Astronauts floating against a starry backdrop. Food suspended in a spacecraft. These images are what have constructed our cultural understanding of space as a place where the gravitational rules are completely different from what we experience on Earth. This has led many to believe that spacecraft are completely devoid of gravity.
This is a picture of what many people call zero-gravity. It comes from the idea that if astronauts in a spacecraft look like they’re weightless, it must be because there’s no gravity weighing them down.
Some quick math calculations will reveal this simply isn’t true. At the height that most spacecrafts orbit, the force of gravity is indeed smaller, but it’s probably not at the level you’d expect. Perhaps this is why you’ve heard people call it micro-gravity? That’s actually wrong too! At a 250 km orbit, there is only a 10% decrease in gravitational force–certainly not enough for you to say it’s micro-gravity. You can see this math here or you can do it yourself, if you have the respective masses and orbit distance. Just plug it into the law of universal gravitation, if you remember that from class!
But we have to go back to that happy student in the picture we saw before. We know that we have weight on Earth because of the planet’s gravity. Why do those astronauts not have weight if there’s still gravity?
Answering this means taking a closer look at what it means to fall. While the spacecraft experiences the Earth’s gravity pulling it down, it is also moving forward at a high speed that brings it into what we call orbit. As vehicles such as the International Space Station circle Earth, they are technically doing a very drawn out fall. Moving forward at the same speed that they’re falling at brings astronauts into what we call free-fall–this looks to us like an absence of gravity. The shuttle’s motion is counterbalancing the Earth’s gravity, which feels to the astronauts like weightlessness. This creates the classic image we have of an astronauts helplessly floating above a dinner they can’t reach.
This means that both zero-gravity and micro-gravity are inaccurate explanations of what’s going on in spacecraft. Weightless is a much better description.