6. "There's no Gravity in Space"
This is one of the commonest misconceptions among otherwise intelligent people. Almost everyone knows that astronauts experience “weightlessness”; today, we see TV broadcasts from the International Space Station, showing the crew effortlessly “floating” along its corridors, and those of my generation and older will remember seeing broadcasts from the Apollo spacecraft on their way to the Moon. But a great many people don’t understand why, and believe that astronauts are weightless, “because there’s no gravity in space”.
Well, if there is “no gravity in space”, then what do they imagine holds the Moon in its orbit around the Earth, and the Earth in its orbit around the Sun? Nothing could be further from the truth! Gravity affects everything in the Universe; in fact, it’s what holds the entire Universe together!
While NASA itself uses such terms as “weightlessness” and “zero-gravity”, these terms are in fact extremely misleading; astronauts in orbit are very much under the influence of gravity, and have nearly as much weight as they do on the surface of the Earth. Understanding why they appear to be “weightless” requires just a little elementary physics, which I’ll explain in this essay.
6.1. “The cruellest hoax”
In recent years, there has been a disturbing worldwide trend for TV programmes in which members of the public are ridiculed and humiliated for other people’s “entertainment”. This trend plumbed entire new depths in 2005, with a series on British TV entitled Space Cadets. This was described by critics as “the cruellest hoax in the history of television”; frankly, those who were stupid enough to fall for it and take part deserved all they got! I didn’t waste my time watching it, but couldn’t avoid hearing about it.
The idea was that two “winners”, from a group of ten contestants, would supposedly be sent into space aboard “the Russian space shuttle”. Ahem. Apart from the sheer absurdity of the notion that anyone could be sent into space for a TV show, anyone with any slightest knowledge of spaceflight knows that no such spacecraft exists! In the late 1980s, the Russians did build an experimental space shuttle; it flew a single and successful unmanned test flight, but the programme was then cancelled after the collapse of the Soviet Union. It never flew carrying humans. To this day, Russia ferries crews to and from the International Space Station using Soyuz spacecraft, versions of which have been in use since 1967!
The viewers were told all along what was really going on; only the contestants didn’t know. Two of the “contestants” were in fact actors, whose job was to pretend to believe that everything was real, and convince the others if they had doubts. The contestants were, they believed, taken to a training base in Russia – which was in fact a disused airfield in England; the plane which supposedly took them to Russia had simply flown in circles over the North Sea for a few hours. There, they were subjected to a programme of “training” for their “mission”; while one was voted off each week, Big Brother style.
When the time came, the two “lucky winners” actually believed that they were going into space. They were taken aboard a mock-up of the mythical “Russian space shuttle”, whose “launch” was simulated with a lot of noise and vibration. ( How did they imagine it took off – were they really so stupid as to not even realise that to launch anything into space requires a rocket? Had they never even seen the launch of a real space shuttle on TV? ) For a couple of days, they “performed experiments”, apparently genuinely believing that they were in space! Only when they “landed” did they learn the humiliating truth, that they had never left the ground.
So now we come to a rather obvious question. How was it possible for anyone, however gullible, to believe that they were in space, when they were still on the ground? Specifically, how could they believe that they were in space, when they were not weightless? The answer, believe it or not, is that they were told that they were “not going high enough to be weightless”!!!!!
Think about that. The “success” of the whole enterprise – getting the contestants to fall for it – depended on them not having any remotest comprehension of what “weightlessness”, or being in orbit, actually means! It all depended on them believing the “there’s no gravity in space” misconception – either believing that gravity somehow magically “disappears” at some particular height above the Earth, or more likely, that it becomes rapidly weaker with height, such that it drops off to zero by the height of the International Space Station.
In fact, such misunderstanding was the primary selection criterion for the contestants. All applicants were initially given a test of knowledge – but unknown to them, those with the worst scores were selected to take part! Anyone who demonstrated any slightest knowledge of spaceflight, or any slightest comprehension of the most basic physics, was rejected.
Of course, gravity does become weaker with distance from the Earth – but nowhere near as rapidly as many people apparently think. In fact, astronauts aboard the ISS still have 89% of the weight which they have on the ground! To understand why they don’t feel it, we need to consider the basic physics of motion. Don’t worry; it’s nothing more difficult than a couple of very simple equations!
6.2. “Weightlessness” on Earth
As I said earlier, the term “weightless” is inaccurate and misleading; that’s why I’m using it in quotes throughout this essay. It’s far more accurate to say that astronauts in orbit are in free fall. There are a couple of ways in which it’s possible to experience free fall on Earth – though the first is definitely not desirable.
Imagine you’re in a lift ( elevator, for American readers ) near the top of a tall skyscraper, and the cables break, and the lift falls freely down the shaft. For those last few seconds of your life, you would experience “weightlessness”, just as astronauts do! I’ll explain exactly why in Section 6.5.
Another way to experience free fall - and live to tell the tale – is aboard an aircraft which makes a steep dive. This is exactly how NASA trains astronauts to cope with “weightlessness”, before they go into space for the first time. They are flown aboard a modified aircraft, which has a large empty hold space, whose walls, floor and ceiling are padded to prevent injury. On each flight, the plane alternately climbs steeply, then goes into a steep dive; during the latter, the trainees are “weightless” for about half a minute. This is done several times in succession. As you might imagine, this is not for the weak of stomach – not for nothing is the plane nicknamed the Vomit Comet!
In the excellent film Apollo 13, most of the scenes inside the spacecraft have the astronauts strapped into their seats – but there are several scenes in which they are “weightless”, and float through the hatch between Command Module and Lunar Module. This was not done by camera trickery; it was done for real! A replica of the spacecraft set was built in the hold of the Vomit Comet; in those scenes, the actors really were in free fall. ( If you watch the film closely, and know what you’re looking for, you’ll see that none of the “weightless” scenes last longer than about 30 seconds – the longest they could shoot in one go, during each of the plane’s dives. )
To understand free fall, we need to consider a little elementary physics.
6.3. Velocity, acceleration and force
In everyday language, the words “speed” and “velocity” are often used interchangeably, and assumed to mean the same thing – but in physics, they have distinctly different meanings. Speed is a scalar quantity, meaning it has only size; velocity is a vector quantity, meaning it has both size and direction. Speed is simply the rate of motion, measured in metres per second ( denoted by ms-1 ), or in kilometres or miles per hour. Velocity is the rate of motion in a particular direction.
Acceleration is the rate of change of velocity, measured in “metres per second squared”, or “metres per second per second” ( denoted by ms-2 ); if we say an object has an acceleration of 10 ms-2, that means its velocity increases by 10 metres per second, every second.
Again, in everyday language – especially that of driving – “acceleration” is normally taken to mean an increase of speed. In fact, it means a change of velocity, either up or down; a decrease of velocity is simply a negative acceleration. Note again the difference between speed and velocity; acceleration is a change of velocity, not necessarily of speed – it can mean a change of speed or direction, or both. When a car goes around a bend without changing its speed, it is in fact accelerating – though you wouldn’t think of it as such when driving – because it’s changing its direction of motion.
To cause an object to accelerate requires a force to be applied to it. If the force acts in the direction of the object’s motion, or the opposite direction, then it will change its speed, but not its direction; if the force acts at an angle to the direction of motion, then the direction will also change. In the example of a car rounding a bend, turning its front wheels at an angle to its direction of motion creates a sideways friction between the tyres and the road; this is the force which causes it to change direction.
6.4. Newton’s Three Laws
The simple physics of velocity, acceleration and force, as described above, is summarised by Sir Isaac Newton’s famous Three Laws of Motion. These are actually quite simple, though it took the genius of Newton to formulate them. So here they are.
First Law: “A body will remain at rest, or move with a constant velocity in a straight line, until that state is changed by the action of a force on the body.”
Second Law: “The rate of change of linear momentum is proportional to the applied force, and occurs in the same direction as that of the force.”
Third Law: “For every action, there is an equal and opposite reaction.”
A body’s linear momentum is its mass multiplied by its velocity, so in the Second Law, the rate of change of momentum is the rate of change of ( mass x velocity ). But the rate of change of velocity is acceleration – so we can say that the force applied to a body is equal to its mass times its acceleration. This is stated mathematically as
F = ma
where F is the force, m the body’s mass and a the acceleration it experiences.
The significance of the Third Law will become clear a little later.
