Orbit = Mind blown

[SixPapaCharlie]

Probably everyone knew this but me.

I was reading up on weightlessness in space because of the space jumper guy today and not sure why I am just learning this and it makes total sense.

For those that missed class:

When you leave the earth's atmosphere, you are not weightless.
All that crap that is orbiting is falling just as if you dropped it here on earth.

From movie depictions, they make it seem like you breach this plane and suddenly are floating. No you are falling and falling fast but moving laterally so fast that you are falling around the curvature.

NEVER KNEW THIS. Or I totally forgot it.

I never put much thought into this and always sort of thought once the ship is out of the atmosphere you start floating like in Space Camp.

Of course the Earth gravity would not be affected by this "in vs out" of atmosphere.

Movies do tend to imply that this is the case.

Sorry if I am the only one that never realized this.


What an Orbit Really Is

The drawings above simplify the physics of orbiting Earth. We see Earth with a huge, tall mountain rising from it. The mountain, as Isaac Newton first envisioned, has a cannon at the top. When the cannon is fired, the cannonball follows its ballistic arc, falling as a result of Earth's gravity, and it hits Earth some distance away from the mountain. If we put more gunpowder in the cannon, the next time it's fired, the cannonball goes halfway around the planet before it hits the ground. With still more gunpowder, the cannonball goes so far that it never touches down at all. It falls completely around Earth. It has achieved orbit.
If you were riding along with the cannonball, you would feel as if you were falling. The condition is called free fall. You'd find yourself falling at the same rate as the cannonball, which would appear to be floating there (falling) beside you. You'd never hit the ground. Notice that the cannonball has not escaped Earth's gravity, which is very much present -- it is causing the mass to fall. It just happens to be balanced out by the speed provided by the cannon.
Getting Into Orbit

The cannonball provides us with a pretty good analogy. It makes it clear that to get a spacecraft into orbit you need to

• Raise It Up (the mountain) to a high enough altitude so that Earth's atmosphere isn't going to slow it down too much. In practical terms you don't generally want to be less than about 100 miles above the surface of the Earth. At that altitude, the atmosphere is so thin that it doesn't present much frictional drag to slow you down.
• Accelerate It until it is going so fast that as it falls, it just falls completely around the planet. The required speed for a particular altitude A can be found from the formula
  
  
  where A is in miles and v comes out in miles per hour. So for example the shuttle, orbiting at 200 miles up travels at
  
  
  At that speed, it takes about 90 minutes to complete one orbit (an hour and a half to go all the way around the Earth!).
  If we place a satellite way up - at an altitude of 22,284 miles, then to stay in orbit, the satellite should travel at
  
  
  At that speed, you can show that it takes 24 hours to orbit the Earth. But since the Earth is rotating once every 24 hours, the satellite is going around the Earth at the same exact rate that the Earth is turning. The satellite stays above the same point on the Earth, or looking at it from the Earth's surface, the satellite stays in the same place in the sky. This is called a "geostationary" orbit, since the satellite seems to be stationary - it looks like it doesn't move! This is great if you have to point your satellite dish to pick up a signal from this satellite. Point it once and you're done.

 

[Sac Arrow]

You lost me after "missed class".

 

[SixPapaCharlie]

You lost me after "missed class".

Oh um if you stare at it long enough, you will see tits

 

[ClimbnSink]

But if you fly further into space you are no longer falling in orbit around earth, yet still weightless.

 

[SixPapaCharlie]

But if you fly further into space you are no longer falling in orbit around earth, yet still weightless.

Right! totally correct.

My research went like this
"Hmmm how are these guys falling back to earth from space?"

So I learned about that then I started wondering how geosynchronous orbits work as they have to be moving laterally but maintain a fixed position relative to the planet.

But yes I am referring specifically to Earth orbits.

We have all seen the movie where it is blue, they climb past that thin layer and suddenly everything is floating.

 

[jesse]

Eh..I'm not sure what you're really getting at here. What you know as the feeling of "weight" is just the feeling of strain from your body in contact with an object that's stopping you from being pulled further towards Earth.

Once you don't have an object stopping you then the feeling of "weight" is gone. You could be in orbit around Earth or you could be far out in space with very Earth playing very little role in things.

Think about it. Weight is just the feeling you're used to because your body is under strain since it's in contact with an object that's stopping it from being pulled further. It goes away in freefall or many other things....

 

[jesse]

Honestly if you want to learn more about Orbit than you ever need. Download the game Kerbal Space Program, buy a bunch of beer, and lose the next week of your life trying to figure out how to build a rocket that can get you to the moon and back. You'll kill many Kerbals trying.

https://kerbalspaceprogram.com/

David and I spent a large quantity of time last winter trying to get to the moon and back. The great space race...

