When the reaction of a top Nasa scientist is “wow” you know it’s time to pay more attention to latest developments coming out of the US space agency. So what elicited such a response?

A Nasa spacecraft edging closer to Pluto has sent back dramatic images of the dwarf planet’s moon Charon – revealing a dark spot at one of the poles.

The New Horizons mission, travelling at about 27,000 miles per hour, will be closest to Pluto on July 14 but is “making new discoveries”, said Nasa, as it gets ever closer.

One of those is that Charon has a “dark pole” – a mysterious dark region which forms a kind of anti-polar cap.

(Nasa/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

The image on the right has been deconvolved – a method which sharpens images. Occasionally, warns Nasa, it can introduce “false details”. More pictures will be taken as the piano-sized spaceship gets closer to confirm the dark spot’s actual existence.

New Horizons won’t land on Pluto, or even enter the planet’s orbit (it’s too pricey to make it slow down) but it will do a fly-by before sailing on to the Kuiper Belt.

During the fly-by, New Horizons will fly over what is being dubbed the “close approach hemisphere”. It has the greatest variety of terrain types seen on the planet so far.

(Nasa/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

“This system is just amazing,” said Alan Stern, New Horizons principal investigator, from the Southwest Research Institute, in Colorado. “The science team is just ecstatic with what we see on Pluto’s close approach hemisphere. Every terrain type we see on the planet – including both the brightest and darkest surface areas – are represented there, it’s a wonderland.

“And about Charon – wow – I don’t think anyone expected Charon to reveal a mystery like dark terrains at its pole,” he continued. “Who ordered that?”

This short video shows some of the most recent pictures captured by the onboard kit – the telescopic Long Range Reconnaissance Imager. Our best images of Pluto before New Horizons came from the Hubble Space Telescope. These are far superior.

New Horizons is about 2.9 billion miles (4.7 billion kilometres) from Earth and just 16 million miles (25 million kilometres) from Pluto. The spacecraft and payload are in good health and operating normally. It launched from Earth on January 19, 2006.