Nasa’s New Horizons spaceship has travelled billions of miles in nine years to answer astronomers’ questions about the tiny world

It is the fastest spaceship ever launched and has been hurtling towards its target at a staggering 36,000mph for the past nine years. But now Nasa engineers are preparing to put their robot craft, New Horizons, on its final course – to the tiny world of Pluto.

Scheduled to reach its target on 14 July, New Horizons has already covered more than three billion miles since its launch, a distance that means signals from the spacecraft now take about 4.5 hours to reach our planet.

And the spaceship is travelling at such a speed that even tiny grains of dust have the potential to inflict serious damage if they strike the craft. “We have got round that problem in a very ingenious manner,” the mission’s principal investigator, Alan Stern, told the Observer. “We have given the craft a bullet-proof jacket. To be more precise, we have covered it in Kevlar, the material used to make body armour. That should protect it.”

Perched at the edge of our solar system, Pluto has remained a mysterious world ever since its discovery in 1930. It has an atmosphere of nitrogen and methane and a surface temperature of about minus 230C. It is so distant that to anyone there, the sun would appear as a single point of light, albeit a bright one. Despite its bitterly cold surface, scientists believe it could possess an underground liquid ocean.

Pluto also has five moons, all named after characters in Greek mythology: Charon, Styx, Nix, Kerberos and Hydra. Very little is known about any of them, and few features can be made out on the planet’s surface.

All that should change in July when New Horizons sweeps past Pluto and its moons, getting to within 6,200 miles of Pluto itself, and within 17,000 miles of Charon. The encounter will be brief, however, for Pluto has insufficient gravity to slow down New Horizons and hold it in orbit. Instead, the probe will indulge in some of the fastest bouts of data gathering seen in modern space travel.

“We will take hundreds of thousands of photographs and spectral images of Pluto and its moons as New Horizons sweeps past,” said Stern. “In fact we will gather so much information about Pluto and its moons that it will take New Horizons until the end of 2016 to transmit all its data back to Earth.”

That information should transform our knowledge of Pluto. At present, astronomers would struggle to fill a single piece of paper with known facts about the planet. By the time New Horizons has completed its mission, they will be writing textbooks about it. “We have already begun studying Pluto as we approach it, though we are still far off,” said Stern. “There have been no surprises so far but I guarantee there will be several by the time we get there.”
After its encounter with Pluto, New Horizons will speed out into the Kuiper belt, a zone of frozen rocks and asteroids that marks the outer solar system, and will rendezvous with another small planet – yet to be selected – in several years’ time. Our knowledge of the outer solar system will be transformed.
It will be an intriguing mission and a fitting tribute to US astronomer Clyde Tombaugh, who discovered Pluto. In accordance with his dying wishes, Nasa has attached a small container – with some of his ashes inside – to New Horizons. In a few weeks, Tombaugh will finally get to visit the very place that made him famous.

New Horizons will achieve another important task, added Stern. Its data should correct “a gross mistake that is an embarrassment to astronomy”, he told the Observer. When the design of New Horizons was being finished 10 years ago, Pluto was considered to be the only planet that had not been studied by a space probe. That is why the probe was funded and constructed.
But a few months after its launch, the International Astronomical Union (IAU) met and decided that Pluto was not really a planet. It was too similar to several other small icy bodies in the solar system and could not be classified a full planet. So it was downgraded to dwarf planet status.

Stern described the decision as anti-scientific. “Astronomers had found several other worlds like Pluto, so the IAU had the choice: call them all planets or downgrade. It chose the latter because it didn’t want to end up with a very long list of planets in the solar system. Children wouldn’t be able to memorise them. And that is ridiculous. We cannot memorise all the rivers or mountains on Earth. Why do we have to remember all our neighbouring planets. That is 19th-century thinking. Pluto is a real planet and New Horizons is going to show it in all its glory in a few weeks. That should change their minds, if nothing else.”