WHAT'S that in the sky? A mayoral vanity project, a £60m tourist attraction or a much-needed river crossing?

When Boris Johnson resurrected the idea of a cable car across the Thames two years ago it seemed to many people like one of his more madcap schemes.

After coming to office he ditched the Thames Gateway Bridge plans leaving people in this part of town with only the Blackwall Tunnel, rail, the ferry or the Greenwich and Woolwich foot tunnels to cross the river. Could the type of transport usually seen at ski resorts really plug the gap?

Well, fast forward to 2012 - while adding £35m to the original £25m cost estimate - and the cable car is a reality with its gondolas seeming to have appeared almost overnight.

It's quite a technical feat and a striking view on a part of the Greenwich peninusla which already features some spectacular-looking buildings.

Testing is underway on the Emirates Air Line but, despite hopes it could be ready for the Olympics, there is still no official opening date and no confirmation of travel prices.

Operation manager Danny Price said: "It was never part of the Olympic travel plan. But we always said we'd be aiming to be open for summer 2012 and that's very much the case.

"Testing will take as long as it needs to. The absolute priority is delivering a safe and reliable service."

Since it was first put forward though, questions have been raised about who really needs to travel from The O2 to the Excel centre in Newham.

Mr Price said: "This is born out of a need to transfer people over the river. It's certainly something that's needed.

"The benefit, I suppose, of a cable car system is within 12 months it's been delivered. It will link two key locations - two growing areas.

"It will also be a real experience for passengers."

And he insisted the cars, which have retractable benches to fit bikes inside, were not just for tourists but "a number of markets", with Londoners able to use their Oyster cards on the system.

Airline Emirates has provided £36m - around 80 per cent of the construction costs - and get their logo on the bottom of each car as reward, along with its name on the stations.

With construction costs set at £45m, an application has been made to the European Regional Development Fund for more money while Transport for London (TfL) has provided the upfront funding for a scheme whose overall cost is estimated at £60m. TfL insists it will recoup the money through sponsorship and retail at the site.

Mr Price said: "If the European funding is not successful we'll have other options to bring in the remainder from other partners.

"At the moment we're very much focused on getting the operation up and running."

CABLE CAR FACTS

Maximum speed - 6 metres per second.

Capacity - 10 people per gondola (or four plus a bike).

Maximum passengers per hour - 2,500.

Travel time - five minutes.

Length - 1.2km.

Height - 90 metres with views for 15 miles.

Construction cost- £45m.

Overall cost - £60m.

Journey - From Greenwich Peninsula to Royal Docks.