Stars Can't Be Seen from Outer Space

originally posted by: Soylent Green Is People

originally posted by: InachMarbank

originally posted by: Phage
a reply to: InachMarbank

Do you know if the ISS must come to halt, from traveling approximately 17,000 miles per hour, to allow these repairs to be done?

The ISS cannot come to a halt.

Do you know if the ISS is still traveling near 17,000 miles per hour when the repairs are being done by space walking astronauts?

Yes , both the astronauts and the ISS are moving at about 17,000 mph all the time -- whether they are inside the ISS or outside of it on a spacewalk. That's what makes them "weightless"...

The astronauts are weightless because they are in a free fall along with the ISS. Both the ISS and the astronauts are falling at generally the same speed and in generally the same direction, which is what accounts for them being weightless -- i.e., they fall together in the same direction, giving the astronauts an appearance of weightlessness relative to the ISS.

The direction that they are falling is "sideways", generally parallel to the surface of the earth. That way as they are being pulled down to the Earth by Earth's gravity, their sideways motion is fast enough (17,000 mph in the case of the ISS), that the spherical Earth curves away from them before they have a chance to impact the Earth's surface.

...And that is what defines an orbit. An orbit is a controlled fall as gravity pulls the orbiting object back to earth -- controlled in such a way that the orbiting object never hits the Earth due to the object's very fast sideways velocity.

If the orbiting object came to a halt relative to the Earth (such as the ISS as you suggested), it would fall straight down to Earth's surface due to Earth's gravity pulling it down.

How is the speed of approx. 17,000 miles per hour sideways continually sustained?

When doing a spacewalk moving 17,000 miles per hour, is it fairly possible, considering friction, to walk on the not enclosed exterior of the ISS, and perform repairs?

a reply to: InachMarbank

How is the speed of approx. 17,000 miles per hour sideways continually sustained?

By Earth's gravity, and it isn't "sideways". The ISS is "falling" around the Earth. Pretty basic stuff.
www.nasa.gov...

When doing a spacewalk moving 17,000 miles per hour, is it fairly possible, considering friction, to walk on the not enclosed exterior of the ISS, and perform repairs?

What friction? There isn't really enough there to create friction (or, has been pointed out, heat). It is entirely possible and is done quite frequently. Very frequently when the station was under construction. It was also done for other purposes.

originally posted by: InachMarbank
is it fairly possible, considering friction

No friction in the vacuum of space.

originally posted by: InachMarbank

originally posted by: Soylent Green Is People

originally posted by: InachMarbank

originally posted by: Phage
a reply to: InachMarbank

Do you know if the ISS must come to halt, from traveling approximately 17,000 miles per hour, to allow these repairs to be done?

The ISS cannot come to a halt.

Do you know if the ISS is still traveling near 17,000 miles per hour when the repairs are being done by space walking astronauts?

Yes , both the astronauts and the ISS are moving at about 17,000 mph all the time -- whether they are inside the ISS or outside of it on a spacewalk. That's what makes them "weightless"...

The astronauts are weightless because they are in a free fall along with the ISS. Both the ISS and the astronauts are falling at generally the same speed and in generally the same direction, which is what accounts for them being weightless -- i.e., they fall together in the same direction, giving the astronauts an appearance of weightlessness relative to the ISS.

The direction that they are falling is "sideways", generally parallel to the surface of the earth. That way as they are being pulled down to the Earth by Earth's gravity, their sideways motion is fast enough (17,000 mph in the case of the ISS), that the spherical Earth curves away from them before they have a chance to impact the Earth's surface.

...And that is what defines an orbit. An orbit is a controlled fall as gravity pulls the orbiting object back to earth -- controlled in such a way that the orbiting object never hits the Earth due to the object's very fast sideways velocity.

If the orbiting object came to a halt relative to the Earth (such as the ISS as you suggested), it would fall straight down to Earth's surface due to Earth's gravity pulling it down.

How is the speed of approx. 17,000 miles per hour sideways continually sustained?

When doing a spacewalk moving 17,000 miles per hour, is it fairly possible, considering friction, to walk on the not enclosed exterior of the ISS, and perform repairs?

As phage pointed out, maybe I should not have used the word "sideways" so loosely.

The 17,000 mph occurs because the object builds up that speed while falling back to Earth. The object in orbit is given an initial velocity at launch that puts it in a position to fall back to earth, but also do so while moving parallel to the curved surface of the Earth (that parallel to the Earth's surface is what I meant by "sideways").

As it falls, it builds up speed, enough speed to continue falling past the point where the Earth curves out from under it.


