A NASA spacecraft will complete a nine-year, three-billion mile journey tomorrow when it zips past Pluto in the outer reaches of the Solar System. 

The robotic mission, called New Horizons, is the first to the remote world which has been a complete mystery since it was discovered in 1930. It is carrying the ashes of its discoverer, Clyde Tombaugh, who died in 1997.

Travelling at 100 times the speed of a jet airliner, New Horizons will fly past Pluto a distance of only 7,800 miles at 07.49 EDT (11.49pm UT). It is travelling so fast that it cannot go into orbit, and will race on out of the Solar System after the close encounter.

In recent days, as it neared its target, New Horizons has sent back intriguing pictures of Pluto showing features resembling a heart and a whale. Other photos show a huge chasm on the largest of its five moons, Charon.

When New Horizons launched on January 19, 2006, Pluto was considered the ninth planet. But in August that year, regulatory authority the International Astronomical Union demoted it to a new class of dwarf planet. The move was highly controversial, particularly in the US, where a campaign continues for its reinstatement. It is supported by Alan Stern, who is principal investigator on the mission.

The probe was the fastest ever sent out into the Solar System. And unlike other planetary probes which slingshot their way past planets to gain speed, it took a direct route in order to get to Pluto as quickly as possible.

It is now so distant that radio signals, travelling at the speed of light, take four and a half hours to reach us from the spacecraft. During the flyby, the probe will focus all its attention on its seven science instruments getting photos and other data from Pluto, Charon and the smaller satellites, Styx, Nix, Kerberos, and Hydra.

Pluto is only 1,472 miles wide and so far away that it takes more than 247 years to make one orbit of the sun. Charon’s diameter is just over half that of Pluto.

Though Pluto was discovered in the US, at the Lowell Observatory, in Flagstaff, Arizona, it was named by an English schoolgirl. Venetia Burney, aged 11, suggested the name Pluto, after the Roman god of the underworld, to her grandfather over breakfast in Oxford. He passed the suggestion to a local professor who cabled it to colleagues in the States. She got a £5 reward for her suggestion.

Venetia, who became a maths teacher at a school in south-west London, died in 2009, aged 90. A dust-counting experiment on New Horizons was named after her.

Mission controllers, at Johns Hopkins University, Maryland, got a scare earlier this month when New Horizons suddenly vanished from their computer screens. Fearing it had hit a chunk of ice and been destroyed, they managed to regain communication and control of the craft via a backup computer. It turned out the main computer had become overloaded as it struggled to carry out scheduled commands from NASA at the same time as it was compressing data to send back home. You can read about the drama here if you are a subscriber to Sen.

Pluto and Charon are thought to have formed together, so mission scientists are already puzzled as to why they look different. Pluto is reddish in colour with contrasting bright and dark areas, while Charon is a more uniform grey.

Pluto also has a thin atmosphere, made of nitrgen, methane and carbon monoxide, while Charon has none.

Chief scientist Stern, of the Southwest Research Institute in Boulder, Colorado, told Skymania today: “Pluto is simply amazing.” He added: “These two objects have been together for billions of years, in the same orbit, but they are totally different.”

Pluto expert Dr Jane Greaves, of the University of St Andrews in Scotland, told Skymania: “It is very exciting now that we are getting really close to Pluto and getting some great pictures. Pluto is in our headlights now.

“Just seeing the terrain close-up will tell us a lot about how it formed, and its history. We know the colours of the rocks, from changes in its light as it rotates, but actually seeing why those colours are there will be really important.

“Pluto’s very thin atmosphere basically comes from the ice on the surface boiling off, so just seeing that in action as the spacecraft flies past will be pretty amazing. Some of the ice is methane, a lot of it is nitrogen, and there’s carbon monoxide. All these things that would be pretty nasty if you transferred them into the atmosphere of the Earth in large quantities.”