Space walk

JohnD · Post by **JohnD** » Sun Sep 16, 2007 9:30 am

All,
APOD 16th September 2007
Great picture of an astronaut totally free of the Shuttle and 100meters away. What an experience!
But I see that he has moved away 'to the side', not above or below the Shuttle. Would these be 'forbidden directions'?

Because;
If he moved straight up, his orbit would be higher than the Shuttle, an orbit that requires a lesser orbital speed to maintain, while he was travelling at the same speed as the Shuttle. So he would be travelling faster than his orbit required and would spiral outwards, away from the Shuttle. Vice versa if he went downwards.
Same predicament as a lone sailor who swims off his boat without a ladder to climb back on board!
Or would the speed differences be so small that his thruster pack could bring him back against this effect?

JOhn

William Roeder · Post by **William Roeder** » Sun Sep 16, 2007 1:51 pm

If he moved straight up, his orbit would be higher than the Shuttle, an orbit that requires a lesser orbital speed to maintain, while he was travelling at the same speed as the Shuttle. So he would be travelling faster than his orbit required and would spiral outwards, away from the Shuttle. Vice versa if he went downwards.
Same predicament as a lone sailor who swims off his boat without a ladder to climb back on board!
Or would the speed differences be so small that his thruster pack could bring him back against this effect?

The two orbits are almost identical. If your above the shuttle, your max hight is slightly higher than the shuttle. What happens for 45 minutes you slowly drift away and then you slowly come back. (This ignores air resistance)

In any case the difference is trivial compared to the thruster pack.

Arthur C. Clark had a short story using this exact problem. They pushed the spy away and told him if he didn't talk quickly they wouldn't be able to fetch him. He talked and they went back inside. They told him he'll be back in 90 minutes and they'll collect him then.

JohnD · Post by **JohnD** » Sun Sep 16, 2007 3:13 pm

Thank you, William!

But why do they drift back again? Left outside, would the hapless astronaut then drift inwards of the shuttle and back? In other words, they have distorted their own orbit to one more eccentrc than the Shuttles's?

Ah! You said 45 minutes. That's half of a 90 minute Shutte orbit (??) so it's the distorted orbit then?

JOhn

Andy Wade · Post by **Andy Wade** » Sun Sep 16, 2007 3:36 pm

JohnD wrote:Thank you, William!

But why do they drift back again? Left outside, would the hapless astronaut then drift inwards of the shuttle and back? In other words, they have distorted their own orbit to one more eccentrc than the Shuttles's?

Ah! You said 45 minutes. That's half of a 90 minute Shutte orbit (??) so it's the distorted orbit then?

JOhn

OMG! Can you imagine watching the shuttle disappear over the horizon?
It would be terrifying, and then exhilirating as you watched it reappear behind you and catch you up.
Stiil, wouldn't the view be great!

JohnD · Post by **JohnD** » Sun Sep 16, 2007 4:18 pm

That's not the scenario that I thought William meant, Andy.
To have the Shuttle go off ahead of you and then come around on another orbit, in effect doing one more owrbit that you have, wouldn't you have you be going a LOT slower and LOT further out from the original orbit, or else to wait a very long time?

John

rigelan · Post by **rigelan** » Sun Sep 16, 2007 5:56 pm

Right, you would just be on a different elliptical than them. It would appear to go further towards the earth, and then back again. But it would always be in sight.

orin stepanek · Post by **orin stepanek** » Sun Sep 16, 2007 9:48 pm

http://antwrp.gsfc.nasa.gov/apod/ap070916.html

Kind of a lonely feeling looking at this. Bet it would be neat in 3-D. Some how it reminds me of some Si-Fi flicks.

Orin

Andy Wade · Post by **Andy Wade** » Sun Sep 16, 2007 11:34 pm

JohnD wrote:That's not the scenario that I thought William meant, Andy.
To have the Shuttle go off ahead of you and then come around on another orbit, in effect doing one more owrbit that you have, wouldn't you have you be going a LOT slower and LOT further out from the original orbit, or else to wait a very long time?

John

Oh I see.
Small, and Great circles eh?
I get it now.
Still a great view of course!

