By Marcus Wilson 30/11/2009

Going back to my comments on the Karman line (100 km about the earth’s surface), I think it’s worth commenting a bit ‘being in orbit’ means. We are familiar with the fact that if we drop something it accelerates downwards and hits the ground. If we throw something away from us, it will still accelerate downwards and hit the ground, but this time at some distance from us. If we throw it hard enough (and I mean, really hard), it will accelerate downwards, but, because the earth curves, by the time it has fallen the earth has dropped away too, so that it is still the same distance above the earth. It’s then in orbit, and will come back and hit you on the head (obstructions and atmosphere being absent).

Put more precisely, a circular orbit results when the acceleration due to gravity matches the centripetal acceleration required to move the object in a circle.

Orbits don’t have to be circular. Kepler worked out that the planets orbit the sun in ellipses, with the sun at one focus. Newton worked out that, if the sun exerted a force on a planet inversely proportional to the distance between the sun and the planet squared, then an elliptical orbit would necessarily result. (Well, actually the orbit could be circular, elliptical, parabolic or hyperbolic – these are all conic sections).  

One curious result of orbit theory is that, the closer a particle is to the focus of its orbit (e.g. the closer a planet is to the sun; Mercury as opposed to Neptune – alas, Pluto no longer counts) the faster it goes. The same applies to satellites orbiting the earth as well. Satellites that are in lower orbit move faster.  These satellites have to contend with more atmosphere as well (remember the edge of space is not distinct, there are still a few air molecules up there). When a satellite hits an air molecule, it loses energy. It drops to a lower orbit (less potential energy) and so it speeds up. We don’t normally think of something that is gaining speed as losing energy, but, in the case of a satellite, that is the case.

To counteract air resistance, satellites have to be given little boosts of energy to keep them in orbit. Without it, they will progressively spiral inwards, until they burn up in the earth’s atmosphere, or, sometimes, hit the earth itself.

0 Responses to “Orbits”

  • I got a question..
    Why when the earth just spins around one way, why is it that satellites can orbit both ways around the earth ?

  • But do they? I doubt there are too many going the “wrong” way( Israel does it but only because if it doesn’t make it, it will drop into the Med) . The fact the earth is travelling at about 1600kph at the equator means that if you can launch closer to the equator you get a tremendous boost and saving in fuel. Taking an awful big rocket to 1600kph is no mean feat. That is why the French use Guyana for there Arianne launches. Cape Canaveral is pretty close to being as close to the equator as the mainland USA can get. For polar orbiting satellites (Landsat, orbiting weather satellites) they get launched on the west coast of California for the exact opposite. They launch to the SW so that mistakes can fall into the Pacific. To go around the poles it is a waste of energy to slow the satellite down to get it into orbit if it is launched from the equator. But there is still a cost just to get it into orbit.

    I would be pretty sure the only satellites that go the “wrong way” might be earth observation and weather polar orbiters on the “front side” of their orbit. eg, on the day they (usually) travel north to south and east to west but as they go south to north (on the way up if you like) they travel west to east – the “right way”.

    There is a whole host of reasons for picking orbits. What is wrong for one is right for the other.

    But the question is a wonderful thought trigger!!!!

  • I quite often watch sky but it is not on the tube. From my angle of the milky way, I quite often see satellites most head from eastish to westish across the sky. But also see them heading the other way in terms of frequency maybe maybe 2 out of ten, they also travel other directions but mostly east to west.
    I might start a journal on good observation nights, would help to get a better idea.
    Lately I have seen what I think is a satellite on a lower orbit, a lot brighter than all the others. Anyone know anything about it ?

  • I had fun recently writing up a simple JavaScript simulation of gravity that can be used (to some extent) to see this change in action. It’s far from perfect and certainly not polished, but I have found it roughly accurate as far as my knowledge of gravity and orbits goes (which was mostly learned from Kerbal Space Program).

    Here’s a link if anyone would like to play around with it: http://codepen.io/Cipscis/full/eiowl

  • Derek/electrickiwi, your comment doesn’t seem to have anything to do with this 1 1/2 year old post. While I agree that the latest information coming from the New Horizons spacecraft is interesting, this behaviour is rude and inappropriate. However, it would be perfectly appropriate for you to share things like this on a blog of your own. Might I suggest you do that instead?

    The comments sections are areas for people to discuss the post, they are not your personal platform to share irrelevant videos. It’s particularly rude of you to post this exact same comment on two separate posts, neither of which is relevant.

  • Mark is right. I’m sure I’ve suggested to you before that you should set up your own blog rather than use someone else’s. It’s not difficult – WordPress do a good job of guiding you through it.