To illustrate this, there was a famous thought experiment devised by Sir Isaac Newton that is known as "Newton's Cannonball":

Image Source

In Newton's Cannonball, the first cannonball "A" has only enough initial velocity to fall back to Earth soon after it is fired. Cannonball "B" has a little more initial velocity out of the cannon that it begins to go beyond the curvature of the earth, and the Earth begins to curve away from it -- but even Cannonball "B" does not quite have enough velocity to keep going...

..Cannonball "C", however, is fired out of the cannon with enough velocity to go far enough out that the surface of the Earth curves away from it -- basically getting out of the way -- allowing the ball to continue without hitting the ground.

That is an orbit.

a reply to: Soylent Green Is People

You know, the concept actually goes back to Galileo. As a result of his observations of the motion of falling objects he began to understand the idea of gravitational acceleration.

a reply to: Phage

As this article seems to explain...
www.idiotsguides.com...

At approx. 210 miles altitude...
Inertia effects straight line motion away from Earth...
(... basically pull up, it sounds like...)
But gravity effecting a pull to the center of Earth also applies...
(... basically push down, it sounds like)

The 2 push and pulls at this altitude force a gravitational fall/orbit that is an approx. 25000 mile circle, around Earth
(... basically a 90 degree angle between up and down, around a circle...)

And as seems to have been commented here, a rocket has to hit this orbit at like a 90 degree angle -- between straight up and straight down -- at approx. 17000 miles per hour, to maintain a pretty consistent altitude of approx. 210 miles, and consistent speed of 17000 miles per hour, after the engines are shut off.

Is that about right?

a reply to: InachMarbank
Yes.

Couple more questions...

1. What is the source of heat for the thermosphere?

2. Not considering the amount of oxygen and other particles in the air or near vacuum, are the gravitational forces in the near vacuum where the ISS is (~210 miles) any different than the gravitational forces in the air where a commercial jet flies (~5 miles)?

originally posted by: InachMarbank
Couple more questions...

1. What is the source of heat for the thermosphere?

i believe its solar radiation.

2. Not considering the amount of oxygen and other particles in the air or near vacuum, are the gravitational forces in the near vacuum where the ISS is (~210 miles) any different than the gravitational forces in the air where a commercial jet flies (~5 miles)?

it should be, the "force" of gravity gets less the further from the surface you are, although at those differing altitudes the "force" of gravity difference would be minimal.

originally posted by: InachMarbank
Couple more questions...

1. What is the source of heat for the thermosphere?

Absorption of highly energetic solar radiation. Temperatures are highly dependent on solar activity, and can rise to 2,000 °C (3,630 °F). en.wikipedia.org...

2. Not considering the amount of oxygen and other particles in the air or near vacuum, are the gravitational forces in the near vacuum where the ISS is (~210 miles) any different than the gravitational forces in the air where a commercial jet flies (~5 miles)?

Vacuum or air has really nothing to do with gravity, only the distance from Earth's surface has. At the level of the ISS, Earth's gravity is only a tiny bit weaker than on the surface.

a reply to: wildespace

Absorption of highly energetic solar radiation. Temperatures are highly dependent on solar activity, and can rise to 2,000 °C (3,630 °F). en.wikipedia.org...

Approximately, the Earth and the thermosphere are both said to be 93,000,000 miles from the Sun.

Why would the thermosphere absorb heat to such a greater degree?
And why wouldn't this heat make it to Earth?

At the level of the ISS, Earth's gravity is only a tiny bit weaker than on the surface.

Then why does the forward momentum on the ISS continue almost perpetually, at the same speed, with only a one-time, upfront thrust, and no perpetual force?

originally posted by: InachMarbank
a reply to: wildespace

Why would the thermosphere absorb heat to such a greater degree?

I don't know the answer to that question, sorry.

And why wouldn't this heat make it to Earth?

For the same reason it doesn't heat up the ISS or spacewalking astronauts - the molecules are few and far between, so there's very little heat transfer going on.

Then why does the forward momentum on the ISS continue almost perpetually, at the same speed, with only a one-time, upfront thrust, and no perpetual force?

Newton's laws of motion: an object in uniform motion will continue that motion, unless acted on by an external force. There is virtually nothing acting on the ISS (apart from extremely small drag from molecules in themosphere), so it continues doing what it does - falling around the Earth at 17,150 miles per hour.

Perhaps your perception of spaceflight comes from sci-fi movies like Star Wars or Aliens, where spaceships basically fly like fighter jets or jumbo jets, constantly firing their engines and being able to bank and turn like airplanes do. It doesn't work like that in the vacuum of space.

