Why is there no gavity in space?

Question

All the planets are held in place by gravity...
how can then..........?

Answer 1

There IS gravity in space --- you simply CAN'T get away from it! Out in deepest space it may be weak; but, if there's one strongest source of gravity nearby, other objects may orbit the dominant one. That's the case with the solar system (the planets orbiting the Sun), or with individual planets and their own sets of satellites or "moons."

The mistaken idea that "there is no gravity in space" may come about because you sometimes hear that astronauts are "weightless" in space. Notice that I carefully wrote "weightless" and not just weightless. There's a VERY IMPORTANT REASON, which I'll explain below:

The terms "weightless" or "weightlessness," often applied to orbiting objects such as astronauts, ought always to be expressed with inverted commas or quotation marks (" ... ") around them when writing them down, as I've just done. (The use of quotation marks like that traditionally means "Watch out! This isn't really true. Think what this means whenever you use it.") The reason is that being "weightless" is a PHYSIOLOGICAL SENSATION or APPARENT EXPERIENCE, even though one STILL HAS WEIGHT! It doesn't mean that you are truly without weight, rather only that under some given circumstances you FEEL that you don't weigh anything.

Why is this? For simplicity, assume a circular orbit, but first:.

Consider an easily demonstrated Earth-based analogy. Every object moving uniformly in a circle must have a FORCE causing it to deviate from straight-line motion. (That force causes an ACCELERATION towards the centre of the circle.) An object tied onto the end of a string will move in a circle if you whirl it around your hand or head, but ONLY because the string provides the tension to do that. (Cut the string, and the object takes off, initially along the straight line tangent to the circle at the moment the string is cut.)

An orbiting object is doing the same thing, with gravity replacing the string. In that sense, then, an orbiting object is always accelerating or "falling" towards some central attracting thing, but never reaching it! And that's as true for the planets orbiting the Sun as it is for the Moon, or for astronauts and their spaceship orbiting the Earth.

If that seems mind-blowing, take comfort in the fact that even the great Isaac Newton didn't really understand that until his detested rival Robert Hooke pointed it out in a letter to him! (For centuries, Hooke did not receive the credit for providing Newton with the key he needed to unlock gravity's secrets.) After learning of this very insightful viewpoint, Newton came up with the following thought experiment:

Mount a cannon on top of the highest possible mountain (imagined above the atmosphere), and fire cannonballs out horizontally. With a low initial velocity, the cannonball's trajectory will just curve over quite soon, because of Earth's gravity, and the ball itself will plop down without going very far. Fire a cannonball faster, it will go further, curving over more gently. Fire it very, very fast, and it will get away from the Earth completely, curving only extremely gently initially, but then ultimately tending to a straight line motion far away from the Earth.

Newton then realized that for a particular speed (now known as the CIRCULAR ORBITAL VELOCITY), the initial curving of the cannonball's path would JUST MATCH THE CURVING AWAY OF THE EARTH'S SURFACE BENEATH IT, in such a way that the cannonball would remain forever moving forward, BUT ALWAYS AT ITS ORIGINAL HEIGHT ABOVE THE EARTH'S SURFACE.

The cannonball would then have achieved EARTH ORBIT !

To understand orbital motion, it's very important to appreciate this concept of "perpetual falling," with the "sideways" or "forward" motion always keeping you from reaching the point of attraction. The effect of gravity, in other words, the TRUE, LOCAL WEIGHT THAT OBJECTS DEFINITELY POSSESS AT ANY POINT IN SPACE, is solely responsible for orbital motion, when it's coupled with the appropriate value of "sideways" or "forward" motion.

So:

What's with this "WEIGHTLESSNESS" thing, then ?? !!!

O.K.; back to "weightlessness" as a sensation or experience:

Consider another Earth-based analogy, though I don't recommend that you actually try this one! Get in a frictionless elevator. Stand there while it's stationary, holding a heavy object out in front of you. You'll experience your normal sensation of weight, and there'll be a tension in your muscles from holding your arm with the weight straight out. Now ask your worst enemy to cut the cable holding the elevator up. The elevator cabin will fall downwards, with acceleration 'g'. So will you, and the object you were previously holding, even when you let go of it! But since both you and the elevator are falling at exactly the same rate, there'll now be no force beween your feet and the floor. The heavy object will also fall at the same rate, but because everything is doing that --- you, the elevator cabin, the object --- there'll be NO relative motion, and both YOU and the HEAVY OBJECT will seem to be "weightless" inside the falling elevator cabin. That heavy object, which certainly didn't cease to be attracted to the earth just because the cable was cut, will nevertheless appear to be hovering there in front of you, AS THOUGH IT WEIGHED NOTHING. And there you have it: (apparent) WEIGHTLESSNESS !

There is a VERY IMPORTANT LESSON in all this. It is this: our Earthly sensation of possessing "Weight" really comes from the REACTION EXPERIENCED BY US FROM OUR SURROUNDINGS. When you stand on your bathroom scales, those scales register the REACTION the scales themselves provide to prevent you from falling though the floor and down to Earth's centre! (They CAN'T register YOUR experience, but rather only whatever the strong springs etc. inside the scales are doing to RESIST your actual weight, the strength with which gravity is attempting to pull you downwards.) Similarly, your body registers that resistance of the scales not by the scales' own internal workings, rather only by the (upward!) force that YOU feel on the lower surfaces (the soles) of your feet.

