be going there now with what mission leader Stern calls “the most sophisticated suite of instruments ever sent to a new planet”. For those of us always straining to see what’s over the next horizon, the long wait was worth it. 

When New Horizons passes by Pluto it won’t only take pictures. Infrared and ultraviolet detectors will probe the molecular composition of surface deposits and map surface-temperature variations. On Earth, scientists will “weigh” the dwarf planet. As New Horizon’s radio messages are beamed home, tiny shifts in their frequency caused by the tug of Pluto’s gravity will allow them to calculate Pluto’s mass.

Not all of Pluto is frozen solid – especially where the surface is warmed by the midday sun. “Pluto has an atmosphere,” McKinnon says. “We expect to see clouds and hazes.” New Horizons’ spectrometers will analyse the makeup of Pluto’s atmosphere. 

Each gas present will absorb certain wavelengths of light, leaving a fingerprint that can be read by analysing sunlight that has passed through Pluto’s atmosphere to identify which wavelengths are missing. “When we slip behind it, we will watch the sunset and sunrise, and that will allow us to determine exquisitely the composition of the atmosphere,” McKinnon says. 

Pluto is not the only world to be examined in the flyby. In 1978, Pluto was found to have a giant moon, Charon – its surface area is more than one-quarter that of Pluto’s. The two are only 19,570 kilometres apart (our Moon is 20 times more distant), which means that if you get close to one, you automatically get close to the other. We will be able to map two new worlds for the price of one. 

In the early 1970s, when I started college, planetary science was moribund. The Solar System was mostly composed of lifeless, geologically dead and largely fuzzy-looking planets – studying them was an exercise in eyestrain. 

But in the last decade we’ve witnessed a renaissance. Sniffing their way across the surface of Mars, the rovers Spirit, Opportunity and Curiosity dazzled us with findings of ancient lakes, whiffs of methane in the atmosphere and organic molecules in the rocks. Meanwhile the Galileo spacecraft, cruising the moons of Jupiter for eight years, found evidence of liquid water beneath the icy crusts of Europa and Ganymede. And Cassini, arriving at Saturn shortly before New Horizons’ launch, reported liquid water geysers from the moon Enceladus and seas of methane on Titan. We’ve witnessed the birth of “planetary geology” and “comparative planetology”, fields of study that would have been inconceivable only a few years before.

If we’re looking for a practical reason to have spent $700 million to travel the five billion kilometres to Pluto, this is probably the best answer. 

“Studying other planetary bodies not only gives us a sense of our place in the Universe but also gives us a chance to study the processes that shape our own planet,” says Ralph Lorenz, a planetary scientist from Johns Hopkins University in Baltimore. 

Even at a distance of 10,000 kilometres from Pluto, New Horizons’ passage will be close enough to fly through the outermost layers of the atmosphere – which scientists believe is slowly being lost to space as a result of Pluto’s low gravity and the buffeting of the solar wind. “We will directly sample the interaction; the goal is to understand how planets lose their atmospheres,” says McKinnon. It’s a familiar tale. Geological evidence suggests that early in its history, Earth somehow lost its original atmosphere. “Mars did the same,” McKinnon says. Studying Pluto may therefore help us understand our own planet’s past. 

In the meantime, planetary geologists have turned their spotlight on Pluto, producing a spate of studies indicating that it is much more than cold, dark and distant. There’s even a chance it may not have always been lifeless. The radioactive cores of Pluto and Charon, combined with the friction generated by their tidal interactions, might once have generated enough heat to melt water into a Europa-like subsurface ocean which supported life. 

Pluto’s interior has gradually cooled but if an underground ocean did once exist, geological relics may be visible – a freezing ocean would have pushed up the overlying surface like an ice-cube bursting its container, producing a distinctive pattern of cracks. 

The first time New Horizons came to my attention was in June 2006, six months after its launch. The International Astronomical Union – the body charged with officially assigning astronomical names – was tackling the question of what to do with two newly discovered moons of Pluto: soon designated Nix and Hydra. The names continued to draw from the Greco-Roman underworld myth. Charon took its name from the ferryman who carried the deceased across the river Styx. Nix was Charon’s mother, the Greek goddess of darkness. Hydra was a nine-headed monster that guarded the underworld. But the names were also picked because they offered an apt acronym. “N” and “H”: New Horizons – already en route to Pluto.

