SPACE

[Absolute_Zero]

It’s a [local] limit on how fast things can move in space, not a limit on the universe itself. The observable universe is by definition the things we can see. And as the universe is only 13.8 Bn years old, it means the observable universe is no more than 13.8 Bn light years round. But the universe is thought to be much bigger than this.

Science: we can embiggen that. The popular figure du jour for the diameter of the observable universe is ~93 billion light years. Using the "cosmological proper distance".

The best estimate of the age of the universe as of 2015 is 13.799±0.021 billion years but due to the expansion of space humans are observing objects that were originally much closer but are now considerably farther away (as defined in terms of cosmological proper distance, which is equal to the comoving distance at the present time) than a static 13.8 billion light-years distance. It is estimated that the diameter of the observable universe is about 28.5 gigaparsecs (93 billion light-years, 8.8×10^26 metres or 5.5×10^23 miles), putting the edge of the observable universe at about 46.5 billion light-years away.

https://en.wikipedia.org/wiki/Observable_universe

Slightly more digestible.
http://www.space.com/24073-how-big-is-the-universe.html

[zip accident]

For drakflam:


http://www.vexxarr.com/Index.php

I really like the idea of walking up to space. People have walked from Land's End to John o'Groats, which is further, so it would be possible, if you had a spacesuit for most of the journey. To the top would be further, but probably doable if you had the time. There is no reason you couldn't route the stairs around the elevator.

[Dio]

Van Allen belts would be a major problem. They're an issue even for a space elevator moving at a couple of hundred miles an hour - that's a 10+ hour trip to geostationary orbit and a bit long to be in a major radiation belt.

[Darkflame]

I still am more sold on the idea of skyhooks and such. Decoupling as much as possible fuel from what you want to lift (or rather accelerate) makes sense, but you dont need a cable all the way to the ground to do that.
https://en.wikipedia.org/wiki/Skyhook_(structure)

Still ""got problems"" but seems vastly more achievable then space elevators.

---

That said, the talk of walking into space has got me thinking of a little gap in my knowledge. Or at least "scales of comparison".
When walking up that staircase the staircase is accelerating you for you - its just gaining the rotational speed from the earth its attached too. So you reach the needed speed ''for free''.

But out of interest how far out of earths gravity well do you have to go so that you don't also need to get upto orbital speeds at all? That the gravitational pull of the earth can be casually countered by some tiny constant rocket, or even a ion-drive.

For that mater, "going high enough for it not to matter" vs "going fast enough to reach orbit" which is the greater cost fuel wise? My gut says orbit, but I have no idea. I think about orbits a lot, but it accors to me I know a lot less about "just going up".

[pogo]

There’s a lot of difference between walking up and along. I’ve walked up the tallest mountains in Hong Kong, which are taller than anything in England. It’s totally worn me out doing so, but it’s only a vertical climb of 1km or 1000m. It’s basically a day out walking, at least for me. Even taller mountains such as those in the Himalayas are major multi-week expeditions just to climb. It would not matter if you were wearing a spacesuit - in fact people normally use bottled oxygen at the top of the tallest mountains – it is still a monumental effort. To climb only 8km vertically.

The main problem still though is building anything up. In theory you could run the first few km up a high mountain, but after that you are up against the same constraints that have stopped anyone building a space elevator.

[gumboot]

Geosynchronous or GTFO.

[zip accident]

But out of interest how far out of earths gravity well do you have to go so that you don't also need to get upto orbital speeds at all? That the gravitational pull of the earth can be casually countered by some tiny constant rocket, or even a ion-drive.

Geosynchronous or GTFO.

Which is quite high. You'd need restrooms on the way up.

[gumboot]

Could probably do the maths to figure out how many bacon sandwiches are required to get you up there... I bet it's a lot.

[Darkflame]

I dont think Geosynchronous is high enough is it? If your not going (speed of the earths rotation at that height) arnt you going to be pulled downwards pretty quick?
ah, hell, I should be able to work this out
*some googling and rough calculations*

Seems at Geosynchronous height you have about 1/30th g pulling you down.
I think.

[zip accident]

I dont think Geosynchronous is high enough is it? If your not going (speed of the earths rotation at that height) arnt you going to be pulled downwards pretty quick?
ah, hell, I should be able to work this out
*some googling and rough calculations*

Seems at Geosynchronous height you have about 1/30th g pulling you down.
I think.

At geosynchronous height, an orbit takes 24 hours. Which is why a space elevator stays up.

[gumboot]

Yeah, assuming that the space staircase winds straight upwards; by the time you reach geosynchronous orbit you're in orbit implicitly. Any lower than that and you'll have to take a running leap to get yourself into orbit.

Merely standing on a platform 100 miles up might be "in space" but you haven't achieved the important bit.

Even Usain Bolt might struggle to get a sufficient run-up to get into orbit from there!

[pogo]

You know how high a geosynchronous orbit is? 35,786km. Rather more than the low-earth orbit of ISS and the Space Shuttle. It’s all hypothetical as no-one is going to build a space elevator any time soon, but if they did you would want to shoot up it speeds more like those of a space rocket than motor car. You would not want to be walking up.

[gumboot]

Good point. If you take a bicycle then you'll also be able to get up more speed off the platform, so you won't need to be as high to get into orbit.

[Darkflame]

I dont think Geosynchronous is high enough is it? If your not going (speed of the earths rotation at that height) arnt you going to be pulled downwards pretty quick?
ah, hell, I should be able to work this out
*some googling and rough calculations*

Seems at Geosynchronous height you have about 1/30th g pulling you down.
I think.

At geosynchronous height, an orbit takes 24 hours. Which is why a space elevator stays up.

Which is basically what I meant by;

When walking up that staircase the staircase is accelerating you for you - its just gaining the rotational speed from the earth its attached too. So you reach the needed speed ''for free''.

However, using a rocket straight up would not be the same. The height is useless without the speed.....unless you are high enough for the earths gravity to be negligible at that distance. (as it falls of inverse squarely with distance)

So I just wondered, with all the fuel being spent accelerating craft up to orbital speeds, if the equivalent fuel was spent just going away from the earth how far out of the gravity well would you reach.

You know how high a geosynchronous orbit is? 35,786km. Rather more than the low-earth orbit of ISS and the Space Shuttle. It’s all hypothetical as no-one is going to build a space elevator any time soon

Indeed.
The height necessary for the cable makes the physics on it rather insane.

A Skyhook, by comparison, can be vastly shorter.

[pogo]

A Skyhook, by comparison, can be vastly shorter.

Never heard of that before. Had a look at WP’s article and it looks impractical in all sorts of ways. Mostly I’m not sure it actually saves you much energy. Yes if you have a massive stationary or rotating tether you can attach a smaller mass to it and have it boosted further into space. But you still lose a few % of the (rotational/orbital) energy of the tether, and need to expend a lot of fuel (as it is massive) getting it up to speed again. And that‘s ignoring the practical aspects of building something so big in space.

The only way it would work is if they could fuel it entirely from space. Find a asteroid and put that in a lowish orbit. But that itself would be very expensive – it takes just as much energy to enter orbit as leave orbit, the energy scaling with the size of the mass involved. And doing anything like that is far in the future. We have only just managed to visit our first asteroid with a small probe.