“Things float in space because there’s no gravity above the atmosphere, right?”
I was thinking about this common misconception, and I thought I’d share it (I just did!) and then explain ‘the truth’. And then it occurred to me that this misconception is so interesting that exploring how/why it is so tenacious and persuasive in the first place is at least as interesting as ‘the truth’. So a lot of this post is in praise of the misconception, and then we’ll correct the record a bit toward the end.
If you think it isn’t a common misconception, then I agree it doesn’t seem to be written about much, but I have met a lot of people who think it… Just try asking unsuspecting people “Why do things float in space?” and see what they say (if you don’t like the word “float”, substitute a better one…).
I know it’s tricky to explain to people the difference between real and apparent weightlessness. The point of this post is to argue that it’s far harder than you realised!
‘Reinforced misconceptions’
First an aside about language. Not scientific terminology – just language.
I have a theory that ‘led’ is the most commonly misspelt word in English (people write ‘lead’ – they really do). Full disclosure – there are many online lists of ‘most commonly misspelt words’ and ‘led’ is never on them. But look out for it yourself in people’s writing and I think you might come round to my point of view. Also, see here (he said, cherry-picking the only bit of corroborating evidence he could find!).
What is the point of this story? Well just bear with me, and I’ll get there in the next paragraph. One website I found claimed that ‘led’ is misspelt so often because the verb ‘to read’ has a past tense ‘read’ which rhymes with ‘led’, and so it is tempting to treat ‘lead’ → ‘led’ in a similar fashion. I’m sure that alone would cause some misspelling. Another site said that the reason for misspelling ‘led’ is that it is a homonym of (sounds the same as) the metal ‘lead’. And that’s another plausible reason which could fox some people. And there’s no criticism here – the English language is nuts (and simultaneously cool).
These are both plausible reasons, but what are the odds of them both existing? (And this is my point) The homonym ‘lead’ (metal) has the same spelling as the present tense of ‘led’! Which reinforces the message coming from the past tense of ‘read’ being ‘read’! It’s hard to get your head round just how intertwined two things can be!
I think that, because the two reasons to misspell are linked, they reinforce each other. In other words, the combination of reasons for the misconception is stronger than the sum of its parts. More full disclosure – I have no qualifications in linguistics, and am making this up. But what are blog posts for?!? And it does lead (!) on to talking about orbits, gravity and the atmosphere.
A very plausible misconception
How? Well, because the topic of gravity and the atmosphere also has separate but intertwined, reinforcing facts that combine to make the wrong conclusion almost inevitable. And I think having more than one reason to ‘think wrongly’ is a powerful thing! So let’s examine those reasons.
Why gravity should stop at the top of the atmosphere
So, obviously gravity should stop at the top of the atmosphere. We know this because:
- The atmosphere is being held on by gravity. So where there is no atmosphere, that’s because there is no gravity. Obviously…
- The fact that astronauts ‘float’ in their space craft means there is no gravity there (and they are definitely outside the atmosphere)
- Planes fly in the atmosphere, and pilots aren’t weightless. Also, planes need wings, to counter gravity. Astronauts fly in space outside the atmosphere and they display the signs of weightlessness. So isn’t the weightlessness a lack of gravity? And doesn’t it correlate with a lack of atmosphere?
It’s well known that the concepts of real and apparent weightlessness are tricky to absorb. But I think it’s harder than people give it credit for, because of all the plausible distraction coming from the existence of the atmosphere, and how human acitivity is different inside it and outside. There’s a complex interplay between distractors that makes the misconception almost irresistible.
So these are the things that we are fighting against when we try to talk to people about the extent of Earth’s gravitational field, and how orbits work. And I think we should acknowledge that difficulty rather than assuming that people will just believe us when we tell them ‘the truth’.
But it doesn’t – there is gravity above the atmosphere
The strength of Earth’s gravitational field, commonly denoted ‘g’, is approximately 9.8 N/kg or m/s2 (these units are equivalent) at Earth’s surface, with a tiny bit of variation due to latitude and altitude. As we rise from the surface of the Earth, gravity does indeed get weaker (as it would if it stopped at the end of the atmosphere!). It gets weaker according to an inverse-square law: g is inversely proportional to the square of the distance from the centre of the Earth. This means that if we get three times further from the centre of the Earth, the gravitational field strength will be one ninth what it was.
