It took a genius like Issac Newton to invent the laws of motion because friction, air resistance, and other forces on Earth complicate things so much that it's hard to see the underlying forces. In space, however, much of this is stripped away, and we can witness physics in a raw form, such as a body in motion remains in motion unless acted on by a force. The European Space Agency (ESA) has provided a great example of this with footage of astronauts inside the International Space Station (ISS) during a reboost.
Rather than stopping suddenly, the Earth's atmosphere gradually gets thinner the higher you go. Even at the ISS's altitude of 400 kilometers (240 miles), there is still a faint drag, which causes the orbit to slowly decay. Consequently, every now and then the ISS needs a boost, in this case, provided by a resupplying cargo craft firing its thrusters for a few minutes to push the station to proper altitude. The astronauts – and anything else not tied down – aren't equipped with rockets of their own, so as the space station moves around them, they continue on their orbit.
To a camera fixed in the ISS's frame of reference, however, it looks like this (the footage has been sped up x 8, according to ESA):
Clearly, the process of having the station accelerate past you is so much fun the astronauts are keen to push off at the temporary bottom to go through the whole thing again, like pandas on a slide. We're not sure why the ESA sped the whole thing up eight times – personally, we'd be happy to watch a four-minute video at actual speed.
In the comments one individual attempts to calculate the acceleration of the station, concluding it's around 0.005 ms-2. If so to the astronauts on board it would feel like a gravitational pull of 0.0005g, that is 0.05 percent of the Earth's gravity. The figure relies on some rough estimates of the time it takes for the astronauts to get from one end of the corridor to the other, and the length they travel. A more precise calculation of a different reboost, based on figures supplied by the ISS agencies, gives a figure of a third that much.
For comparison, astronauts on the Moon experience 0.17g. Even on Ceres gravity is 0.03g. To replicate this feeling of almost, but not quite, weightlessness on an extended basis you would need to visit a small moon or the asteroid-like Martian moons Phobos and Deimos. Alternatively, we might get the experience on the way there, if future Mars missions choose ion thrusters over quick-firing rockets.
