This is Sean, AKA @SSquire on Twitter. I first met Sean at the NASATweetup for the launch for STS-135 at Kennedy Space Center, so I was very happy to see him again!
Sean is in the Multi Axis Trainer in this video. The Multi-Axis Trainer simulates the disorientation one would feel in a tumble spin during reentry into the Earth’s atmosphere. The MAT is patterned after the MASTIF ( Multiple-Axis Spin/Space Test Inertia Facility), a series of cages within cages, used for astronaut training during the Mercury program. The astronauts used this to condition themselves for disorientation that might occur in emergency conditions during flight.
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