In and out of orbit: Difference between revisions

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By most definitions it's ''at least'' 100km up.  
By most definitions it's ''at least'' 100km up.  


Yet this answer seems to come from people just really liking simple answers, more than any one good reason.
It's a nice round number. And it's simple. But it doesn't point at a particular thing, or threshold.
 


If you take space to mean 'a place with no atmosphere and no gravity', you may want a number much, ''much'' higher than that. Because...
If you take space to mean 'a place with no atmosphere and no gravity', you may want a number much, ''much'' higher than that. Because...
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Above 6km or so becomes impractical for people to breathe enough oxygen.
Above 6km or so becomes impractical for people to breathe enough oxygen.


Above 10km or so it's ''inefficient'' to fly with jet engines, though the record of trying really hard is rather higher, order of 35km.  
Jet planes cruise around 12km because the decreased drag makes for better fuel economy.
...and not much higher, for a mix of reasons (a few of them related to pressure).
 
The record of trying really hard to go up is rather higher, order of 35km.  
The same 35km-ish is also the highest recorded balloon flight{{verify}}
The same 35km-ish is also the highest recorded balloon flight{{verify}}


Above 100km or 200km the gas is thin enough that it would barely slow you down.
Above 100km or 200km the gas is thin enough that it would barely slow you down.
The likes of sputnik, MIR, ISS, and Hubble are somewhere in the 200 to 600km range.
The likes of sputnik, MIR, ISS, and Hubble are somewhere in the 200km to 600km range.


You can detect some tiny amount of gas ''at all'' up to order of 10000km{{verify}}, but you'd only care when you want your vacuum to be ''really'' empty.
You can detect some tiny amount of gas ''at all'' up to order of 10000km{{verify}}, but you'd only care when you want your vacuum to be ''really'' empty.
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Earth's radius is around 6300km.
Earth's radius is around 6300km.


On that scale, 100km away is comparatively close, and gravity is still roughly 95% of that at the surface.
On that scale, 100km away is comparatively close, and the force of gravity there is still roughly 95% of the force it is at the surface.
 
Even at 2000km {{comment|(far above e.g. the ISS (at ~400km))}} it's still roughly half.


Even at 2000km, far above the ISS, it's still roughly half.


While mathematically you could pretend it never quite goes to zero, yet in practice there is a distance at which the force becomes easy to counteract with small amounts of energy.
While mathematically you could pretend it never quite goes to zero, in practice there is a distance at which the force becomes easy to counteract with small amounts of energy.


This guides our understanding of '''microgravity''' (which has no exact definition - it's basically 'little, but not nothing').
This also guides our understanding of '''microgravity''' (which has no exact definition - it's basically 'little, but not nothing').




Assuming fuel is not an issue, it might be feasible to keep something up just by pointing a rocket down - yet given current technology, the point at which this becomes feasible with fuel you can actually take there and have is - ballpark - not closer than 200000km away[https://en.wikipedia.org/wiki/Micro-g_environment#Absence_of_gravity], which is ''well'' on your way to our moon.
Assuming fuel is barely an issue, it might be feasible to keep something up just by pointing a rocket down - yet given current technology the force is low enough that you can keep this up for a long time is -- ''ballpark'' -- not closer than 200000km away[https://en.wikipedia.org/wiki/Micro-g_environment#Absence_of_gravity], which is ''well'' on your way to our moon.  
So in practice fuel is ''still'' dumbly practical issue and is why we can't really do this for any amount of time.
 
So in practice fuel is almost always a dumbly practical issue, and is why we can't really do this for any amount of time.




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: most [[https://en.wikipedia.org/wiki/Low_Earth_orbit low earth orbits] are between 200km and 2000km or so,  
: most [[https://en.wikipedia.org/wiki/Low_Earth_orbit low earth orbits] are between 200km and 2000km or so,  
: [https://en.wikipedia.org/wiki/Medium_Earth_orbit medium earth orbit] at the order of 20000km (~12 hour orbits),
: [https://en.wikipedia.org/wiki/Medium_Earth_orbit medium earth orbit] at the order of 20000km (~12 hour orbits),
: [https://en.wikipedia.org/wiki/Geostationary_orbit geostationary orbit] at approximately 35000km (~24 hour orbit, so when we make it rotate the same way as earth this seems fairly stationary to us),  
: [https://en.wikipedia.org/wiki/Geostationary_orbit geostationary orbit] at approximately 35000km (~24 hour orbit, so when we make it rotate the same way as earth this seems fairly stationary to us, hence the name),  
: [https://en.wikipedia.org/wiki/High_Earth_orbit high earth orbit] at the order of 100000km (and orbits lasting hundreds of hours).
: [https://en.wikipedia.org/wiki/High_Earth_orbit high earth orbit] at the order of 100000km (and orbits lasting hundreds of hours).


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So, 100km is not ''that'' far away.
So, 100km is not ''that'' far away.


Sure, it's well above commercial airplanes which typically cruise around (10km), but that's just because they're specifically designed to fly in air - airfoils or jet engines won't work at 100km, so getting high with a plane must come largely from momentum, and is a little easier if you've strapped on a rocket too.
Sure, it's well above commercial airplanes which typically cruise around (~10km), but that's just because they're specifically designed to fly in air - airfoils or jet engines won't work at 100km, so getting high with a plane must come largely from momentum.  It helps if you've strapped on a rocket.
 
So yes, it takes a bunch of energy, and also a bunch of coordination.


Yet in theory, if you can somehow keep pointing a moderate rocket down, it's within the realm of manageable,
something you can do with enough determination, and somewhat-specialized airplanes have done this since roughly the sixties.


Sure it takes a bunch of energy, and a bunch of coordination, and some determination,
but pointing a moderate rocket down, is within the realm of manageable,
and only-somewhat-specialized airplanes have done this since roughly the sixties.


...which have then quickly come back down.
...which have then quickly come back down.
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As mentioned, at 100km up, gravity is still roughly 95% of that at the surface.  
As mentioned before, at 100km up, gravity is still roughly 95% of that at the surface.
 
 
So if you're at 100km, and want to stay at 100km, and point a rocket down, you're spending ''almost'' the same amount of fuel as you did at launch. And you have to keep it permanently on, if you want those forces to balance and ''stay'' there.
 
 
So it will run out of fuel quickly, probably within minutes.
No, you can't just take more, because fuel has weight, so takes more fuel to get and keep up there.  


Mathematically this is an nonlinear relationship.
I can save you some of the math: practically that relation is ''not'' going to make you happy.


So if you're at 100km, and want to stay at 100km, and point a rocket down, you have as bad of an issue as you did at launch.  
{{comment|(Even a lot of ''fictional'' spaceships aren't made for this. Some ignore it, some handwave it away.  Other acknowledge it by saying their spaceship isn't made to work in atmosphere, or work around it by having some way to sustain high thrust ''and'' to power for long enough to be practical)}}
You have to keep it on permanently on, so it will run out of fuel quickly, probably within minutes (you can't just take more, because fuel has weight, so takes more fuel to get and keep up there. This is an nonlinear relationship, but it's not one that is going to make you happy).


{{comment|(Even a lot of ''fictional'' spaceships aren't made for this.  Some ignore it, some handwave it away.  Other acknowledge it by saying their spaceship really isn't made to work in atmosphere, or work around it by having some way to sustain high thrust ''and'' to power that, both for long enough to be practical.)}}




It turns out that if you want to stay up there for a longer time,
then probably the single most energy-efficient way is to fall sideways so fast that it balances with the falling downwards.


It turns out that if you want to stay up there for a longer time, probably the single most energy-efficient way to stay up there for a long time is to fall sideways so fast that it balances with the falling downwards.
We like to call that 'orbit', because that sounds less terrifying.


We like to call that 'orbit', because that sounds less terrifying. Also it's fewer letters.


This is also roughly why escape velocity is called escape ''velocity'', not escape distance.
This is also roughly why escape velocity is called escape ''velocity'', not escape distance.
{{comment|(for the KSPers: escape velocity is 11km/s, because you're not in orbit yet)}}
''Approximately'', escape velocity is the minimum speed of going sideways for that to start working (the height at which that starts working is less relevant{{verify}}).
{{comment|(for the KSPers: earth's escape velocity is 11km/s, because you're not in orbit yet)}}




Because close to earth you have most of the gravity, you still need to be going sideways pretty fast.
 
You can imagine that the higher you are, the lower the falling-sideways speed needs to be.  
 
At the same time, we've learned the falling-down force doesn't decrease very quickly.
 


'''Low earth orbit''' (maybe 300 km to 2000 km) is basically the smallest distance it becomes feasible to stay up this way ''at all''.
'''Low earth orbit''' (maybe 300 km to 2000 km) is basically the smallest distance it becomes feasible to stay up this way ''at all''.




The ISS, at around 2000km from earth, is still going (ballpark) 7km/s. Yes, per ''second''. That's 25000 km/h or 15600 mph, because there are 3600 seconds per hour.
The ISS, at around 2000km from earth, is still going (ballpark) 7km/s. Yes, per ''second''. That's 25000 km/h or 15600 mph {{comment|(because there are 3600 seconds per hour)}}.
 
Further out you can go slower, but not by very much actually.
 
Geosynchronous orbit -- which basically ''means'' ~24hr orbit time -- is 35000km away for earth (several earth diameters away from earth)), and still implies about 3km/s.
High earth orbit is still 2km/s, and even the moon, 380000km away (a dozen years if you had to walk it), still goes 1km/s in its orbit around us.
 


Further out you can go slower, but not by very much actually.
tl;dr: if you want an orbit, there's no getting out of going sideways fast.
Geosynchronous orbit means ~24hr orbit time, which is 35000km away for earth (several earth diameters away from earth)), and still implies about 3km/s. Hhigh earth orbit is still 2km/s, and even the moon, 380000km away and a dozen years if you had to walk it, goes 1km/s in its orbit around us.


In fact, when you have a rocket getting you to orbit, most of its fuel doesn't get you up high, it gets you moving sideways fast enough. [https://what-if.xkcd.com/58/]


So it turns out that when you have a rocket getting you to orbit, most of its fuel doesn't get you high, it gets you moving sideways fast enough.




You can now also guess ''part'' of the reason why ships burn up on re-entry: the exact same thing, but in the other direction.


https://what-if.xkcd.com/58/
If you hit air with at a few kilometers a second, it is going to hurt. A ''lot''.


It's not really air friction that bothers you, it's more the fact that air cannot get out of the way fast enough, so you're compressing air in front of you enough for that compression ''itself'' to be a noticeable source of heat, a thing we call [https://en.wikipedia.org/wiki/ aerodynamic heating].


Aerodynamic heating starts being noticeable above mach 2 or so.


You can now also guess ''part'' of the reason why ships burn up on re-entry: the exact same thing, but in the other direction.
Which sounds high, until you realize low earth orbit (and no braking) means your relative speed to earth starts off around mach 23.
If you hit air with that speed keeping you up there, it is going to hurt. A ''lot''.


It's not really air friction that bothers you, it's more the fact that air cannot get out of the way fast enough, so you're compressing air in front of you enough for that compression ''itself'' to be a noticeable source of heat, a thing we call [https://en.wikipedia.org/wiki/ aerodynamic heating].  
On the upside, you start in ''very'' thin air, but the whole point of the exercise is to get down,  
so there's still plenty of trouble inbetween - and at that speed, very limited time.


That effect starts being noticeable above mach 2 or so.
Which sounds high until you realize  that low earth orbit plus no braking means your relative speed to earth starts off around mach 23.


On the upside, you start in very thin air but there's still plenty of trouble between that and .
"Can't I be more aerodynamic, lessen the amount of compression?"


That sorta works.


In theory you can be more aerodynamic to lessen the amount of compression,
...very temporarily, because it just postpones the problem.  
but it really just postpones the problem. A decent factor even, but when you start with 23, yeah, nooooo.
You're still going mach 23, and will soon be in thicker air.


Because we're specifically slowing down ''less'',
you might get down to regular pressure still going mach 10, 20.


On the upside, this is also why most meteorites burn up in the atmosphere instead of harming us.
{{comment|On the upside, this is also why most meteorites burn up in the atmosphere instead of harming us.
Atmospheres are great - at protecting us, and at re-entry being a thing without spending even ''more'' energy.}}


Atmospheres are great - at protecting us, and at re-entry being a thing without spending even ''more'' energy.




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In concept that is a great idea, actually.   
In concept that is a great idea, actually.   
You still have to deal with falling pretty hard, but you'ld avoid a lot of compressive heating, and parachutes or similar are manageable.


You still have to deal with falling pretty hard, but now we're merely talking terminal velocity, which might be mach 0.1 for your object.
Not a speed you want to hit anything at, but you'ld avoid a lot of compressive heating, and parachutes or similar are ''manageable''.


So it's great - if you cancel all that 8-ish km/s to nothing.


...while still in space, so with nothing but thrust.  
Great - '''if''' you cancel all that 8-ish km/s to nothing.
 
...while still in space.
 
...so with nothing but thrust.  


...meaning rocket.
...meaning rocket.
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...meaning fuel, in an amount comparable to (only somewhat less than) the amount you took to get up into this orbit.
...meaning fuel, in an amount comparable to (only somewhat less than) the amount you took to get up into this orbit.


...and that's a good amount, which you woul have needed to carry up here. Using more fuel.  
...and that's a good amount, which you would have needed to carry up here. Using more fuel.  
 
 
There are ''layers'' to how impractical and inefficient that is, for very dumb practical reasons.
There are ''layers'' to how impractical and inefficient that is, for very dumb practical reasons.
Fuel has weight.  
Fuel has weight.  
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It turns out that while a heat shield (to not die in a fire) is expensive to do well, it is still much cheaper than that, and currently the most feasible solution until someone has a better plan.
It turns out that while a heat shield, a.k.a. "try to not die in a fire", is expensive to do ''well'',  
it is still much cheaper than that, and currently the most feasible solution until someone has a better plan.
 


It turns out it's a little more detailed than this[https://aviation.stackexchange.com/questions/21981/how-does-the-space-shuttle-slow-down-during-re-entry-descent-and-landing], but that's the broad strokes.
It turns out it's a little more detailed than all that[https://aviation.stackexchange.com/questions/21981/how-does-the-space-shuttle-slow-down-during-re-entry-descent-and-landing], but that's the broad strokes.




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[[Category:Nerding around]]
[[Category:Nerding around]]
[[Category:Physics]]
[[Category:Space]]

Latest revision as of 16:10, 7 May 2024


How high up is space?

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