Do space travellers experience 0 gravity?

02 July 2019

Interview with 

Mathew Hall, Naked Scientists

INTERNATIONAL-SPACE-STATION

The International Space Station (ISS) in orbit, photographed from the attending space shuttle Discovery

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Matthew Hall has been investigating the controversial claim that “space travellers experience zero gravity”...

Three, two, one, zero...

Matthew - Space: it hosts countless environments that are detrimental to our health but ironically, it's home to one of the most sought after astronomical phenomena to experience - zero gravity. Space is a mere 100 kilometres away from us at all times, yet only those trained as sufficiently as astronauts have the privilege to feel weightlessness in space.

If you are willing to pay a considerable amount of money, there is the option to experience simulated weightlessness using an aeroplane travelling in parabolic flights. The dips in the trajectory of the plane have been designed to recreate zero gravity, but this isn't real zero gravity, right? Buying tickets for these parabolic flights would be like buying grandma's cookies in a store, they'll get the job done but they are not the authentic, gooey, mouth watering cookies from grandma.

Well, that's where the misconception lies; the idea of zero gravity or no gravity at all, doesn't exist. Let's look at the International Space Station; contrary to popular belief the weightlessness of the astronauts in the ISS is not due to reduced gravitational forces. The pull from gravity at their height is actually almost the same as here on the ground. The truth is they're actually... falling. Every single second the ISS is in orbit, the football field-sized hunk of metal is actually in freefall and so are the astronauts inside. And yes, I refer to an American football field.

The fact that these astronauts have not plummeted to their doom is thanks to the speed at which the ISS orbits the Earth - a casual 7 1/2 kilometres per second. The station moves so fast that the Earth's surface curves away from their freefall path. Essentially, the ISS is a large projectile that keeps missing its target.

To help better visualise the effect, imagine a canon on top of a really tall mountain that has an unobscured view in front of it. You want to make the cannonball go as far as possible into the clear space ahead of you but you can only fire horizontally. How do you do it? By increasing the speed at which the cannonball is launched, it can then cover more horizontal distance before gravity pulls it to the ground.

Now imagine your canon has limitless power and you keep adding more energy to increase the speed of the cannonball, eventually the ball will reach a speed great enough to circle around the Earth and land behind you. Add more and more speed and the ball will keep circling and never touch the ground, despite the fact that it is falling. That is essentially what an orbit is, an object in a seemingly endless freefall.

One other key concepts to debunking zero gravity is that gravity is literally everywhere. Look at the Moon for example, Earth's gravity is what keeps it in orbit around us. The ISS is between the Moon and Earth, which means that the station still experiences the force of gravity from Earth. Say you acquire a rocket and blastoff away from Earth's gravity, the Sun has a gravitational pull that extends a couple of light years in all directions.

Let's boost up the power of that rocket and say we make it out of the Sun's gravitational field, even still there's gravity from the Milky Way galaxy keeping you and billions of other stars in orbit. Let's get even more ambitious and say we leave the Milky Way's ring of gravitational influence, at that point there are so many billions of other galaxies floating in the universe that you will have entered their ring of influence. No matter how far you could theoretically go, gravity will always be there lurking in the background making the idea of zero gravity impossible.

So go ahead and buy those plane tickets because that weightlessness is exactly the same as seen in space. It won't be an endless freefall like the ISS, but it is an out of this world experience nonetheless.

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