6.5. Mass, weight and gravity
Next, we need to consider Sir Isaac’s most famous law, the Law of Gravity – or to give it its correct name, the Law of Universal Gravitation. This states that:
“Every object attracts every other object, with a force which is directly proportional to the product of their masses, and inversely proportional to the square of their distance apart.”
It can be expressed mathematically as follows. The force F, mutually exerted between two bodies of masses m1 and m2, separated by a distance r, is given by
F = G m1 m2 / r2
where G is a constant, called the Universal Constant of Gravitation. If the masses are expressed in kilograms, the distance in metres and the force in newtons, the value of G is 6.67 x 10-11 Nm2kg-2.
The r2 on the bottom of the equation shows us that gravity obeys the inverse square law. If the distance apart is doubled, the force is reduced by a factor of four; if the distance is multiplied by ten, the force is reduced by a factor of 100, and so on.
The fact that G is such a tiny number shows us that gravity is actually a very weak force! You might not think so if you fell off a cliff – but that’s because the Earth is an extremely big mass. Think about it – when you pick up a nail with a tiny magnet, the attraction of that magnet is overcoming the gravitational attraction of the entire Earth!
Now let’s consider gravity on or near the surface of the Earth. In the physics of motion, any object can be considered as a point mass located at its centre of mass, or centre of gravity. So for an object on the surface of the Earth, to calculate the force between it and the Earth, we can say that the “distance apart” in the above equation is the radius of the Earth. If we call that radius R, and the mass of the Earth M, then the force exerted on a body of mass m is
F = GMm / R2
But remember that force is equal to mass times acceleration; so if we divide this by m, we see that the body experiences an acceleration of
g = GM / R2
This is what we call the acceleration due to gravity, denoted by g. The mass of the Earth is 5.98 x 1024 kg ( 1024 means a one followed by 24 zeroes ), and its radius is 6380 km, or 6380000 metres. So the value of g at its surface is
g = ( 6.67 x 10-11 ) x ( 5.98 x 1024 ) / 63800002
If you do the maths, this comes to 9.8 ms-2. So near the surface of the Earth, any object which falls under gravity accelerates at a rate of 9.8 ms-2 – that is, its velocity increases by 9.8 ms-1 every second.
Every falling object, regardless of its mass, falls at the same rate, with that same acceleration. Galileo first realised that in the 17th Century; legend has it that he demonstrated it by dropping objects from the top of the Leaning Tower of Pisa. It doesn’t always appear to be the case; very lightweight objects, such as a feather, fall much more slowly, but only because they are carried by air currents. In a vacuum, a feather would fall at the same rate as a stone or a hammer. In 1971, astronaut Dave Scott demonstrated this by dropping a hammer and a feather together on the surface of the Moon.
In everyday language, the words “weight” and “mass” are often used interchangeably – but again, in physics, they are not the same thing. The mass of any object is always the same, whether it’s on Earth, on the Moon or in a spacecraft in orbit. But its weight depends on where it is; an object’s weight is the gravitational force acting on it – more correctly, the mutual gravitational force between it and the planet on which it’s standing – which is equal to its mass multiplied by the acceleration due to gravity. So an astronaut on the Moon has the same mass as he does on Earth, but one sixth of the weight.
In everyday language, we usually express weight in the same units as mass, such as pounds or kilograms – but in physics, this isn’t correct. As stated above, weight is in fact a force, so it’s measured in the units of force, newtons. A mass m, at the surface of the Earth, has a weight of m times g, so a mass of one kilogram has a weight of 9.8 newtons. It’s also valid to say it has a weight of “one kilogram force”.
Obviously, the value of g does decrease with distance from the Earth – but nowhere near as rapidly as those Space Cadets contestants thought! For a spacecraft at a height of h above the ground, its distance from the centre of the Earth is ( R + h ), so we can substitute that for R in the above equation, and we see that
g = GM / ( R + h )2
The International Space Station ( ISS ) orbits at a height of about 400 km, or 400000 m. So at that height, the value of g is
g = ( 6.67 x 10-11 ) x ( 5.98 x 1024 ) / ( 6380000 + 400000 )2 = 8.68 ms-1
This is 89% of the value of g on the surface – so we see that the astronauts aboard the ISS still have 89% of the weight which they have on the ground!
