"Imagine the explorers of old who crossed the oceans," says Dr Alan Stern, the principal investigator of the New Horizons mission. "They will have had some of the same feelings we are having after their long journeys. I just tell people, if you are anything like me then you will fasten your seat belt because I expect that you will be in for one heck of a ride."
Pluto's history has already been on something of a rollercoaster. Hypothesised by US astronomer Percival Lowell in the early 1900s, it was finally discovered by Clyde W Tombaugh on 18 February 1930. A month later, this object believed to be the ninth planet was officially named, but it was stripped of its planetary status by astronomers in 2006.
Today Pluto is classified as a 'dwarf planet' since its highly elliptical orbit overlaps with that of Neptune. But that matters little to Dr Stern and his team at the Southwest Research Institute, which was granted the mission by NASA following an open competition in 2001. For, regardless of its status, Dr Stern views New Horizons as a giant leap forward. "At one point we even considered naming the mission One Giant Leap," he reveals. And he firmly believes the wait will be worth it: "Space missions are usually over in less than nine years but ours is only just beginning."
To get ready for the mission, Dr Stern and his team had to work fast. As soon as they were chosen to head the mission, they knew they needed a Jupiter gravity assist if they were to get to Pluto within nine-and-a-half years. "But that meant getting this built and launched in four years and two months because the Jupiter gravity assist that we used to go to Pluto was the last one for a decade," he says. It represented a record: Cassini-Huygens, which was sent to Saturn, took a decade to be built, as did Galileo, which studied Jupiter. In order to achieve it, the team worked nights and weekends for four years straight.
The results have been impressive, though. Five times less expensive than Voyager, New Horizons has not relied on the invention of new spacecraft technology but it has a good set of seven instruments on board. "We put all of our technology into better cameras, better spectrometers, more capability to return data; says Dr Stern. In February 2007 as New Horizons swung past Jupiter for a gravity boost, the team was able to test the spacecraft systems and instruments on a practice flyby. More than 100 scientific papers using data from the Jupiter flyby were published and it also made the cover of Science magazine. When asked what he expects to find on Pluto, though, Dr Stern laughs: "If I knew, I wouldn't have to do the mission."
New Horizons came out of hibernation for the last time on б December 2014 as it made its final approach. Since then it has observed Pluto for an entire 6.4 Earth-day rotation, watching it wobble slightly thanks to the gravity of its largest moon, Charon. It has also revealed a large bright area at the pole of Pluto. The science team cannot know for certain what it is until the composition is tested but it suggests the presence of a polar cap. With broad dark and bright regions across the icy world, even at a distance of 113 million kilometres (70 million miles), it was viewed as a major discovery.
"Pluto showed dramatic markings, telling us that it is a special place; that is different from most of the others especially because only the Earth has had anything similar," says Dr Stern. When the spacecraft gets closer, things will become even clearer. That's when discoveries will be made at a rate of knots with astronomers keen to find out if there are other possible moons and even whether there are rings around Pluto.
"If we did see rings then what we could conclude would depend on what we found, the number and locations and densities of them, but it would be exciting," says Dr Stern. "The possibilities are many." Indeed, there are many mysteries waiting to be found and explained: the surface composition is said to include nitrogen, carbon monoxide, methane and water ices, for instance, but many other materials may be present and undiscovered.
There is even a possibility, no matter how small, of life. Pluto is two-thirds rock and one-third water in the form of water ice. It has more than three times as much water as all of the Earth's oceans. If either have an ocean, meaning the water would be warm enough to be liquid, there could theoretically be life on Pluto or Charon. If that was the case they would, like Saturn's moon Enceladus and Jupiter's moon Europa, go on the list of potential astrobiological targets. What is just as fascinating is that, during the five months it will spend observing Pluto close-up, the dwarf planet will barely have moved. Pluto only orbits the Sun once every 248 Earth years but it's not the only focus of New Horizons' mission. Its secondary objective is to make the first exploration of the small bodies in the disc-shaped Kuiper belt region, which Dr Stern says "turns out to be the largest class of planet in the Solar System."
The Kuiper belt contains bodies of mainly ice and rock. But there are potentially trillions of Kuiper belt objects, hundreds of thousands of them believed to be larger than 100 kilometres (62 miles) in size, plus many, many more as small as one kilometre (0.62 miles). Eris is one of the icy worlds and several of the dwarf planets have tiny moons. "There are more known Kuiper belt planets than there are gas giants and terrestrial planets combined," says Dr Stern. "So this is a very important new and unexplored class of targets in the Solar System."
Pluto remains the primary focus, though. As New Horizons approaches, the spacecraft's dust and charged particle instruments have been taking measurements. Operating every day and sending home data every week, they have been telling scientists about Pluto's wider environment. When scientists get close-up data, they will be able to interpret it in context.