In space, the firing of engines accelerates the craft, and the longer you fire the engines, the more acceleration you give to the craft. Once you shut the engines down, the craft will continue moving in the same direction (bent only by gravity) and at the same speed that it had at the moment of engine shutdown. This is what allows us to send robotic craft to distant parts of the Solar System: they just mostly coast there under their own momentum. They would have to carry gigantic fuel tanks if they had to fire their engines all the way!

originally posted by: InachMarbank

Then why does the forward momentum on the ISS continue almost perpetually, at the same speed, with only a one-time, upfront thrust, and no perpetual force?

at the altitude the ISS orbits at, air friction is basically non-existant.

the velocity/speed it is travelling at has no net force to change its forward momentum.

a reply to: wildespace

Let’s see if I understand this…

Heat from the sun travels 93,000,000 miles, hits the thermosphere, and heats up that region to 2000 to 2500 Celsius. There are almost no particles in the thermosphere, but the few particles that there are, absorb the heat and get to a degree of 2500 Celsius. Then, in less than the final 0.001% of the heats distance traveled, the temperature drops drastically, and is made bearable to live in at Earth's lower atmosphere (troposphere and stratosphere).

Since the heat in the thermosphere comes from the Sun, and the very few particles that are in the thermosphere absorb this heat, why doesn’t the ISS?

And, what is the mechanism in the lower atmosphere (troposphere and stratosphere) that cools the heat down to a bearable temperature? Clouds? Something else?

Oh, and another matter that has occurred to me…
Since there are almost no particles in the thermosphere, does that mean there is no oxygen?

And on to the second matter of discourse…

Is it in accordance with Newton's laws, that gravity is a force pushing toward the center of Earth? (Or at least a straight line fall toward Earth, unless friction sways)?

And as has been stated the gravity force barely changes 210 miles above Earth, but the friction pretty much disappears.

And have the following facts been verified?

In a vacuum, with an initial thrust, you can basically create your own forward gravity. Accelerate to 17100 mph, get an appropriate inclination to Earth, shut off the engines (initial thrust) and orbit is attained. A forward gravitation, in perpetuity, because of the vacuum, combined with whatever co gravity force is pushing down, so that the fall is at a steady arc.

Ok… can anyone help me out with the math?

If an object in orbit is traveling 17100 mph forward,

and the earth is rotating 1040 mph

and the object is moving forward in the same direction the earth is rotating,

and the object attained orbit at an altitude of 210 miles,

and an inclination of 51.6 degrees

and the earth is 24800 miles in circumference

how fast is the object also falling?

Doesn’t there have to be at least these 2 factors:

1. Forward momentum speed,
2. Downward gravity speed,

to make the arc of the fall/orbit?

Or can the arc be established in the acceleration process (initial thrust) from an altitude of 210 miles?

- what I love when few people become uncomfortable for majority of the others! I like your style! let's see how deep and how far are you minded to go!

originally posted by: InachMarbank


Since the heat in the thermosphere comes from the Sun, and the very few particles that are in the thermosphere absorb this heat, why doesn’t the ISS?

The ISS does absorb heat. So does anything else that is in sunlight in space, including a spacewalking astronaut. In the case of the astronaut, spacesuits have a coolant system that circulates cooled liquid through tubes that keep the astronaut cool.

In a vacuum, with an initial thrust, you can basically create your own forward gravity. Accelerate to 17100 mph, get an appropriate inclination to Earth, shut off the engines (initial thrust) and orbit is attained. A forward gravitation, in perpetuity, because of the vacuum, combined with whatever co gravity force is pushing down, so that the fall is at a steady arc.

I don't know what you mean by "create your own forward gravity". Gravity isn't created by something moving. Gravity is a force that all matter intrinsically has whether it is moving or sitting still (relative to a point of reference) -- a pebble has gravity, as does trillions of pebbles altogether in a ball the size of a planet.

Two equally-sized things sitting near each other in space would have an equal gravitational attraction to each other and both would move toward each other by equal amounts. However, something with much more mass than another much smaller thing would have a greater gravitational pull on the other, smaller thing. The ISS, being matter and having mass, has gravity, but the Earth has so very much more mass than the ISS that the gravity of the Earth overwhelms the ISS, so any calculation of the gravitational effects on the ISS from the earth really only needs to take the Earth's gravity into account.

But I don't get what you're saying when you wrote "In a vacuum, with an initial thrust, you can basically create your own forward gravity"

originally posted by: InachMarbank
Ok… can anyone help me out with the math?

