Image caption The lower slopes of Mount Sharp can be seen at top-left. The northern rim of Gale Crater is at top-right. This mosaic was assembled and coloured by Ken Kremer and Marco Di Lorenzo (www.kenkremer.com)

Nasa's Curiosity Mars rover has finally begun the long drive to its primary mission destination - Mount Sharp.

For the past seven months, it has been investigating a site just east of its August 2012 touchdown point, drilling rocks and analysing their composition.

But scientists have now decided it is time for Curiosity to get rolling.

On Friday, engineers commanded the vehicle to make an 18m drive. On Monday it travelled 40m. It will however take many months to reach Mount Sharp.

The rover has to steer clear of a long bank of sand dunes that represent a potential trap. The intention also is to get to a specific site where satellites have indicated there are layers of sediment that were most likely laid down in water.

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All up, Curiosity could have to roll roughly 8km to get to the places of key scientific interest. And if the cameras onboard spot unusual rocks along the way, the rover will be commanded to park and examine them. This could put the eventual arrival date deep into next year.

The mission team announced in early June that its extended stay at the small depression known as Yellowknife Bay was almost over.

Although a diversion to the main mission, it proved to be an extremely productive one. Curiosity found mudstones that were very probably deposited billions of years ago in a lake. And that followed the identification on the drive into Yellowknife Bay of rocks sculpted by flowing water in the distant past.

The rover - also called the Mars Science Laboratory (MSL) - was put down in Mars' equatorial Gale Crater on 6 August last year.

Its mission goal is to probe the crater's sediments for evidence that past environments on the Red Planet could once have favoured microbial life.

Image caption The rover has spent recent months examining rocks that owe their origin to the action water

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos