[thedoc (OP)]

How can a spaceship change direction in space when – unless I was misinformed - I was told it is a vacuum.  So it’s got nothing to thrust against? And the second question is, I'm old enough to remember Sputnik 1 going up, and it didn’t stay up very long, although I was told it was about 500 miles up. But now, I understand that satellites we have got up there now are not as high, but they stay up for longer.  How is that?
Asked by David from Fincham, over the telephone


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[thedoc (OP)]

We answered this question on the show...



Dave -  Okay, I’ll start off with the first question. The way a spaceship changes direction, accelerates, and decelerates in space is by pushing on something.  It does push on something but because there's nothing up there, it’s going to take the thing it’s pushing on with it, and the thing it pushes on is fuel, so it maybe burns hydrogen and oxygen, those two react, get very hot and you get very, very hot water flying very quickly out of the back.  
It’s a bit like if you've ever fired a gun, the gun fires a bullet out one way, but the gun gets kicked backwards into your shoulder in the other direction with an equal and opposite amount of momentum.  So if you imagine a spaceship is a bit like a gun firing a bullet in one direction, so it gets pushed in the other direction.  
As for satellites, things like the International Space Station which is flying at about a couple of hundred kilometres up, it does fall downwards relatively quickly but it has a rocket on it which keeps pushing it up occasionally and they'd have to send up more fuel for that quite regularly which is one of the major things they have to do to keep it running as well as taking up food for the astronauts and things for them to do.  
The other thing is that just because the orbit was at the maximum 500 miles, it doesn’t mean the whole orbit was at 500 miles.  It could mean it was quite elliptical.  So rather than being exactly circular, if it’s very elliptical, the lowest point of the orbit could’ve been very low, maybe 150 km up, or something like that.  At which point, when it came in the lower bit, it would go too close to the atmosphere, it will get slowed down a lot and so, its orbit would decay away quite quickly even though the highest point of its orbit was relatively high.

[MikeS]

I don't think the above post makes this particularly clear.  The hot fuel expanding out of the rocket motor pushes on the motor which is similar to a tube sealed at one end with a flared skirt at the other.  The burn is like a continuous explosion but the pressure exerted on opposite sides of the tube are equal and cancel.  The effective push is against the sealed end and the flared skirt.  By changing the angle of the flared skirt (rocket motor nozzle) the spaceship can change direction.

 

[yor_on]

A little like a solar sail is expected to get 'pushed' by 'photons', although what pushes inside a rocket engine does it as Mike described it.

It's a constant expansion of a 'force', being at rest with your rocket, having one easy way out which is the exhaust, where it don't have anything to push on, as space is a empty vacuum classically.

The other side, in the direction of your motion, being the engines wall that the radiation, molecules, whatever hit and impart a momentum to, that transfer and translate into a 'motion' for your rocket. And there the construction of that rocket chamber in where it explodes/burn, as well how the burning is planned to expand in itself should be quite important.

[CliffordK]

A little like a solar sail is expected to get 'pushed' by 'photons', although what pushes inside a rocket engine does it as Mike described it.

I think the Solar Sail is supposed to be pushed by Protons, not Photons.

Or, perhaps a combination of the two.

[JP]

You can design charged sails to work by reflecting charged particles or (optically) reflective sails to work by reflecting photons.  You can even design magnetic sails which deflects charged particles via their magnetic fields.