A matter of gravity

weekly column from the Dominion Astrophysical Observatory




We have all heard terms like “zero gravity”and “microgravity” used to describe conditions in space or in spacecraft.  We hear of things being done under zero gravity, and astronauts on the International Space Station performing experiments under microgravity. Firstly, a term like zero gravity is just plain wrong. There is no place in the universe where there is no gravity. It is the common force that binds the whole thing together. It’s everywhere. Secondly, microgravity experiments are not done where gravity is weak; they are done where the effects of gravity are small within the framework of the experiment. That sounds a bit obscure, but we’ll get to that later on. However, if there is gravity everywhere, then how do those astronauts float around inside and outside the International Space Station, or work on the Hubble Space Telescope, which also seem to be floating? It’s simple; none of those things are floating. They are falling.

Imagine we have built a tower 1,000 km high. When we step out onto the observation platform at the top (wearing space suits of course), we notice we still have weight and can walk around. We feel a little lighter, about three-quarters of what we weigh on the ground, because as we move away from the Earth the gravitational attraction gets weaker. If we step off that platform we will fall straight down, just as if we stepped off a more conventional high building, with the same ultimate result.

Now imagine we have a nice big cannon at the top of the tower. We put in a load of gunpowder and then a cannonball. When we fire it, the cannonball comes out of the muzzle flying almost horizontally. Then its path curves downward as gravity gets to work. Finally it hits the ground some distance away. So we load the cannon again, cramming in a lot more gunpowder before putting in the cannonball. This time the cannonball leaves the muzzle much faster, its path curves downward much more gently, and it hits the ground further away. So we cram in even more powder. Incidentally, this is a thought experiment. Don’t try it. If we make the cannonball leave the muzzle fast enough, its curve downward under gravity will be so gentle that the Earth’s surface curves away underneath it, so that the cannonball never hits the ground. It simply loops around the Earth. It is in orbit.

In the same way, the Hubble Space Telescope, the International Space Station, thousands of satellites, millions of pieces of space junk, and the odd astronaut are all falling, round and round the Earth, because they are moving horizontally fast enough for their curving paths never to touch the ground. The most truthful way to describe their condition is that they are in “free fall”. In the same way, the Moon is free-falling around the Earth, and the Earth free-falling around the Sun. If we are in the International Space Station, we and the station are falling, just as if we were in a freely falling elevator. Without a window to look through, there is no way to tell the difference. If you are falling freely, you feel weightless. In the space station, or in that elevator, we are in a microgravity environment. In both cases we can do experiments requiring microgravity conditions, except that in that falling elevator they had better not take too long.

Saturn is low in the southwest after sunset. Jupiter rises around midnight and Mars about 3 a.m. The Moon will reach First Quarter on the 6th.


Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton.