h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The Edited Entry on man-made satellites A248591 says that low orbits are slow ones and in higher ones, the orbiting object is faster.

I thought it was the other way around. As you get further from the earth, the force of gravity is less, so you don't have to go as fast to stay in orbit.

Can somebody please confirm this, before I start fixing the Edited Entry.

You are correct

http://www.freemars.org/jeff/speed/index.htm

So the further out ones are travelling at a lower speed but covering a greater distance in the same amount of time? How does that work?

Just realised I am thinking of geostationary ones rather than just being in orbit.

Hi Gnomon

As I understand it a low earth orbit requires a faster orbital speed to overcome greater gravitational pull and a geostationary orbit requires less speed. If a body in orbit slows down that balance is disrupted and the orbit will decay (think Skylab and many other low earth orbit objects - including Hubble eventually). If however you want to move an object into a higher orbit you need to speed it up.

Orbital Mechanics is much more complicated than I can explain but I'll try an example. When the Space Shuttle launches it has to achieve escape velocity of 11.2km/s to get out of Earth's gravity well. Then it has to catch up with the ISS at an average height of 340.5km and a speed of 7.7km/s. In order to catch up the shuttle has to boost (accelerate) up to the height if the ISS and then catch it up. The 11.2km/s is upwards and the shuttle has to change direction to a circular orbit at the right height. It could boost up off the Earth to a greater height and in front of the ISS and then slow down to drop into the right orbit and then let the ISS catch it up but to do this it would use more fuel and have to drop below the ISS and then accelerate to get back to a point where they could dock.

There is a table at the bottom of this page ( http://en.wikipedia.org/wiki/Orbital_speed ) that gives the speeds for different types of Earth orbit.

Makes you realise why we need h2g2 Guide Entries.

t.

Kelli, you are assuming all satellites complete one orbit in the same amount of time. They don't.

As indicated by my second paragraph...

>If however you want to move an object into a higher orbit you need to speed it up.<

This is where it could get complicated. But I don't think that's really necessary for Gnomon's purposes.

I think it is more correct to say you need to give it more energy. Then you can start worrying about the angular velocity and such like...

I did say..."Orbital Mechanics is much more complicated than I can explain but..." and I meant it too.

I don't understand the science and maths and struggle to explain it although I can picture it in my head.

Maybe what I should have said was >If however you want to move an object into a higher orbit you need to accelerate it into a new position<. Trouble is that one then gets into vector thrust etc.

Where's Arnie when you need him?

t.

>>"Just realised I am thinking of geostationary ones rather than just being in orbit."<<

Geostationary satelites can only work at a particular distance from the Earth.

Yes - 35,786 km (22,240 statute miles) with zero inclination with respect to the equator.

t.

I've put this into Peer Review as an update: A37762158. It's the updated version of the Man-Made Satellites entry, which was completely wrong in its description of orbits.

If would be great if someone who understands orbits checked it out.

One sentence jumped out at me as being wrong in there and that's the one about diamond being the strongest known substance. That sadly is incorrect, diamond is the 'hardest' known substance which is not the same thing. Hardness is the resistance to being scratched by another substance.
Strength is a fairly ill defined term as it is roughly speaking, resistance to being broken or deformed, so balsa wood for example has low tensile strength but very good compressive strength, but hey, you're an engineer so I'll stop teaching you to suck eggs I guess.
Diamond, I would have thought, would be a rubbish substance to build the elevator out of as it is relatively easily shattered - which makes it brittle...

Good point. I hadn't really gone into the rest of the entry in any great detail.

I was unaware of this entry when I wrote my own entry on the Space Elevator, and there is a lot of repetition between the two entries. I'll have to combine the two at some point, but in the meantime I want this one to be correct.

I'll have a look at the bit about diamonds.

I've rewritten the paragraph about diamond slightly and marked it in red.

Traveller in Time confused
"There is this substance 'carbon fiber' they use to make bodies for race cars . . .

is a bit off topic regarding orbits. Still makes sense in relation to the entry. "

I've never know what carbon fibre is. Perhaps Orcus knows. But I don't think it is worth mentioning the entry.

As far as I'm aware, "carbon fibre" is long carbon chains (effectively massive oil molecules with the hydrogen stripped off), woven together to form matting, which is then treated with resin to stabilise the mat. How you make and lay the weave of the matting, together with the resing you use, and how you 'cook' the result affects the properties of the resulting "carbon fibre".

http://en.wikipedia.org/wiki/Carbon_fiber

That's OK Gnomon, but it now looks a little bit random if you know what I mean, there doesn't really seem to be any need to mention diamond at all the way you put it now.

That carbon fibre entry on wikipedia certainly reinforces the final point about how strong buckytubes might be if you could make them in the size required.

>>looks a bit random

I see what you mean. More work needed...