But the centre of the Earth is a very long way away from the surface (in everyday terms), so everyday journeys above the Earth involve only a very small fractional change in distance from the centre. For example, at the top of Everest, 8850 m up, we are only 0.14 % further from the centre of the Earth than we were at sea level (just read that again – the Earth is pretty big!).
At the top of Everest, the value of g, 9.79 m/s2, is 99.7 % that at sea level. And yet, as we saw in our post on the atmosphere, at the top of Everest the atmosphere has thinned to about a third of its sea level density. Both of those facts are shown in the graph below. Also shown for perspective is the Karman line, which is the official boundary of ‘outer space’.
Although basically all the atmosphere is contained in this graph, we haven’t gone far enough ‘up’ to see much decrease in ‘g’. So let’s go further, up way past the International Space Station (ISS) and see what happens to g at these heights. Note: (a) how quickly the atmosphere drops off at this scale – it’s the green curve, (b) how much gravity there still is, well past the ISS, (c) the size of Earth on this scale – shown in grey…
So if there is loads of gravity above the atmosphere (despite all the sensible objections in the previous sections), what on Earth (!) is weightlessness?
Real and apparent weightlessness
You may well have expected the value of g at the ISS to be zero. That would have been reasonable because the visual evidence is on your side. After all:
- the ISS is way above the atmosphere (which in previous paragraphs was persuasive in itself)
- we see ISS astronauts floating around and drinking spherical globules of escaping orange juice, and
- it is often described as a zero-g environment.
In fact, the value of g at the ISS is 89 % that at the surface. So what’s going on?
Well, the ISS astronauts are experiencing apparent weightlessness, not true weightlessness. True weightlessnes occurs at g=0, and we know that isn’t the case. Instead, the astronauts are orbiting the Earth, in effect in free fall. But so is the ISS in which they orbit. So the astronauts and ISS follow the same path, and neither exerts a force on the other – the astronauts ‘fall’ within the walls of the ISS. You have felt a miniaturised version of this yourself when a lift (elevator) accelerates downwards and you feel ‘your stomach in your mouth’. Apparent weightlessness occurs whenever your ‘container’ follows a free-fall path and you do too. The ISS is one example. A lift is a partial example (a full example if the cables break and it hurtles downwards with an acceleration of g). The vomit comet is another. The vomit comet is a plane that follows a parabolic path in Earth’s atmosphere – the path it would have taken had it been launched from a cannon and then left to its own devices in free fall. There is then a limited time (before the plane dives into the ground) of apparent weightlessness because the plane is taking the same free fall path as the people inside it. When you see film actors pretending to be astronauts, generally they haven’t been to space – they have been in a vomit comet a number of times (respect to them!).
It is possible to take all that in without really incorporating it into your world view. So here’s a reiteration: when you see ISS astronauts floating around, they still have 89 % as much gravitational force acting on them as they did before they launched! The floating thing is to do with free fall, not a lack of gravity.
Have humans achieved true weightlessness?
So how far up does gravity extend? Well, to set the scene, here’s a scale drawing of the Earth and moon…
If you can’t see the moon, you may need to rotate your phone! Now let’s add some values of Earth’s gravitational field strength to this picture.
Unless you are a moon-landing-hoax-conspiracy theorist, you will agree that a few human beings have traversed that whole diagram. And that, by the way, is the furthest human beings have been. You can see that about one-sixth of the way to the moon the value of g has dropped from 9.91 to 0.1 m/s2, which is a 99 % decrease. At this level of gravity, I think we could reasonably claim that astronauts were experiencing true weightlessness.
For real sticklers, who want a value of g closer to zero, an interesting thing occurs at the red cross labelled the ‘neutral point’. At this point, the gravitational forces on an astronaut from the Earth and the moon are equal in magnitude and opposite in direction. They cancel out and there is no force acting – true weightlessness! Astronauts wouldn’t notice – by this stage they have been very nearly truly weightless, and also apparently weightless (since they shut their rockets down) for days…
And by the way, as far as the moon landing hoax conspiracy theory is concerned, here is a detailed flight plan of the Apollo mission, from the Smithsonian Institute. It’s amazing. If you go to their site you can access a version that is zoomable, and you can read every detail of what happens in the mission. Quite frankly, if you are going to go to the trouble of making a flight plan as detailed as this, you might as well carry it out… What more evidence of the moon landings do you need? 🙂