If an object in orbit is traveling 17100 mph forward,

and the earth is rotating 1040 mph

and the object is moving forward in the same direction the earth is rotating,

and the object attained orbit at an altitude of 210 miles,

and an inclination of 51.6 degrees

and the earth is 24800 miles in circumference

how fast is the object also falling?

relative to the earth, the object is falling towards earth at roughly 8.6m/s/s (about 88% of 9.81m/s/s)
all those numbers you have given are fairly irrelevant unless you want to work out the forward velocity of the object relative to the earth?

Doesn’t there have to be at least these 2 factors:

1. Forward momentum speed,
2. Downward gravity speed,

to make the arc of the fall/orbit?

Or can the arc be established in the acceleration process (initial thrust) from an altitude of 210 miles?

the arc is obtained from the acceleration process. just change the forward momentum so that it is just the right amount to equalise the drop from gravity, kind of like a balancing act.

originally posted by: sadang
- what I love when few people become uncomfortable for majority of the others! I like your style! let's see how deep and how far are you minded to go!

And what I love more is when such people's questions are answered with solid, tried and tested science.

The rocket scientists have all those spacecraft and space stations successfully doing their thing in space, whether you accept it or not. Even little kids can understand a lot of that science, funny how many adults don't.

originally posted by: InachMarbank
a reply to: wildespace

Let’s see if I understand this…

Heat from the sun travels 93,000,000 miles, hits the thermosphere, and heats up that region to 2000 to 2500 Celsius. There are almost no particles in the thermosphere, but the few particles that there are, absorb the heat and get to a degree of 2500 Celsius. Then, in less than the final 0.001% of the heats distance traveled, the temperature drops drastically, and is made bearable to live in at Earth's lower atmosphere (troposphere and stratosphere).

I'm not really knowlegeable in that area, but I'd guess that lower down in the atmosphere, there are significantly more molecules to absorb that heat, so overall the energy of the sun is more "diluted". For comparison, going for a swim in the sea on a hot sunny day still makes the water feel rather cold, because water is so much denser (and thus a bigger heat sink) than air.

Since the heat in the thermosphere comes from the Sun, and the very few particles that are in the thermosphere absorb this heat, why doesn’t the ISS?

The ISS, the various spacecraft out in space, and spacewalking astronauts, all get hot in the sun, which is why they all employ some sort of thermal insulation and a cooling system. Things in space in direct sunlight do get rather hot!

Oh, and another matter that has occurred to me…
Since there are almost no particles in the thermosphere, does that mean there is no oxygen?

The few particles that are there do include oxygen. Oxygen molecules up there is what creates the airglow and some of the aurora's colours.

a reply to: Soylent Green Is People

The ISS does absorb heat. So does anything else that is in sunlight in space, including a spacewalking astronaut. In the case of the astronaut, spacesuits have a coolant system that circulates cooled liquid through tubes that keep the astronaut cool.

So since it seems concluded here, anything in the thermosphere would absorb heat, and particles in the thermosphere can heat up to 2500 degrees Celsius, should we consider this?
According to the known melting points of all the 118 elements listed here:
www.lenntech.com...
there are only 6 elements that have a known melting point higher than 2500 degrees celsius, enough to withstand the heat of the thermosphere. These elements, and there melting points in Celsius are:
Molybdenum, 2617
Tantalum, 2996
Osmium, 3045
Rhenium, 3180
Tungsten, 3410
Carbon, 3500
Carbon seems the best choice, but it conducts heat very well, too, so I'm not sure yet if any person could live inside a carbon space ship in the thermosphere without being vaporized. But the carbon itself could probably withstand the heat, right?
Isn't the ISS built from aluminum?
The melting point of aluminum is 660 degrees Celsius, and at 2500 degrees Celsius the aluminum would eventually become vapor wouldn't it?

Gravity is a force that all matter intrinsically has whether it is moving or sitting still (relative to a point of reference) -- a pebble has gravity, as does trillions of pebbles altogether in a ball the size of a planet.

Two equally-sized things sitting near each other in space would have an equal gravitational attraction to each other and both would move toward each other by equal amounts. However, something with much more mass than another much smaller thing would have a greater gravitational pull on the other, smaller thing.

OK now I'm confused.
I'm matter. Do I have gravity? Can I attract a smaller object toward me, without actually picking it up?

But I don't get what you're saying when you wrote "In a vacuum, with an initial thrust, you can basically create your own forward gravity"

I meant, according to what has been stated here, in a vacuum, all you need to create perpetual motion is a one-time thrust;
I can't think of many other perpetual motions, other than those effected by gravity;
so it's like saying, in a vacuum, you can be like gravity... create a perpetual motion from a one-time thrust...
Oh, actually, scratch that, this seems like a pretty convincing perpetual motion machine, so long as the magnets keep working.
www.youtube.com...