# how do orbits (eg satellite, planetary orbits) work? **URL:** \ **Category:** Factual Questions\ **Created:** [November 27, 2002, 12:41pm UTC](https://boards.straightdope.com/t/how-do-orbits-eg-satellite-planetary-orbits-work/139089 "2002-11-27T12:41:41Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![mooka](https://avatars.discourse-cdn.com/v4/letter/m/b5a626/32.png) [@mooka](https://boards.straightdope.com/u/mooka)\ **Post date:** [November 27, 2002, 12:41pm UTC](https://boards.straightdope.com/t/how-do-orbits-eg-satellite-planetary-orbits-work/139089/1 "2002-11-27T12:41:41Z")
From my A level physics i can remember that orbits are stable because gravity constantly accelerates objects in orbit downwards, and this exactly counteracts the fact that their sideways motion would normally whizz them off away from the object. But how do orbits start? When they launch satellites, do they need to give them a sideways push to get the orbit started? Surely if you just launched them up there and left them they should just fall back down again? But then if this is true, why are so many things in orbit? You have planets orbiting the sun, galaxies orbiting each other, which is strange because you’d think the most likely outcome would be for the two objects to simply collide. If the earth had no atmosphere, could you get something orbiting the earth say 100 metres up just by giving it enough sideways velocity? ( Assuming no mountains etc in the way) Sorry for all the questions - its just i got myself in a bit of a muddle thinking about it! ---
**Author:** ![grimpixie](https://avatars.discourse-cdn.com/v4/letter/g/ecb155/32.png) [@grimpixie](https://boards.straightdope.com/u/grimpixie)\ **Post date:** [November 27, 2002, 1:03pm UTC](https://boards.straightdope.com/t/how-do-orbits-eg-satellite-planetary-orbits-work/139089/2 "2002-11-27T13:03:03Z")
Remember that the earth is turning at the time when the object is launched, so it already has a west to east velocity of [just over 1000 miles per hour](http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970401c.html) at the equator - which is why rockets are launched from Florida rather than Alaska… As far as planets and what not are concerned - the solar system is believed to have been created from a condesing cloud of gas which must have had a small rotation to start with which resulted in a stable, orbiting planetary system. There are many theories - some given on [this page](http://www.oarval.org/section3_3.htm) - apologies for the colouring!! > [@](#): > > We do believe, however, that we understand the overall mechanism, which is that the Sun and the planets formed from the contraction of part of a gas/dust cloud under its own gravitational pull, and that the small net rotation of the cloud was responsible for the formation of a disk around the central condensation. Doubtless orbit-less systems also arrive, but they don’t stick around for long enough… According to [this site](http://liftoff.msfc.nasa.gov/academy/rocket_sci/orbmech/vel_calc.html) an orbit of 100m (less than 185km) is not stable. For a orbit of 200km a speed of 7.78 km/sec is needed. Grim ---
**Author:** ![G.Cornelius](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@G.Cornelius](https://boards.straightdope.com/u/G.Cornelius)\ **Post date:** [November 27, 2002, 1:11pm UTC](https://boards.straightdope.com/t/how-do-orbits-eg-satellite-planetary-orbits-work/139089/3 "2002-11-27T13:11:57Z")
Take the example of firing a cannon from somewhere on the surface of the earth. The cannon ball is instantly given a velocity - that is a speed and a direction. In this situation, the ball will always take a path that is an elipse and one of three things will happen: 1. The ball will be going so fast that it will go off into space and never come back (the elipse is infinitly large). 2. The ball’s velocity will be changed by an event such as hitting the surface of the earth (the elipse intersects the surface of the earth). 3. The ball will traverse the complete elipse and hit the back of the cannon. You may have heard that the cannon ball tajectory will be a parabola - near the earth’s surface that is a very good approximation of the actual eliptical trajectory - it does not take into account the fact that gravity gets weaker as the ball gets further from the centre of the earth. So, yes, to put something into a proper orbit is does need a sideways ‘push’. Sideways velocity alone can in principle give you an orbit, but the orbit will always have a closest point that is the same as your launch height. ---
**Author:** ![coffeecat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/coffeecat/32/3405_2.png) [@coffeecat](https://boards.straightdope.com/u/coffeecat)\ **Post date:** [November 27, 2002, 2:06pm UTC](https://boards.straightdope.com/t/how-do-orbits-eg-satellite-planetary-orbits-work/139089/4 "2002-11-27T14:06:42Z")
> [@](#): > > \*Originally posted by grimpixie \* > \*\* > According to [this site](http://liftoff.msfc.nasa.gov/academy/rocket_sci/orbmech/vel_calc.html) an orbit of 100m (less than 185km) is not stable. > Grim \*\* The NASA site says that for earth orbit. I believe that is because of atmospheric drag. If the earth had no atmosphere, you could indeed get a 100 meter orbit. ---
