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Up, up and away! High-altitude balloons called Stratollites might soon be giving the US military and NASA permanent and relatively low-cost eyes in the sky wherever they want.

Developed by US firm World View, Stratollites are uncrewed, helium-filled balloons that tour the stratosphere at heights of between 10 and 46 kilometres. As the stratosphere is layered with winds blowing in different directions, a Stratollite鈥檚 path The balloons are controlled remotely from the ground.

World View has used dozens of flights to refine the computer models and algorithms used to navigate the balloons. Along the way, the company claims to have carried out the biggest controlled altitude-change manoeuvre ever by a balloon, switching nearly 8 kilometres in one sweep.

鈥淢ultiple times we鈥檝e demonstrated the ability to stay within a very tight radial area 鈥� under 50-kilometre radius for over 24 hours,鈥� says Andrew Antonio, World View鈥檚 director of marketing. The firm aims to have balloons that can permanently linger over a very small spot, with on-board solar arrays providing all the power they will need.

World View released images from one of its Stratollites this month with a resolution of half a metre. This is enough to distinguish between a pick-up truck and a car, and is comparable to .

Current spy satellites revisit a given point less than once a day, but a Stratollite can theoretically keep watch 24/7. And in terms of cost, Antonio says that a one-month Stratollite mission will cost hundreds of thousands of dollars, compared with millions for a satellite.

Government eyes

鈥淲e think this has the potential to be a game-changer for us,鈥� says US admiral Kurt Tidd, commander of , which handles US military operations in Central and South America and the Caribbean.

Southern Command has been testing Stratollites since last August, and is interested in using them for combating drug trafficking and piracy in remote areas, something it currently uses drones for. Once deployed, Stratollites and drones can both be spotted from the ground by a keen observer, but Stratollites are silent.

NASA is also testing Stratollites. The space organisation has operated stratospheric balloons for decades to monitor everything from cosmic rays to Earth鈥檚 surface, but once the balloons are in the sky, the direction of travel isn鈥檛 controllable. Alan Stern, who leads NASA鈥檚 New Horizons mission, which completed a fly-by of Pluto last year, is also a co-founder of World View.

Alphabet, Google鈥檚 parent organisation, has its own stratospheric balloon project called . It plans to use the balloons to provide mobile communication in remote areas. Last year the company deployed balloons to provide mobile phone communications in Puerto Rico after Hurricane Maria destroyed infrastructure.

Steering Stratollites isn鈥檛 always going to be easy. In some places prevailing winds may dominate over countervailing winds, says Neal Butchart at the UK . 鈥淪uccess is likely to vary with the latitude band and hemisphere the balloon is flying in,鈥� he says.