That must be completely mind blowing.

And they get paid to do it too. 8)

ChrisO · Post by **ChrisO** » Mon Sep 17, 2007 3:45 am

Sunday's picture:
http://antwrp.gsfc.nasa.gov/apod/ap070916.html
shows an absolutely great photo of the space walk, but I have to ask, where are the stars in the background? Perhaps they are too faint with the required exposure for this photo?

Many thanks
-Chris O

"Back in 1984, they didn't have iPods"

dpenney · Post by **dpenney** » Mon Sep 17, 2007 3:54 am

Hi All:

A trivia question I love to ask is: Why does an astronaut float free in space? The usual answer is, "because there's no gravity up there." I then ask, how come the moon is attracted to the earth, and the earth to the sun, to stay in orbit? Gravity is obviously at play there. I explain further that, at the elevation of the space shuttle above the earth, the gravity is about 92% of that on earth. Gravity on the moon is only one sixth. So how come the astronaut "floats" in, or, in this APOD case, near his/her space craft?

The answer, of course, is that the sucker is flying at 28,000 km/hr around the earth and is, while doing that, falling like a lead weight toward the earth! The reason he/she doesn't sink down into the atmosphere is because he/she is speeding along at a trajectory that is the same curvature (more or less (stay with me here)) as the upper atmosphere of the earth. If he/she slowed down, he/she would sink into the atmosphere and enter it at 28,000 kph. Hence, the friction and burn up. If he/she were to speed up, then bye bye! It's off out into space (at least for a while.)

So it always irritates me when the text of such APOD messages talk about "floating" in space. There is no "floating", only speeding. Very high speed, at that!

Let's stop giving the general public the impression that gravity ends at 300 km above the earth's surface.

auroradude · Post by **auroradude** » Mon Sep 17, 2007 7:16 am

Between a subject in full sun verses getting stars on film, exposure requirements differ by a factor of a few tens of thousands.
By my experience: a sunlit subject might require an exposure of approx 1/100th second at f/11 and ISO 100 (A fella in a bright white suit enjoying the thrill of extended zero G's aginst a blue-white Earth might require a bit less exposure even).
On the other end: to capture several stars at the same ISO of 100 could require 10 seconds of exposure - more or less.
Given this huge difference in exposures it probably would leave the sky absolutely black and devoid of stars when properly exposing for the astronaut. (I think I see him smiling!)

Case · Post by **Case** » Mon Sep 17, 2007 10:58 am

dpenney wrote:A trivia question I love to ask is: Why does an astronaut float free in space? The usual answer is, "because there's no gravity up there."

The usual answer may be wrong, but that doesn't make the use of the term "to float" wrong.
The floating merely points to (a) the equilibrium between going up and down and (b) the likeness to being suspended in a liquid.
- Clouds float through the air, not by lack of gravity.
- Small particles float in turbid water, but there's still gravity present.

William Roeder · Post by **William Roeder** » Mon Sep 17, 2007 4:44 pm

Still a great view of course!
That must be completely mind blowing.

And they get paid to do it too. Cool

Listen to them complain when the weather's bad and they have to stay up an extra day

[/quote]

iamlucky13 · Post by **iamlucky13** » Mon Sep 17, 2007 6:05 pm

Orbital mechanics can really screw with your head. The Gemini astronauts had a lot of trouble in their first rendezvous attempts getting it figured out.

One key is to remember that you're anytime you apply a thrust, you're changing an elliptical orbit.

If you apply a thrust upward, you've made the orbit more eccentric. You will gain altitude, but still be moving the same velocity around a larger radius. You would see the shuttle drop below and move ahead of you. Then as you fall back around to other side of the planet, that reverses and you end up at a lower altitude and higher speed, and you'd catch back up. This repeats each orbit.

If you thrust forward, you've gained velocity and your momentum carries you into a higher altitude (due to the earth's curvature). However, in the process of gaining altitude, the tug of gravity reduces your velocity. This, coupled with the increased radius again causes the shuttle to move below and ahead of you. Because, the thrust was only in one direction only, as you swing around to the other side of the planet, you will again gain speed and drop down, but this time only to your original altitude. To stay at that altitude, you must thrust again to circularize the orbit.