I spent more time then I'd like to admit building a rocket to recover a Kerbal that I accidentally shot out of Earth's orbit. Took a **** load of planning to figure out how I was going to intercept that Kerbal who was flying away from Earth at some crazy velocity. I did save him though

 

[SixPapaCharlie]

Eh..I'm not sure what you're really getting at here. What you know as the feeling of "weight" is just the feeling of strain from your body in contact with an object that's stopping you from being pulled further towards Earth.

Once you don't have an object stopping you then the feeling of "weight" is gone. You could be in orbit around Earth or you could be far out in space with very Earth playing very little role in things.

Think about it. Weight is just the feeling you're used to because your body is under strain since it's in contact with an object that's stopping it from being pulled further. It goes away in freefall or many other things....

Point being.
If you are 100 miles up and stop moving laterally, you will hit the ground
I had surmized that once you got out of the atmosphere you just float.

You don't. you fall. but the lateral direction keeps you from hitting the ground

 

[jesse]

Point being.
If you are 100 miles up and stop moving laterally, you will hit the ground
I had surmized that once you got out of the atmosphere you just float.

You don't. you fall. but the lateral direction keeps you from hitting the ground

You're making associations though that just aren't there. You can be weightless in the Earth's orbit, in the Earth's athmosphere, or 10 thousand light-years from Earth.

The only thing that makes you not weightless is when you come into contact with a force that stops you. Like the ground.

You can be weightless floating along in space for a very long time until you get sucked into something and smash into it.

This is the part you wrote that is rather misleading:

When you leave the earth's atmosphere, you are not weightless.

From Wikipedia on weightlessness:

Weightlessness, or an absence of 'weight', is in fact an absence of stress and strain resulting from externally applied mechanical contact-forces, typically normal forces from floors, seats, beds, scales, and the like.

So if by weightlessness, you mean, a state where you can spend a life time without feeling your weight (like you could in a good orbit) then sure. But weightlessness doesn't mean that. It means you don't feel the resisting mechanical force at a precise moment in time...achievable in your airplane/

Weightless doesn't even have anything to do with being in the Earth's atmosphere. It has to do with you getting stopped by something like the ground. Although the atmosphere does provide resistance but so does other things in space...like crashing into an Asteroid belt.

Ok..I'm done..someone smarter than me can point out everywhere I'm wrong

 

[SixPapaCharlie]

Hard to put into words
I (stupidly) had thought once you left the atmosphere, you would just float there.

I can't find a clip on youtube of what I mean but it is in a lot of movies
If you moved perpendicular to the earths surface and got outside the atmosphere

you would fall right back down to earth.

 

[jesse]

Hard to put into words
I (stupidly) had thought once you left the atmosphere, you would just float there.

I can't find a clip on youtube of what I mean but it is in a lot of movies
If you moved perpendicular to the earths surface and got outside the atmosphere

you would fall right back down to earth.

I see. Well yes if you just take an object and stick it 20,000 km orbit it would just get pulled back to Earth. It needs to be moving at a certain velocity to avoid that. Need to crank that up to tens of thousands of mph depending on distance to avoid the crash.

 

[SixPapaCharlie]

Scroll to 7:20
Tom Skerrit is pulling for them, you actually hear them rip through the atmosphere and start "floating"

It seems to imply "floating" is based on ripping through that layer

 

[denverpilot]

Mass attracts.

 

[denverpilot]

Scroll to 7:20
Tom Skerrit is pulling for them, you actually hear them rip through the atmosphere and start "floating"

It seems to imply "floating" is based on ripping through that layer

Oh lord. You had to link to that stupidity.

I'd managed to carefully carve that retarded movie out of all my memory cells with copious amounts of Scotch over many years.

 

[SixPapaCharlie]

Oh lord. You had to link to that stupidity.

I'd managed to carefully carve that retarded movie out of all my memory cells with copious amounts of Scotch over many years.

Oh come on it has the back to the future mom, the dude that dated Rachael on Friends, the black guy from Iron eagle, and Joaquin phoenix

Recipe for perfection

 

[wanttaja]

We used to have a large thermal vacuum chamber where I work ("large" as in three stories tall). Had a VIP visitor once who thought anything in the chamber would be weightless as soon as the air was sucked out.

The key is "free fall." Imagine you're standing on a bathroom scale in an elevator high in a tall building. All safety devices suddenly break, and the elevator car falls (assume a frictionless shaft). Gravity is now working equally on you and the scale, and the scale drops from your svelte weight to zero. If you've got M&Ms in your pocket, you can fling them around and chase them. Until the elevator reaches the bottom of the shaft, of course.

This is identical to what's happening on orbit...with the key point that if you're going fast enough, you aren't going to crunch into the bottom of the shaft and there's no air to slow you up (assuming you're high enough to have cleared the atmosphere).

You must remember, though, that you still have *mass*. It will take effort to get you moving (relative to the elevator car) and the same amount of effort to stop you.

The amount of actual gravity "pull" is in inverse relationship to the square of your distance from the center of the planet. Remember, you're about 3444 nautical miles from the center of Earth when you're standing on the surface, so being in an orbit a couple hundred miles higher does not affect the amount of gravitational pull much.

Ron Wanttaja

 

[ClimbnSink]

Cool jump pilots on mostly empty loads can give a nice dose of floating around.

 

[warthog1984]

Hard to put into words
I (stupidly) had thought once you left the atmosphere, you would just float there.

I can't find a clip on youtube of what I mean but it is in a lot of movies
If you moved perpendicular to the earths surface and got outside the atmosphere

you would fall right back down to earth.

If you are lucky. The alternative is that the earth moves away and you don't. Nothing good happens in that case.

 

[denverpilot]

This guy may have felt pretty weightless for a short time today...

http://mobile.nytimes.com/2014/10/2...baumgartners-world-record.html?_r=0&referrer=

 

[MAKG1]

I used to explain orbiting to undergrads by misquoting Douglas Adams.

The art of orbiting is to throw yourself at the ground and miss.

The "weightless" stuff happens when you consider a comoving reference frame. In an inertial frame, there is most definitely a force acting on the orbiting body, making its path curve.

 

[Sac Arrow]

F=MA muthas.

 

[Dr. O]

In orbit you are neither weightless nor floating - you are in microgravity

The simple proof of that is the shuttle tether experiments for generating power, where they reeled out a weight on the end of a 20 KM line.
It ended badly (of course) but that is not the point. The point is that IF you were actually free floating the weight would not have pulled itself DOWN.
It would have gone any which way except down (999 times out of a thousand) and it would not have had enough pull (weight) to keep the line taut. It would merely drift about.

True weightlessness is difficult (if not impossible) to find this close to massive bodies. Remember, gravity is one of the weak forces and therefore the field is infinite. I suspect it will require a minimum of 10 times the distance from the Sun to Pluto (and probably more) before a true floating tethered body stops stabilizing in the direction of the Sun (i.e. down) given enough time.

 

[Geico266]

Even more interesting that weightless is gravity. We still don't know what causes it. As you set your cup of coffee down this am think about that.

 

[Captain]

Where do you suppose this correlation between atmosphere and gravity comes from? We've all seen Neil Armstrong and his colleagues bouncing around on the moon with zero atmosphere so clearly air isn't needed for Gravity to work.

The OPs erroneous correlation is not unique, I've seen it before and am baffled by it.

 

[tonycondon]

Falling with style

 

[SixPapaCharlie]

We used to have a large thermal vacuum chamber where I work ("large" as in three stories tall). Had a VIP visitor once who thought anything in the chamber would be weightless as soon as the air was sucked out.

The key is "free fall." Imagine you're standing on a bathroom scale in an elevator high in a tall building. All safety devices suddenly break, and the elevator car falls (assume a frictionless shaft). Gravity is now working equally on you and the scale, and the scale drops from your svelte weight to zero. If you've got M&Ms in your pocket, you can fling them around and chase them. Until the elevator reaches the bottom of the shaft, of course.

This is identical to what's happening on orbit...with the key point that if you're going fast enough, you aren't going to crunch into the bottom of the shaft and there's no air to slow you up (assuming you're high enough to have cleared the atmosphere).

You must remember, though, that you still have *mass*. It will take effort to get you moving (relative to the elevator car) and the same amount of effort to stop you.

The amount of actual gravity "pull" is in inverse relationship to the square of your distance from the center of the planet. Remember, you're about 3444 nautical miles from the center of Earth when you're standing on the surface, so being in an orbit a couple hundred miles higher does not affect the amount of gravitational pull much.

Ron Wanttaja

You lost me at "Frictionless shaft"

 

[SixPapaCharlie]

shuttle tether experiments for generating power, where they reeled out a weight on the end of a 20 KM line.
It ended badly (of course) but that is not the point. The point is that IF you were actually free floating the weight would not have pulled itself DOWN.
It would have gone any which way except down (999 times out of a thousand) and it would not have had enough pull (weight) to keep the line taut. It would merely drift about.

How is that possible?
If the shuttle is traveling x miles per hour so is the weight and tether it brought with it.

You drop it and it should continue right along it will never fall on it's own relative to the shuttle.

