It's tough to slow down spacecraft descending through Mars' thin atmosphere at supersonic speeds, as they need to drop to a speed that allows them to land in one piece. This is why NASA is developing lightweight inflatable flying saucers that will fit around the outer rims of spacecraft such as human habitats, inflating as the habitats descend to permit a safe landing. The technology will allow astronauts to land bigger and heavier spacecraft on Mars without needing to carry massive atmospheric shields or huge amounts of extra fuel.
Landing spacecraft on Mars is tricky, as the planet doesn't have enough atmosphere to let friction help slow things down. NASA has mostly employed parachute-based deceleration systems, but the parachutes need to be gigantic. More importantly, there's an upper limit to how much of a payload they can deliver. Since it's impossible to scale this method up, landing next-generation rovers and habitats for future missions becomes problematic.
This is where NASA's inflatable saucers could tip the scales. Called Low-Density Supersonic Decelerators (LDSD), the devices use the principle of rapid inflation – just like the Hawaiian pufferfish – to expand their size without adding mass, enhancing drag on the spacecraft as it descends.
Not only will the saucers enable astronauts to land larger payloads with more precision on the Martian surface, but according to NASA, they'll also allow landings at higher altitudes, making more of the planet's surface accessible. The decelerators could additionally be adapted to land payloads on other planets.
NASA is currently developing three devices as part of the LDSD project, all designed to perform at supersonic speeds: two inflatable saucers called Supersonic Inflatable Aerodynamic Decelerators (SIADs) and an extremely large supersonic parachute. The 20 ft (6 m)-wide SIAD-R decelerator is being geared for robotic missions while the 26 ft (8 m)-wide SIAD-E is being developed for payloads related to human missions.
Both these devices are being designed to fit on the outer rim of a capsule-like atmospheric entry vehicle, and when deployed, will decelerate the vehicle from Mach 3.5 (4,280 km/h or 2,660 mph) or more to Mach 2 (2,450 km/h or 1,520 mph). The 110-ft (33.5-m) supersonic parachute will then be deployed to further slow down the spacecraft to subsonic speeds.
Typically, drag-enhancing devices are tested in supersonic wind tunnels, but since these LDSDs are too large to fit inside existing ones, NASA plans to use rocket sleds to test them out. The first test is scheduled to take place this June, at the Pacific Missile Range Facility at Kauai, Hawaii.
The SIAD-R will be deployed along with the supersonic parachute at an altitude of 120,000 ft (37 km), where the air is thin enough be comparable to the Martian atmosphere. If all goes well, NASA expects to deploy the LDSDs on missions as early as 2020.
There's more information in the video below.