When a spacecraft takes off, it obviously needs to accelerate at a rate greater than g. The Saturn V rocket, which launched the Apollo spacecraft, accelerated at a rate of about four times g, and the Space Shuttle at about three times g. So during their ascent, shuttle crews had three times their normal weight; it’s said that they experienced an acceleration of “3G”, or “three gravities”. This is where the misleading term “zero-G”, or “zero-gravity” comes from – because when a spacecraft reaches orbit and its engines shut down, the crew go from feeling three times their normal weight to not feeling any. But that doesn’t mean they don’t have any! ( Fighter pilots and racing drivers also experience significant “G forces”. )
Now we return to the concept of free fall. To understand it, consider Newton’s Third Law – “For every action, there is an equal and opposite reaction.” On the Earth, as we have seen, your weight is the gravitational force between yourself and the Earth. That force is always trying to pull you towards the centre of the Earth; what prevents it doing so is the resistance of the ground or floor on which you’re standing. So what you feel as your weight is in fact the reaction of the ground to your weight!
Now think again about the example of the falling lift ( Fig. 7 ).
When the lift is operating normally, its floor exerts a reaction to your weight. But when the lift is falling freely, you are falling with it, at the same acceleration – so the floor no longer exerts any reaction. That’s why you feel “weightless”!
So finally, let’s see how all this relates to spaceflight.
6.6. Orbits and free fall
I once read a particularly stupid comment in an internet forum: “Of course there’s no gravity in space – that’s why the International Space Station doesn’t fall down!” Comments like that, together with the aforementioned TV series, show that many people don’t have the slightest comprehension of what being in orbit means. This is where everything I’ve said so far comes together.
Think of any spacecraft launch you have ever seen on TV. The rocket doesn’t just go straight up vertically, does it? It initially rises vertically, but once it gets to a height of a few kilometres, it tilts over to a shallow angle. That’s because in order to get into orbit, a spacecraft needs to reach a certain horizontal velocity, with respect to the ground. That velocity is about 28000 kilometres per hour.
If a rocket did simply ascend vertically, then as soon as its engines shut down, it would fall back to the ground; that’s exactly what sounding rockets do. In fact, the only reason that a spacecraft needs to ascend to a height of hundreds of kilometres is to get above the atmosphere. On the Moon, it would be possible to put a spacecraft into orbit at a height of only about 10 km – just high enough to clear the tops of the highest mountains! This obviously can’t happen on Earth, as any object travelling at 28000 km/h within the lower atmosphere would be vaporised by the friction of the air.
Imagine you throw a ball vertically upwards. It rises to a certain height, depending on how hard you throw it, then falls back, also vertically, to its starting point. In throwing it, you give it a certain speed in the vertical direction; it’s then slowed down by gravity until its speed falls to zero. Then it falls back, accelerating under gravity, until when it returns to your hand, it has the same speed with which it started, but in the opposite direction – that is, the opposite velocity. This is what happens with a sounding rocket – though its descent is slowed by a parachute.
If you throw the ball at an angle, instead of vertically, then it travels a certain distance horizontally before it falls to the ground. That’s because you have given it a certain speed in the horizontal direction, as well as in the vertical. We can say that its velocity has components in the horizontal and vertical directions.
The easiest way to visualise what going into orbit means is the way Sir Isaac himself did, three centuries ago. He imagined a cannon on the top of a mountain, firing horizontally. The bigger the explosive charge, the greater the cannonball’s speed as it leaves the muzzle, as it’s accelerated by a greater force. And the greater its initial speed, the further it will travel before falling to the ground. As it travels horizontally, it also falls vertically under gravity; the time it takes to fall to the ground depends on the height of the mountain, and the distance it travels is that time multiplied by its horizontal speed.
Newton realised that if a cannonball could be fired at a high enough speed, then the point at which it fell to the ground would lie beyond the horizon – so it would, at least in theory, never fall to the ground, but would “miss” the curve of the Earth, and “fall” in a circle, all the way around the Earth! In reality, of course, it would be slowed down by air friction; in fact, as stated earlier, it’s impossible even to accelerate any object to the required speed close to the Earth’s surface, as the heat generated by friction would vaporise it. But on the Moon, or any body without an atmosphere, Newton’s thought experiment describes exactly what would happen.
