The difference in gravity is negligible. The radius of the Earth is 6378km, and gravity inversely scales with the square of that radius. At 30km altitude, you're at 6408. The difference in gravity would be approx 1%. There are balloons that can go higher, but they tend to be much larger, and much much MUCH more expensive. If you're lucky and have a small payload, a typical met-balloon might reach 120,000ft, but not significantly more than that.

Doug

Local gravity at 33km altitude is less than it is at the earths surface but only by about 1%. You can find out all you need to know about how local gravity changes with altitude (and latitude, since the earth is not a perfect sphere) at the Wikipedia page on Earth's gravity.

I'm no expert, but it looks like there is a misconception
that the fact that astronauts in orbit are weightless means
that Earth's gravity at that elevation is zero. In fact, the
weightlessness is the result of the orbital path that allows
the astronauts to fall toward the Earth at the same rate
that their craft descends around the curve of the Earth.

A misconception held by whom? I doubt anyone here would think that.

WIKIPEDIA



I don't understund this quite well.
On 400 kilometers is 90% of Earth greavity.??? That is a little bit strange because i read some text on Internet "everybody who rich 100 km is astronaut".

There is Karaman Line (http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line) at 100 km

Ok thx for explanation but can you tell me why we have a limit at 120k ft? only because of lower pressure?
Do you have some example what ballon they use for that altititude? ('much larger, and much much MUCH more expensive.")

thanks again

The key to understanding this is free-fall, Vladimir. An object that was stationary 400 km above the Earth would experience 90% of the surface gravity. However, an object in orbit is in effect falling around the Earth (i.e., there's a substantial vector of motion transverse to gravity), so there's very little acceleration towards the Earth...certainly far too little to be noticed by a human observer.

Free fall is the key way of describing it. A satellite in orbit is always falling towards the Earth, at the standard rate at which any object falls toward the Earth from that height. It's just going fast enough in a vector, as Nick pointed out, transverse to "down" that the Earth's curved surface falls away from the satellite as fast as the satellite falls toward it.

You can experience free-fall without being in orbit -- most well-known is the KC-135 "vomit comet" trainers that NASA flies, which basically have the airplanes fall freely along a diving arc for about 40 seconds at a time. But you can observe the same effect in an elevator that falls down a shaft without touching the walls. Both you and the elevator would be falling at the same rate, so you would experience "weightlessness" while both of you fell. (You'd experience very short-term but extremely high G-forces when you got to the bottom of the shaft, though...)

-the other Doug

Interesting now i understund much better.
Can you tell me why then if we have some balloon at (for example) on 30-40km altitude (when burst) payload must fall on Earth? And then something above 100km never fall?

Objects that are orbiting the Earth at an elevation of
many hundreds of killometers will fall if their
forward orbital motion is stopped or slowed.

...and if something is launched upwards to 100 km, but does not have orbital velocity, it will come back down.


(if I put in my 2 cents to a comment from centsworth_II, does that equal four cents?)

There's a transatlantic balloon attempt underway right now.

The plan is to get a small balloon across the Atlantic. It launched a few hours ago from Knoxville.

Don't think it has any cameras but seems to be well instrumented.

The telemetry is available at: http://tracking.spiritofknoxville.com/dash...ashboardus.html

(I had better luck at getting all the displays to run with IE than with Firefox but YMMV.)

My concern, looking at the telemetry, is that they have dumped most of the ballast and are still not high enough. If that's the case, come nightfall they won't be able to maintain altitude (I believe the lift is coming from a combination of Helium and solar). OTOH, I may have wrongly interpreted things -I've only just found the site!

Rob

Look on the bright side for that spirit of knoxville attempt: it seems they have a good chance of recovering the flight hardware. :^) (altimeter showing 4094 feet and falling, location over virginia heading farther inland)

(OMG, it's coming right for me! )

It is cloudy, cold and dreary all over the Piedmont: 48 F, and solar energy less than 150 W m-2. I don't think there's much chance of any solar help.

But a really cool attempt. I hope they try it again!!

[967 ft as of 15:00 EDT, not a good trend]

-Mike

I think the 967 number is meters, it says 3172 ft on the US version. That value hasn't updated in a while, though the climb rate is about zero now. Really like the layout of that website. It makes it very exciting so see this live