To really understand orbital mechanics, you have to remember that conservation of energy requires keeping track of both kinetic (U = 1/2 mv^2) and gravitational potential (V = mgh) energy.

Wow...I just found the coolest Java widget. You can use this to stick your own planets in orbit around the sun by clicking and dragging. It'll show some of the effects above.
http://galileoandeinstein.physics.virgi ... epler6.htm

JohnD · Post by **JohnD** » Mon Sep 17, 2007 7:06 pm

Thank you ial13,

Your explanation reminded me of a Larry Niven mantra from the Integral Trees, which goes something like "Down takes you out, out takes you up, up takes you back, back takes you down." I never understood it then.

But searching for that lead me to an old thread on BAUT:
http://www.bautforum.com/astronomy/6175 ... orbit.html

John

auroradude · Post by **auroradude** » Mon Sep 17, 2007 7:37 pm

[quote="auroradude"
On the other end: to capture several stars at the same ISO of 100 could require 10 seconds of exposure - more or less.
[/quote]

That should actually read," ten seconds of exposure at f/2 - more or less."

So, simply put: The exposure of the astronaut is a fast exposure with the lens closed down. The exposure of the stars would have to be long with the lens wide open.

craterchains · Post by **craterchains** » Tue Sep 18, 2007 12:56 am

Just the simple physics of motion JohnD.

Try to dock a sail boat under sail at a dock on a flowing river and you will understand a more complex concept of motions.

Nereid · Post by **Nereid** » Tue Sep 18, 2007 1:37 am

Two threads on the same APOD merged.

craterchains · Post by **craterchains** » Tue Sep 18, 2007 8:38 am

You missed this one Nereid, , ,

http://asterisk.apod.com/vie ... ebb27a9bdb

JohnD · Post by **JohnD** » Tue Sep 18, 2007 11:41 am

craterchains wrote:Just the simple physics of motion JohnD.

Try to dock a sail boat under sail at a dock on a flowing river and you will understand a more complex concept of motions. :P

I know EXACTLY what you mean, cc!
If you can't get the jib to go across at the right moment- disaster looms!
Fortunately for astronauts, the orbital mechanics are much simpler than motion in a chaotic sea, but still counterintuitive.

John

geckzilla · Post by **geckzilla** » Tue Sep 18, 2007 4:41 pm

It's a good thing gravity is instant or we'd be arguing whether it actually happens or not too. hehe. Or maybe I should say we would be arguing about it much more... I'm sure someone out there has found a way to argue with gravity.

NoelC · Post by **NoelC** » Tue Sep 18, 2007 8:58 pm

Current relativistic thinking is that the force of gravity is not instantantaneous, but is propagated through space at the speed of light. Recent experiments involving Jupiter have added weight to this viewpoint. Thus, if the Earth were swept out of existence (e.g., into an alternate universe), it would be a small time before the astronaut's (and moon's) floating paths went from curved to straight. In fact it would be just at the time the astronaut saw the Earth disappear. Depending on the manner of the Earth's demise, the astronaut may not even feel the difference.

And gravity is arguably not anything at all but an abstract way of thinking about the perturbation in space-time made by a collection of mass.

Argumentative enough for you?

-Noel

NoelC · Post by **NoelC** » Tue Sep 18, 2007 9:06 pm

Man, I've heard of "getting away from it all" but McCandless has really taken the cake with this one!

Interesting, the discussion on how the orbit would become eccentric if he should go above or below the shuttle, and he would "float home" after a time.

I wonder what happens to the Nitrogen molecules from the MMU. Do they continue to orbit as molecular space debris, or dissipate over time and return to the Earth as they are slowed by other collisions with gas molecules?

-Noel

Starship Asterisk*

Space walk - forbidden directions? (APOD 16 Sep 2007)

Space walk - forbidden directions? (APOD 16 Sep 2007)

Astronaut 'floating' free in space: I think not (16 Sep 07)

Re: Astronaut "floating" free in space: I think no