Giant gas bag poised

Lake Wanaka and mount Aspiring. Photo / Getty Images

Nasa to launch stadium-size test balloon from Wanaka.

Aballoon the size of Dunedin's Forsyth Barr Stadium is scheduled to lift off above the scenic Central Otago town of Wanaka on Tueday and float 34km into space.

The National Aeronautics and Space Administration (Nasa) heavy-lift super-pressure balloon (SPB) has the potential to circle the globe for up to 100 days as it tests ground-breaking scientific instruments before they are considered for free-flying spacecraft.

Wanaka was selected over other Southern Hemisphere options because of its normally settled weather and low-density population. Take-off was scheduled for today but the potential effects of Cyclone Pam led Nasa to delay the launch for at least 48 hours. The mission will begin on Tuesday at the earliest.

A 2km safety zone will be in place around the launch site at Wanaka Airport and residents inside the cordon are being asked to stay inside.

The head of Nasa's balloon programme, Debbie Fairbrother, said meteorologists would monitor weather conditions carefully to ensure it was safe to launch.

Once they gave the go-ahead, the balloon would be partly inflated with helium and released from its launch vehicle.

"Because of the change in the atmosphere, the helium will expand until it's fully inflated and is about the same dimensions as the Dunedin stadium," Fairbrother said.

The pumpkin-shaped SPB is made from about 9ha of thin film resembling sandwich bags, reinforced with load-carrying tendons. Drifting at an altitude of about 33.5km carrying 2300kg of tracking and communication instruments, it was expected by Nasa to circumnavigate the globe once every one to three weeks.

It was hoped it would eventually come down in Argentina, although Nasa would have little control over the balloon once it was airborne.

"We can drop ballast or open a valve and let out helium, so from that perspective we can change our altitude slightly. But in general we're following the winds," Fairbrother said.

"Our target is [to stay airborne for] 100 days but if we get only 10 or 20 days I'd be ecstatic."

As the balloon travels around the Earth it may be visible from the ground, particularly at sunrise and sunset. Nasa launches up to 15 similar balloons annually. If the Wanaka launch goes well, Nasa will consider returning to the southern tourist town.

Track the balloon's progress here.