So this is exactly what happens when a spacecraft is launched into orbit! The rocket carries it to a height at which it’s above most of the atmosphere, and the friction of the air becomes negligible. ( Of course, there is no “edge” to the atmosphere; it just becomes steadily less dense with increasing height. ) As it climbs into less dense layers of the atmosphere, it also accelerates until its horizontal velocity reaches 28000 km/h. It also changes its direction of flight until, on reaching the desired height of orbit, it becomes horizontal – that is, tangential to the ground. Then the engine shuts down, and the spacecraft is in orbit. Just as in Newton’s thought experiment, the spacecraft is, in effect, “falling” under gravity in a circle around the Earth!
So an astronaut inside the spacecraft feels “weightless”, for exactly the same reason as in my example of the falling lift, or the trainee astronauts aboard the Vomit Comet – because he is also falling under gravity, at the same rate as the spacecraft, and so its “floor” doesn’t exert any resistance to his weight.
To understand exactly what causes a spacecraft to “fall” in a circle, think back to Newton’s First and Second Laws. When a force acts on an object, it causes it to accelerate, or change its velocity. Remember that acceleration is a change of velocity; it can mean a change of speed, direction or both. So an object travelling in a circle, at constant speed, is accelerating, as it’s constantly changing its velocity.
If a force acts on an object in the direction of its motion, or the opposite direction, only its speed changes. If it acts at a right angle to the direction of motion, only the direction of motion changes. At any other angle, both its speed and direction change.
To make an object move in a circle requires a force which is always at a right angle to its direction of motion – that is, the force has to act in a direction towards the centre of the circle. This is called centripetal force. This can be demonstrated by a simple experiment; if you want to try this for real, then do it outdoors, and not near any windows! Attach a small weight to a length of string, and swing it around your head. The weight travels in a circle at constant speed ( Fig. 8 ). If you let go of the string, the weight will fly off in a straight line, in a direction tangential to the circle at the point of release.
In this case, the tension in the string provides the centripetal force, which causes the weight to constantly change its direction of motion. When you let go of the string, the force ceases to act on the weight, so it ceases to accelerate, in accordance with Newton’s First Law.
In the case of a car turning a corner, the centripetal force is provided by the sideways friction between the tyres and the road. And for a spacecraft in orbit, it’s provided by – what else? – gravity! Obviously, the gravitational force between the Earth and the spacecraft acts in the direction towards the centre of the Earth – so we have our centripetal force.
Note that the aforementioned velocity of 28000 km/h, known as orbital velocity, is that which a spacecraft must reach, in order to go into low Earth orbit, at a height of around 200 km. The further out a spacecraft orbits, the more slowly it moves – just as the planets move more slowly, the further they are from the Sun. To reach a higher orbit, a spacecraft has to accelerate against gravity to reach the height, then slow itself down to the required speed.
For any given radius of orbit, the time taken to go around the orbit is always the same; the bigger the radius, the longer the orbital period. It follows that there must be some particular orbital radius, at which a spacecraft takes 24 hours to complete an orbit – so if it orbits above the Equator, it appears to hover above the same point on the Earth’s surface. The advantage of such an orbit is obvious; it’s now occupied by thousands of communications satellites, including the one from which you receive your satellite TV signal. This is called the geosynchronous or geostationary orbit, also known as the Clarke orbit, as the science fiction writer Sir Arthur C. Clarke was the first person to realise that such an orbit must exist, and calculate its radius. ( He did so as early as 1945, 12 years before the first satellite was launched. )
Naturally, the orbital velocity of 28000 km/h only applies to the Earth. For any given planet or other body, it depends on the body’s mass and radius; for the Moon, it’s a mere 5300 km/h.
Note also that in low Earth orbit, there is still enough residual atmosphere to exert a significant drag on a spacecraft, which slows it down. At a height of 200 km, a spacecraft can only remain in orbit for about ten days, before atmospheric drag slows it down to below orbital velocity, and the resultant heat of friction will destroy it. This is called orbital decay; to prevent it, a spacecraft must fire its engines periodically to increase its velocity. The higher the orbit, the longer it takes to decay – but even the ISS, at a height of 400 km, has to boost its velocity once every few months to counteract drag. This is done by means of the attached Soyuz spacecraft firing its engine.