**Author:** ![mooka](https://avatars.discourse-cdn.com/v4/letter/m/b5a626/32.png) [@mooka](https://boards.straightdope.com/u/mooka)\ **Post date:** [November 27, 2002, 2:20pm UTC](https://boards.straightdope.com/t/how-do-orbits-eg-satellite-planetary-orbits-work/139089/5 "2002-11-27T14:20:15Z")
Thanks for the replies pixie and cornelius - thats made it a lot clearer for me! I liked that second link you posted pixie, despite those awful webpage colours, especially the following quote: " The earliest accounts of how the Sun, the Earth and the rest of the Solar System were formed are to be found in early myths, legends and religious texts. None of these can be considered a serious scientific account. " Brought up slightly surreal images of astronomers sitting around seriously discussing the " god coughed up the universe as phlem " theory etc! Anyway your answers did bring up one more question: Why aren’t orbits around earth of less than 185km stable? Cornelius said that a cannon ball fired from a cannon always forms an ellipse, which may or may not be complete. If the cannon is fired from just above the surface of the earth (again assuming no air resistance) with the right velocity then it would follow an ellipse around the earth. If this is possible, why isn’t a circular orbit at less than 185km? ---
**Author:** ![mooka](https://avatars.discourse-cdn.com/v4/letter/m/b5a626/32.png) [@mooka](https://boards.straightdope.com/u/mooka)\ **Post date:** [November 27, 2002, 2:24pm UTC](https://boards.straightdope.com/t/how-do-orbits-eg-satellite-planetary-orbits-work/139089/6 "2002-11-27T14:24:04Z")
Oops - looks like coffeecat answered the question i posted before i even posted it! Thanks for the reply coffeecat, you’ve made it clearer now… Its strange to think of something being only 100m up in orbit! ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [November 27, 2002, 2:32pm UTC](https://boards.straightdope.com/t/how-do-orbits-eg-satellite-planetary-orbits-work/139089/7 "2002-11-27T14:32:31Z")
> [@](#): > > \*Originally posted by grimpixie \* > \*\*the solar system is believed to have been created from a condesing cloud of gas which must have had a small rotation to start with which resulted in a stable, orbiting planetary system. \*\* The same is true for binary (or multiple) stars, star clusters and galaxies. All formed from a cloud of matter which already had initial motion, enough to keep them from collapsing. Or you could say that only the matter which had enough speed for a stable orbit has remained, and the rest have “fallen” to the center. ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [November 27, 2002, 3:45pm UTC](https://boards.straightdope.com/t/how-do-orbits-eg-satellite-planetary-orbits-work/139089/8 "2002-11-27T15:45:46Z")
> [@](#): > > _mooka wrote:_ > you’d think the most likely outcome would be for the two objects to simply collide. Actually, it’s not that easy to get to things to collide. Let’s say you happened upon a planet in the middle of nowhere, and you wanted to it it with a baseball (from an appreciable distance). Figuring out exactly where to throw it would be pretty difficult, what with your relative motion and everything. And if you missed, the baseball would be in orbit. Probably an elliptical orbit. And if you weren’t even _trying_ to hit it, you can see that orbit is much more likely than collision. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 27, 2002, 10:14pm UTC](https://boards.straightdope.com/t/how-do-orbits-eg-satellite-planetary-orbits-work/139089/9 "2002-11-27T22:14:57Z")
It’s hard to get sattelites into orbit around the Earth, because they start off on the Earth. If you have something that starts off elsewhere in space, it’ll almost certainly end up in orbit. The orbit will probably not be circular, at first, but it’ll be an orbit. Trying to hit a target from a distance is about as difficult if the object has gravity as it is without. Since most things in space make for pretty small targets, most things don’t collide. ---
**Author:** ![SandWriter](https://avatars.discourse-cdn.com/v4/letter/s/8491ac/32.png) [@SandWriter](https://boards.straightdope.com/u/SandWriter)\ **Post date:** [November 28, 2002, 4:37pm UTC](https://boards.straightdope.com/t/how-do-orbits-eg-satellite-planetary-orbits-work/139089/10 "2002-11-28T16:37:35Z")
If things don’t collide, then what is the hysteria over earth killing rocks hitting the planet and wiping out life as we know it. Doesn’t the moon and the earth have lots of evidence of things colliding? What’s the straight dope? ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [November 28, 2002, 4:51pm UTC](https://boards.straightdope.com/t/how-do-orbits-eg-satellite-planetary-orbits-work/139089/11 "2002-11-28T16:51:22Z")
Given enough chances, even [unlikely things](http://home.epix.net/~rplr/99GettysPerspective-5.htm) are bound to happen. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [November 28, 2002, 4:53pm UTC](https://boards.straightdope.com/t/how-do-orbits-eg-satellite-planetary-orbits-work/139089/12 "2002-11-28T16:53:14Z")
**Chronos** said _most_ things don’t collide, which is correct. The problem is twofold: First, there are many, many, many things in space, so even if only a tiny percentage of them collide, that’s a lot if any collision of large objects is dangerous. Second, the Earth’s gravity affects orbits (perturbs them, to be exact) so that objects become more likely to intersect Earth’s orbit, resulting in a collision.