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The world has been abuzz with news of Rosetta's arrival at its comet target recently, with the mission making headlines across the globe.

But next year on 14 July 2015 an even more remarkable mission will reach its climax - Nasa's New Horizons probe will flyby Pluto in the culmination of a nine-year journey, returning the first ever close-up images of the dwarf planet.

And Nasa has announced that the spacecraft has now crossed the orbit of Neptune, the last planetary crossing on its way to Pluto, in record time.

The New Horizons probe (artist's illustration shown) has passed the orbit of Neptune on its way to Pluto. Officials at Nasa in Washington DC heralded the milestone as a key moment for the mission. This was the largest major planetary crossing before the spacecraft arrives at Pluto on 14 July 2015

The piano-sized spacecraft, which launched in January 2006, reached Neptune's orbit in just eight years and eight months.

That beats the previous time to travel this distance of nearly 2.75 billion miles (4.4 billion km) taken by Voyager 2 by nearly four years.

And in a rather fitting tribute to its predecessor, the milestone comes on the 25th anniversary of Voyager 2's encounter with Neptune on 25 August 1989.

'It's a cosmic coincidence that connects one of Nasa's iconic past outer solar system explorers, with our next outer solar system explorer,' said Dr Jim Green, director of Nasa's Planetary Science Division at Nasa Headquarters in Washington.

'Exactly 25 years ago at Neptune, Voyager 2 delivered our "first" look at an unexplored planet.

'Now it will be New Horizons' turn to reveal the unexplored Pluto and its moons in stunning detail next summer on its way into the vast outer reaches of the solar system.'

While New Horizons is crossing the orbital path of Neptune, the mission was not designed to encounter the planet - instead, Pluto is the primary target of the mission.

In fact, Neptune is actually 2.48 billion miles (4 billion km) from New Horizons at this moment, and with the spacecraft heading out of the solar system it will never encounter the gas giant.

Nonetheless the milestone is important - it marks a key stage of the mission and the last 'hurdle', with all the major planets now crossed ahead of the rendezvous with Pluto next year.

New Horizons' journey is so long that when it first launched on 19 January 2006 Pluto was still classified as the ninth planet of the solar system.

It was controversially reclassified as a dwarf planet in August 2006 when another body, Eris, was found to be larger.

This led to the International Astronomical Union (IAU) redefining what it means to be a planet, and in the third category Pluto fell short - namely, it had not cleared the neighbourhood around its orbit.

But whatever its designation, the mission is of huge importance.

Barring a few blurry images from Hubble and other long-distance images, no human has ever truly seen what Pluto looks like.

Although Neptune (pictured by Voyager 2 in 1989) was nowhere near at the time, the crossing of its orbit is still an important moment in the New Horizons mission. It also came on the 25th anniversary of Voyager 2's first look at the gas giant back in 1989

This diagram shows New Horizons' position now. For reference, 1 AU (astronomical unit) is defined as the distance from Earth to the sun. The diagram also shows how it takes more than eight house to communicate with the spacecraft as it is so far away

Just like Voyager 2's visit to Uranus and Neptune, which returned the first ever images of those worlds, New Horizons' discoveries promise to be just as groundbreaking.

What Pluto looks like is still unknown, with some estimating it might have a vast icy surface, possibly even with a cloudy atmosphere, while others say it will be more barren like Neptune's moon Triton.

'Nasa's Voyager 1 and 2 explored the entire middle zone of the solar system where the giant planets orbit,' said Dr Alan Stern, New Horizons principal investigator at the Southwest Research Institute in Boulder, Colorado.

'Now we stand on Voyager's broad shoulders to explore the even more distant and mysterious Pluto system.'

Like Voyager 1 and 2 before it, New Horizons is also on a path towards the Kuiper Belt after it has flown by Pluto.

The Kuiper Belt is a disc-shaped region of icy objects past the orbit of Neptune, but it still remains somewhat of a mystery.

In fact Nasa had been concerned that rogue Kuiper Belt objects could pose a threat to New Horizons if it encountered one on the way.

In addition, while Pluto is the main goal, officials had been hoping to find a suitable target in the Kuiper Belt the spacecraft could visit.

However, none has yet been find - and it will be a race against time to find one before the spacecraft leaves the vicinity of Pluto next year.

This image taken by the Hubble Space Telescope in 2012 shows Pluto and its five moons. Styx, also known as P5, is the most recently discovered. The main targets for the New Horizons mission will be Pluto and its largest moon Charon, with the spacecraft planning to map the surface of the to a resolution of 25 miles (40 km)

New Horizons captured this view of the giant planet Neptune and its large moon Triton on 10 July 2014 from a distance of about 2.45 billion miles (3.96 billion kilometers) - more than 26 times the distance between Earth and the sun. It's thought Triton may bear some similarities to Pluto