How Does NASA Simulate Zero Gravity on Earth to Test Equipment?

Before any equipment or hardware is sent to space, it's usually rigorously tested to make sure it can withstand the extremes of space. From cold temperatures to radiation, space is hard on equipment, but hardware also functions differently in space than on Earth because of differing gravity. How, then, do organizations like NASA go about simulating zero gravity on Earth?

Astronauts test their mettle with the Neutral Buoyancy Laboratory (NBL), a giant pool that holds 6.2 billion gallons of water. Thanks to the size, astronauts can work with life-sized mockups to help them prepare for their journey into space. But the NBL is only for astronaut training, not for testing equipment to see how it would fare in space.

This begs the question — can we create zero gravity on Earth? And how reliable is it for testing equipment that will be sent into outer space? Let's find out.

Parabolic flights help with replicating zero-gravity conditions on Earth by providing up to 40 seconds of "freefall." The origins of parabolic flight to simulate zero gravity go back to 1950, when it was first proposed by researchers at Brooks Air Force Base in Texas. By 1959, pilots were able to achieve 10 to 15 seconds of freefall in a C-131B cargo transport, which was large enough to both train astronauts and test equipment. This was a part of the Air Force's Reduced Gravity Program, which started in 1957 and has been run by NASA since 1973.

These flights work by flying in the shape of a parabola. The aircraft begins accelerating, and then the pilot pulls up. When the plane is going up at a fast enough speed, the pilot performs a pushover, reducing thrust, which allows the occupants and equipment on the plane to fall at the same speed as the aircraft.

Parabolic flight is a low-cost and often-used method of simulating zero gravity on Earth (or to simulate gravity on the moon or Mars), but it is limited by duration.

Gravity offload systems work by using an overhead crane-type device to offload the weight of a human or equipment to simulate zero gravity, lunar gravity and the gravity on Mars. The Active Response Gravity Offload System (ARGOS) is a large device located at NASA's Johnson Space Center. It actively tracks and follows motion using a cable angle sensor. This means that equipment and spacecraft can be moved around and interacted with, and ARGOS will maintain the simulated gravity level. Similar effects can be achieved with a complex weight and pulley system to offload the effects of Earth's gravity — a system that engineers used to test the James Webb Space Telescope.

While ARGOS is a large device, there are smaller gravity offload tables perfect for testing equipment. NASA's Goddard Space Flight Center has a table that consists of a large piece of polished and leveled granite. They’re actually able to make test equipment "float" on top of the table thanks to the sled they’re placed on — air bearings produce a thin layer of air underneath the sled, which makes it float on top of the table. "It's basically like an air hockey table in reverse," said Robotics Demonstration and Test Engineer, Joe Easley of NASA Goddard.

The downside to gravity offload systems is that they don't always offer the six directions you can move things in zero gravity — for example, the gravity offload table only offers three degrees of freedom, which can limit accuracy.

NASA also has a specific Zero Gravity Research Facility at Glenn Research Center that's been in operation since 1966. It works by using a 467-foot-long steel vacuum chamber. A vacuum pump reduces the pressure to 0.05 torr. (For comparison, standard atmospheric pressure is 760 torr.) Then, a crane drops the equipment or experiment into the vacuum chamber and researchers have 5.18 seconds to study it in freefall. Like parabolic flight, the downside to this method is the short amount of time available, as well as a weight limit of 2,500 lbs.

Researchers are always looking for new ways to test hardware and experiments before sending them to space. But these days, as the costs of access to space get lower, the International Space Station is often used as a testbed for hardware rather than trying to figure out how can we create zero gravity on Earth.

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