The plane flies in a
parabolic path – the same path that a projectile would take of it was simply thrown
horizontally. Inside the plane the astronauts are also moving with the plane but because
the plane is flying in the parabolic trajectory the astronauts are just falling with it. There is
no reaction between them and the floor of the plane and so they feel weightless. This
simulates the condition that they would feel in a space capsule in orbit round the earth or
in deep space.
This condition only lasts for a short time until the plane has to
pull out of its dive to avoid crashing into the ground.
Astronauts are also trained
in large tanks of water where the buoyancy provided by the water simulates partial
weightlessness.