Finally, how does a manned spacecraft return from orbit? Well, it simply fires its engine in the opposite direction to its direction of motion, to slow itself down to below orbital velocity. As it falls into denser atmosphere, friction slows it down further; this generates intense heat, so the spacecraft has to be protected by a heat shield, and the angle of its re-entry has to be carefully controlled. Re-entry slows it right down to a few hundred km/h, allowing it to land by means of parachutes – or land like an aircraft, in the case of the now obsolete space shuttle.
6.7. The stupidity of Gravity ( the film, that is )
In 2012, after Felix Baumgartner performed his world record free fall parachute jump, I read another idiotic comment in an internet forum: “What would be really impressive would be someone doing it from the International Space Station!”
Er – what????? I hope you can now see the utter stupidity of that statement! Whoever wrote it clearly doesn’t share the “there’s no gravity in space” misconception – but he clearly doesn’t have a clue what being in orbit means either! What would actually happen, if someone “jumped” from the ISS? Answer – not a lot! As we have seen, the astronauts inside the ISS are moving with it – falling under gravity with its orbital velocity. So if someone “jumped” from the station, he would simply remain in the same orbit alongside it!
But that person certainly isn’t alone in his delusion. What really baffles me is how someone could make a “blockbuster” film, with a budget of hundreds of millions of dollars, set in space, without bothering to do even the most elementary research into the subject – and like that commenter, without having the slightest comprehension of the concept of being in orbit!
A couple of years ago, I wasted two hours of my life watching the film Gravity – thankfully only on TV; I didn’t waste good money to see it at the cinema. In my opinion, this is a leading contender – in the face of much strong competition – for the title of the most ludicrous load of utter drivel ever to come out of Hollywood! Of course, it’s one of those films in which the special effects are everything, and the story is incidental – but come on! They didn’t even make any effort to get the absolute basics right!
The entire film consists of one absurdity after another, but I’ll comment here on the most idiotic one of all. After their space shuttle is destroyed while repairing the Hubble Space Telescope, the two surviving astronauts, played by George Clooney and Sandra Bullock, fly from the HST to the ISS, using their jet packs. ( “It’s just over there!”, says George. ) They tether themselves together, to prevent themselves drifting apart.
This scenario is itself totally absurd. The HST and ISS are in completely different orbits, at different heights and with different velocities, and never even come within a couple of thousand kilometres of each other. It’s pretty common knowledge that the shuttle itself wasn’t capable of travelling from one to the other, as the required change of velocity would have taken more manoeuvring fuel than it could carry. This was why, after the 2003 Columbia disaster, NASA initially planned to abandon the HST and cancel any further servicing missions; it was decided that all future shuttle flights would be restricted to orbits which would enable them to reach the ISS, so they could use it as a refuge in case of any damage to the shuttle. But the film has George and Sandra doing with their jet packs what the shuttle itself couldn’t do… yeah, right!
When they reach the station, Sandra grabs hold of part of its structure, and comes to a stop – as does George, as they are tethered together. But now we come to the truly moronic part… Sandra is now holding onto the station, but George is “hanging” from her by the tether; she can’t support his weight, which is threatening to pull her away from the station. So George, in true “macho hero sacrifices his own life to save the girl” tradition, releases the tether, and falls away to his death.
Er – what????? “Falls away” to where, exactly????? Again, I hope you can now see why this is so stupidly, staggeringly wrong!
Think about it. Throughout the entire scenario, the two astronauts are in orbit. They are in free fall. While working outside their shuttle, they shared its orbital velocity; the tiny thrust of their jet packs would have simply added a tiny bit to the velocity they already had. When Sandra grabs hold of the ISS, they come to rest, with respect to the station. But with respect to the Earth, they are still travelling at 28000 km/h! They are already “falling” under gravity, together with the station, and together with each other.
So Sandra would not be “supporting George’s weight” at all, because neither of them would feel any weight! She would simply have to give a gentle tug on the tether to pull him towards her. And if George released the tether, he would not “fall away” anywhere; he would simply stay where he was alongside her, still sharing the same orbit. DUH!!!!!!!!!
So while the film is called Gravity, its makers clearly didn’t have the slightest comprehension of that very phenomenon!
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