Suddenly, Pluto had evolved from a wannabe planet to an entire system.

Space missions generate plenty of angst. Though New Horizons is armoured with Kevlar to protect it against high-velocity impacts with dust grains and other small particles, the main concern is that it may hit a tennis-ball sized object as it approaches Pluto. There could be plenty. In 2011, a tiny fourth moon named Kerberos (a guard dog from the underworld) and only 30 kilometres wide, was discovered by researchers using the Hubble Space Telescope. That resulted in an even more intense search for hazards and the 2012 discovery of another moon, now known as Styx, which has a diameter of about 20 kilometres. 

Suddenly, Pluto had evolved from a wannabe planet to an entire system. The big concern was that if Pluto has many moons, it might also have rings like those of Saturn, Jupiter, Uranus and Neptune which are composed of tiny particles of rock or ice, but still big enough to puncture New Horizons’ armour. 

Nobody had worried about rings in New Horizons’ path, but neither had anyone expected Nix, Hydra, Kerberos and Styx. If there are rings, it would be best to avoid them.

The potential hazard means the mission’s route past Pluto remains undecided. As it closes in on its target, New Horizons’ own instruments should soon be able to pick out obstacles. If they are discovered, it’s not too late to make a course correction. 

Mission control originally believed the safest place to detour would be into the orbit of Charon, which Stern compares to “a giant vacuum cleaner” that would gravitationally suck up every moonlet or other particle that might stray into its orbit. But nobody wants to swing farther out from Pluto than is strictly necessary. “Any option like that comes at a huge cost in the science,” Showalter told me. 

Another option, says Stern, could dive much closer to Pluto, where atmospheric drag clears debris particles out of the way, much as it does on Earth.

There comes a point when it’s too late for bailouts. The longer you wait to make a course correction, the more dramatic the evasive manoeuvre has to be and the more fuel it takes. Furthermore Pluto is so far away it takes radio signals nine hours to make the round-trip journey. If mission control spotted a hazard only a few hours before the encounter, there would be no way to dodge it.

The final stages of the flyby will be under automatic pilot. And for the 24 hours of the most intimate encounter between the spacecraft and Pluto, Earth will lose contact with New Horizons altogether. That’s because the spacecraft’s antenna is in a fixed position. It is aimed by “pointing” the entire vessel in the desired direction – a cost-saving measure that will doubtless cause anxiety on the crucial day. “We have to choose between talking to Earth or looking at Pluto,” Stern says.

This type of communication blackout has happened before. When the Curiosity rover parachuted to Mars in 2012 under automatic pilot, there were “seven minutes of hell” when NASA controllers could do nothing but stand by and wait. For New Horizons, it will be more like an entire day. If anything goes wrong, we will only know if the spacecraft fails to re-establish contact. 

Even if all goes well, it will take 16 months to beam all the data back to Earth. This longed-for information will come back in dribbles – rather like letters from sea captains in the days of the tall ships. 

Meanwhile, New Horizons will have travelled an additional half-billion kilometres out from the Sun. And that’s where this great voyage will have one last shot at fame. Pluto’s demotion from planethood kicked it from the fringe of our more familiar Solar System. It is now part of the Kuiper Belt, where astronomers have spotted enormous numbers of equally mysterious objects floating on the Solar System’s dark edge. Two of the larger Kuiper Belt objects have been designated as potential post-Pluto bonus targets. Billions of kilometres beyond Pluto, they are reachable by course corrections – presuming New Horizons doesn’t have to waste too much fuel on a collision-avoidance manoeuvre.

And New Horizons’ ultimate fate? This last great explorer of our generation, it will continue transmitting data back to Earth, and will eventually move beyond the confines of our Solar System to join the two Voyager spacecraft in the depths of outer space. But for now, the Voyagers are still within the outer limits of  the Solar System, and call home regularly. They each carry a greeting card to aliens in the form of golden phonograph records containing sounds and pictures of Earth. New Horizons offers other souvenirs: an American flag, a 1991 US postage stamp bearing the image of Pluto and the words “Not yet Explored” – and a tiny vial of Clyde Tombaugh’s ashes, the man who saw Pluto’s flickering light from an observatory in 1930.