"It has confirmed models of the expected dust density but that was no surprise." says Dr Stern. "It has amplified on the kind of information that the Voyager spacecraft obtained when it went through the Kuiper belt. The instrumentation that we are carrying to study the solar wind and other charged particles is vastly more sensitive, so we can see many more details. Even this far from the Sun, the spacecraft is crossing interplanetary shocks, supersonic shocks in the solar wind, which Pluto also crosses - and so do the other Kuiper belt objects. So we are just characterising the space weather environment out there."
Additionally, it will also be a good opportunity to find out more about the aspects of Pluto astronomers are certain about. It is understood Pluto has low surface gravity, an estimated temperature of -233 degrees Celsius (-387 degrees Fahrenheit) and its atmospheric pressure is 100,000 times less than Earth's. "We know for a fact that Pluto's surface markings have moved and changed since the Nineties and we know for a fact that Pluto's atmospheric pressure has more than doubled since the Eighties, so we know that Pluto is dynamic; it's changing," says Dr Stern.
But Pluto's moons represent more of an unknown. When New Horizons was being planned, Charon was Pluto's only known moon. But, months before the mission launched, the Hubble Space Telescope (HST) suggested Pluto had two extra moons: Nix and Hydra. There were more to come too. In July 2011, HST found a fourth moon and, a year later, a fifth. They were called Kerberos and Styx.
"There is circumstantial evidence that there could be surface activity on the big moon, Charon. We see that because we have detected crystal and water ice there - and crystal and ice can't survive for long space." says Dr Stern. "The radiation in space changes the crystal structure so the fact that we see hexagonal crystals tells us that ice must be young. We also see. compositionally. a material on the surface of Charon called ammonia hydrate, which often is the result of cryovolcanism. a cold form of volcanism. Those are hints Charon may be active."
The discovery of the extra four moons changed the mission in two significant ways. Not only was the team able to schedule observations of them into the mission - "if we hadn't known about them we couldn't have done that" - they had to rethink the logistics of the flyby. "We realised these four satellites could present hazards, not because we are going to hit one but when there are craters, the shrapnel that comes out of the hole can get into orbit around Pluto and be a danger," says Dr Stern. "So we put in place a seven-week search for hazards that New Horizons initiated in May, lasting all the way down until we are almost at Pluto. It will be searching to find out if the path that we plan to fly is safe or not. If it is not then we will fire the engines and move the spacecraft trajectory to a safer place."
A 93-second thruster burst has already adjusted the trajectory of New Horizons. It slowed the velocity of the spacecraft by 1.14 metres (3.7 feet) per second on 10 March and this shifted the course sideways by 3,442 kilometres (2,139 miles), pushing it towards the desired close-approach point of Pluto. But the moons continue to fascinate. On 18 February this year. New Horizons' Long-Range Reconnaissance Imager (LORRI) provided its first long-exposure views of Nix and Hydra at distances ranging from 201 million to 185 million kilometres (125 million to 115 million miles). By 13 May, it had brought Kerberos and Styx into view. Together with Charon, which had already been seen due to its size and greater brightness, it has meant all five moons have now been detected.
When the spacecraft gets close to the moons, it will seek to map their surfaces at a specified resolution and map their surface composition at another resolution. It will do the same when it nears Pluto Itself. "We will map the surface geologies, the surface temperatures," says Dr Stern. "We'll map the composition, which means we will obtain spectra at literally hundreds of thousands of locations on the disc. We will map the surface topography because we will do stereo imaging so there are a whole variety of different maps that we will make.
"By combining them - looking at the geology, the composition of the different types of features or whether there are thermal hotspots, for example - it can inform us what is going on in the planet, what's going on on the surface. It will tell us how Pluto has evolved and how it originally came to be."
At the same time, Dr Stern insists the spacecraft is not there to answer questions even though many can be posed. "We don't know what questions to ask." Dr Stern explains. "We are there to collect certain data sets that will allow us to explore the Pluto system. It's what the early space programme was about but done with 21st century technology."
More than 1,000 images are being sent to the ground team on its way to the encounter and while LORRI may be the least sophisticated instrument on the spacecraft, it will arguably produce some of the most exciting data. "It's got a big telescope in front of it so it's high magnification," says Dr Stern. "But it only has one charge-coupled device (CCD) and it's black and white, what we call panchromatic."
Those high-resolution images will be backed up by snaps taken from a digital imager called Ralph. Ralph will make the maps that show what Pluto, Charon and the Kuiper belt objects look like at light levels 1,000 times fainter than daylight at Earth. On 14 April this year, it took the first colour image. Eventually it will deliver images showing surface features as small as a few miles across. "It has eight CCDs, both colour and black and white, both scanning and framing cameras, and inside it also has an infrared mapping spectrometer with 64,000 pixels that is dramatically advanced compared to the Voyager devices of the same type," Dr Stern explains.
This will help to ensure that space enthusiasts and astronomers are glued to their TVs and the internet as more images and news niters through. "This mission is the capstone to the historic first era of reconnaissance, the equivalent of the summiting of Mount Everest," says Dr Stern. "It is just as significant as when the last of the continents of Earth were conquered... I can't be more proud of the team."