The fastest spacecraft ever built is on its final approach for a pioneering rendezvous with Pluto, but scientists will not know what it has witnessed until it is beyond the dwarf planet.

The New Horizons probe is closing in on Pluto at 34,000mph.

At 12.49pm on Tuesday it will fly-by Pluto at a distance of 7,800 miles, slipping between the icy world and its five known moons.

Over the last nine years it has travelled three billion miles.

For the mission to be successful the spacecraft now needs to fly through a target circle of just 200 miles.

Dr Martin Archer, a space physicist at Queen Mary University of London, said "very little" is known about Pluto.

"All we've really got is a series of images taken from the Hubble space telescope, which are incredibly blurry if you look at them," he said.

"So what we want to do is get some really good snaps of the planet, work out what its geological structure is, what it's made of, what the atmosphere is made of.

"All of these things that we know tonnes about about all the other planets, we just haven't a clue with Pluto."

Seven months into the probe's journey, Pluto was downgraded by astronomers from a planet to a dwarf planet.

New Horizons will take high resolution photos at a variety of light wavelengths, as well as take temperature readings and analyse Pluto's complex atmosphere.

The spacecraft will be collecting so much data that NASA mission control suspended communication at 4am on Tuesday.

And they will not know whether New Horizons has made it out the other side until it makes contact just before 2am on Wednesday.

Until recently scientists had to make do with images of Pluto that were a few pixels across.

But by Wednesday night they should have pictures that are so detailed they can make out features the size of a football pitch.

Once the spacecraft has screamed past Pluto it will tilt its cameras towards Charon, the largest of the moons.

The satellite was once thought to be featureless.

But images taken over the last week suggest Charon has chasms, craters and geologic activity.

The mission should help scientists understand the mysterious Kuiper Belt, a two-billion mile wide zone of remnants from the formation of the planets.

The Belt is estimated to contain 100,000 objects larger than 50 miles across - and Pluto is the largest.

Many comets originate in the Belt and scientists believe it contains clues to the formation of the solar system.

Dr Archer added learning more about Pluto could help "put our place in the universe into perspective".

"We're trying to find out so much more about planets, particularly ones around other stars," he said.

"We know dwarf planets should be able to exist around other stars, but we really don't know anything about them.

"The big question, why are we here, that I think everyone's had, even since cavemen times, this is where it feeds into."