In June, while beachgoers in Hawaii sit blissfully unaware, a flying saucer will descend over the island of Kauai.
It will not be a stunt for an alien invasion movie. Nasa is gearing up to conduct the first test flight of a disc-shaped spacecraft designed to land heavy loads and, one day, people on Mars.
The Low-Density Supersonic Decelerator (LDSD) will be lofted into the stratosphere from the Navy's Pacific Missile Range Facility on Kauai. The inflatable technology is intended to help slow down vehicles after they enter the thin Martian atmosphere at supersonic speeds.
"It may seem obvious, but the difference between landing and crashing is stopping," says Allen Chen at Nasa's Jet Propulsion Laboratory in Pasadena, California, who oversaw the landing of the one-tonne Curiosity rover in 2012. "We really only have two options for stopping at Mars: rockets and aerodynamic drag."
Until recently, Nasa had used parachutes and air bags for most robotic landings on Mars. But the heavier the load, the harder it is to come in softly. For the car-size Curiosity, Nasa invented an ambitious system called the sky crane, which combined parachutes with landing gear powered by retro rockets that could lower the rover to the surface on tethers.
However, Curiosity pushed the weight limits of that technology, and human landings could require 100 tonnes per mission.
The LDSD design uses a balloon-like decelerator and a giant parachute twice the size of Curiosity's. The decelerator would attach to the outer rim of a capsule-like entry vehicle. When the capsule is travelling at about 4,300km/h, the device would rapidly inflate to increase surface area. The added air resistance would slow the capsule down to about 2,400km/h, at which point the parachute could safely deploy.
To simulate Mars's thin atmosphere on earth, the team in Hawaii will first lift a test vehicle fitted with the LDSD system to about 37 kilometres above the Pacific Ocean using a high-altitude balloon. The craft will detach and fire a small rocket to reach a height of 55 kilometres.
As it falls back to earth, the system will inflate, and moments later the parachute will fire. The saucer should gently splash down in open water.