These experienced mutual forces produced by your weight, felt by the scales, and in turn felt by you on the soles of your feet are all examples of Newton's Third Law of Motion: Action and Reaction are EQUAL and OPPOSITE. Your downward weight is Mg; the scales' upward reaction is Mg; the upward force on your feet is that same Mg. This means that you DON'T experience gravity as a DOWNWARD force when you're standing stationary on the ground! Your brain REGISTERS the REACTION or UPWARD force produced by the ground, but only because there's an UPWARD force on your feet! You've probably always been told that gravity pulls you down towards the Earth and is responsibe for you having weight (both of which are true statements). Nevertheles, Your registering that experience is a much more complicated affair. You spend far more of your time on your feet than you do falling, and so, strange as it may seem, whether you're just standing around, sitting still, or even lying there in bed, it's because of an UPWARD resistive force that you can say "I KNOW there must be a source of DOWNWARD gravity around here somewhere!"

(Actually, as Einstein realized with his famous EQUIVALENCE PRINCIPLE, you could nevertheless stand inside an elevator cabin in empty space, ideally FREE FROM ALL GRAVITATIONAL ATTRACTION, and yet still EXPERIENCE EXACTLY THE SAME "GRAVITATIONAL EFFECTS" !! All it would take would be to have some little imp or deus ex machina ACCELERATE that elevator cabin in some direction with acceleration of magnitude 'g'. The direction of the acceleration would define "upwards," while the opposite direction would be "downwards." An apple that had previously been floating in front of you, "staying put" in space, would still "stay put" in that original space itself. However, because of the newly induced "upward acceleration" of the elevator's floor, it would seem to you, now experiencing an "upward" force of Mg on your feet, that the apple would be "falling" towards the floor with acceleratin of 'g' "downwards." This would mean there was some kind of equivalence between gravity and pure acceleration. At first Einstein thought of this equivalence --- of gravity's effects on Earth, and those due to the same strength of acceleration in empty space --- as only applying to dynamical phenomena. [This is now called the WEAK Equivalence Principle.] But he soon extended the indistinguishability of these effects to ALL physical phenomena --- the very powerful STRONG Equivalence Principle. THAT can be made the basis of a derivation of GENERAL RELATIVITY, in all its full complexity.)

Returning to our relatively simple sensation of weight, before that last paragraph: If this idea is new to you, it may take some getting used to, but believe me, this is the right way to think about it. And when the elevator cable is cut, that's when there's no longer any reaction from your surroundings, so you then feel "weightless."

The same thing happens when in orbit in "truly empty" space, or indeed during whatever you're doing provided that your spaceship isn't using any propellent so that you are technically in "free fall." Both you and your space ship are then accelerating at the same rate, whatever it might be, and there's therefore no reaction between you. And that's WHEN and WHY you FEEL "weightless," under "free fall" conditions in space.

I hope that this rather complete explanation has helped you understand the basic ideas behind what is experienced insidea space ship out in space. It is truly a mind-blowing set of concepts, but perhaps it will open your mind to wonderful new vistas, as indeed it did for me when I first encountered it. Good luck!

Live long and prosper.

POSTSCRIPT Note that, despite what others have written:

1. Your weight isn't "taken away" in space --- only your SENSATION OF "WEIGHT" is taken away (that's very different); and

2. It DOESN'T "take power" to REMAIN IN ORBIT. Orbits involving "free fall" need ABSOLUTELY NO POWER to be provided by the spaceship. Power IS used to move to a DIFFERENT orbit, to combat the frictional effects of the vestigial atmosphere, or even to DE-ORBIT, but provided frictional effects are negligible, one can "coast" in orbit essentially indefinitely.

Many misconceptions are bandied about in discussions of these topics, as you can amply see above in some of the other answers ! (A few have the essence right, I'm glad to say.)

Answer 2

There is gravity in space. The astronauts are weightless due to their orbital motion, not due to the lack of gravity. Just like you are weightless during free fall, for example on a free fall ride at an amusement park. It is the motion of the fall that takes away your weight. An orbit is basically one long continuous fall that keeps missing the ground due to the high speed sideways motion and the curvature of the Earth.

Answer 3

There IS gravity in space. Just not Earth's gravity. You will be drawn towards any nearby object of sufficient mass. If you're in orbit around the Earth, you're basically continually 'falling' into the Earth, but around it instead, if that makes sense - so there doesn't appear to be gravity - but that's what's keeping you in your falling orbit.

Answer 4

Oh, it's there, it is just very weak. According to the Universal Gravitational Constant, the force of gravity is related to the product of the masses involved (along with a constant) divided by the square of the distance between them. Theoretically, every particle in the universe is gravitationally attracted to every other particle in the universe, although most attractions are so minute they can't be detected.

Answer 5

The planets are held in place by the gravity of the sun, and all celestial objects have gravity, even asteroids and comets.

Answer 6

There is gravity in space, including earth's. Earth's gravity keeps you in orbit. While in orbit you are constantly falling so you don't feel your weight. That is why outer space is weightless not gravity less.

Answer 7

yea there defintiely is gravity, in the space station or whatever is freee falling toward earth and they need to use power to stay in place, but thats why you seem "weightless" cause youre falling so you can float

Answer 8

http://en.wikipedia.org/wiki/Gravity :)