 

[azure]

I never put much thought into this and always sort of thought once the ship is out of the atmosphere you start floating like in Space Camp.

Of course the Earth gravity would not be affected by this "in vs out" of atmosphere.

Movies do tend to imply that this is the case.

Sorry if I am the only one that never realized this.

You aren't. I go through this with every new crop of students. I remind them that if gravity didn't work outside the atmosphere, the Moon wouldn't stay in orbit, nor would our artificial satellites. I tell them about the Vomit Comet. I ask them to remember how light they feel for a moment when the elevator starts moving down, and to imagine what it would be like if the cable snapped. Most of them eventually get it, but about 10% never do.

Pre-Newtonian ideas die hard.

 

[RotorAndWing]

Waiting on henning to chime in and tell us about his weightless experience while in orbit........

 

[wanttaja]

How is that possible?
If the shuttle is traveling x miles per hour so is the weight and tether it brought with it.

You drop it and it should continue right along it will never fall on it's own relative to the shuttle.

Tethers take advantage of the fact that gravity varies with the inverse square of the distance from the center of the Earth. The end that's 20 km closer is under more gravitational pull, so the tether pulls taut.

Spacecraft achieve (a rather tenuous) attitude stability the same way; they run a boom out and the difference in gravity tries to hold the spacecraft in alignment. This is called Gravity Gradient stabilization. The Orbcomm satellites use it...

Ron Wanttaja

 

[brentee]

Another good question to ask anyone who's just getting interested in this:

How is it possible, then, to have sattelites suspended in geostationary orbit?

 

[Henning]

Another good question to ask anyone who's just getting interested in this:

How is it possible, then, to have sattelites suspended in geostationary orbit?

Simple, choose the correct altitude so the speed required for orbit is the same arc-speed of the surface.

 

[MAKG1]

Simple, choose the correct altitude so the speed required for orbit is the same arc-speed of the surface.

Not enough.

Inclination must be zero and the orbit circular.

You're describing a geosynchronous orbit. Not the same.

For some more orbit fun, suppose you threw a killer fastball off the international space station, for the sake of argument parallel to Earth's surface. Where would it go?

 

[Henning]

Not enough.

Inclination must be zero and the orbit circular.

You're describing a geosynchronous orbit. Not the same.

Oh, I see what you're getting at, best way to do that I guess would be 'space elevator' style.

 

[MAKG1]

Oh, I see what you're getting at, best way to do that I guess would be 'space elevator' style.

Aside from space elevators using geostationary orbits, it has nothing to do with it.

To stay over one spot on the earth, you must be over the equator (zero inclination) and your speed must be constant (circular).

There are a ton of geostationary satellites out there.

 

[Seanaldinho]

Another good question to ask anyone who's just getting interested in this:

How is it possible, then, to have sattelites suspended in geostationary orbit?

The satellite must be over the equator and around roughly 22,300 miles from the surface. Than they match the rotational speed of the earth to keep it over one spot. Of course the satellite is moving much faster than the earths rotation because it is farther out. Kind of like how the end of the propellor is moving much faster than the hub.

 

[wanttaja]

Another good question to ask anyone who's just getting interested in this:

How is it possible, then, to have sattelites suspended in geostationary orbit?

Others have given good answers; the main factor is that the satellite isn't "suspended." Its orbit merely matches the angular motion of the point below it. It's like a plane following the interstate...if you're in a car, you can travel fast enough that the plane is motionless, relative to you. Doesn't mean it's suspended, though.

Again, others have mentioned that one can only be "geostationary" over a point on the equator. Satellites orbit relative to the center of the Earth, thus the center must lie within the plane of the orbit. So your orbit cannot "circle" at, for instance, 45 degrees of latitude....the orbit plane wouldn't include the center. If you've got an orbit period of 23 hours 56 minutes, you'd prescribe a big figure-8 on the Earth, varying from 45 degrees north to 45 degrees south, crossing the equator twice a day.

With that orbit period, you've got a geosynchronous orbit, but not a geostationary one. Reduce the orbit tilt (inclination) to zero, and you're then geostationary.

Ron Wanttaja

 

[Seanaldinho]

Physics is fun.

 

[wanttaja]

You lost me at "Frictionless shaft"

Tch. It's the friction that makes it fun.

Ron Wanttaja

 

[Dan Thomas]

It's like a plane following the interstate...if you're in a car, you can travel fast enough that the plane is motionless, relative to you.

Or if you are in a Fly Baby, the plane is motionless with respect to a car on a city street in a school zone.

Sorry. Couldn't resist. I fly a Jodel that cruises at 90 MPH, too.

Dan