Orbits. They're mandatory!
Hello fans! I hope the five of you are doing well.
It's time to mess with another control on the old blog control panel. It's been a while since I've posted and some new info needs to be imparted. Today I'm going to turn the orbit knob. Let's begin shall we?
Zero gravity doesn't exist in the strictest sense and here's why. I think it was Newton who explained things this way. Imagine firing a cannon ball, tangential to the Earth's surface, from a cannon. It flies a certain distance and then hits the ground. If you fire it faster, it flies further, then hits the ground. If you could fire it with enough velocity, it would try to hit the ground, but the curvature of the Earth would match the cannon ball's downward path and the cannon ball would essentially fall off of the Earth. This theoretical cannon ball would be in orbit. I apologize if you've already heard this much used example.
If you were the cannon ball, you would feel as if there were no gravity. In fact, what you would be feeling is a constant state of free fall, like skydivers experience when they jump from a plane. Gravity is very much effecting you, so it's not zero g. Orbiting is essentially a constant state of free fall. Even in deep space there is always some amount of gravity from distant planets and stars, even if it is immeasurable. Incidentally, I think it is just fine to use the term 'zero g' because there really is no such thing as zero g we might as well define it as the experience of 'near zero g' or of constant free fall, right? It's essentially the 'feeling' of weightlessness. And here's a fun fact. In a vacuum a cannon ball and a feather do NOT fall at the same rate. I'm always surprised at how many shows like to point out that wind resistance is the only thing that keeps the feather from falling at the same rate as the cannon ball. The fact is that the force of attraction between two bodies depends on the mass of BOTH bodies. So unless the feather is as massive as the cannon ball, its attraction to the Earth will be smaller. The difference is so minute that it is completely discarded by scientists in their examples, but if you want to get technical, the cannon ball is faster!
A big difference in free fall experienced by a sky diver and an astronaut is the effect of wind resistance. If you actually fired this theoretical cannon ball from the Earth's surface, it would be subject to drag caused by the wind. Drag is kind of a big deal, too. It increases with the square of the velocity. Double the velocity, quadruple the drag. It would be hard to launch a cannon ball into orbit. First you would have to be above the highest object in its path, to keep it from hitting a mountain or other structure, then it would have to have enough velocity to overcome wind resistance but not so much that it destroyed the cannon ball due to heating and compression from the increased wind pressure. Finally, I'm pretty sure its orbit would be a ballistic orbit, meaning that it would intersect with the Earth after going into space. I'll leave it as an exercise for the reader to figure it out. But if you actually try the experiment, contact me. I want to see it. That said, in the world of sci-fi, you could realistically place an object into orbit at an altitude of 10 or 20 feet if you had the right circumstances. What if you were on a very round, flat planet, or better yet, an asteroid, that had a smaller gravitational pull and no atmosphere? This has already been postulated by sci-fi authors, but the idea of a satellite whipping by, just over your head every 30 minutes or so, is really cool, isn't it? Oh and just because the astronaut is in outer space does not mean he isn't subject to atmospheric drag. As it turns out the majority of the Earth's atmosphere ends at about 100 km above the Earth, but there is still a decreasingly faint wisp of atmosphere that extends out thousands of kilometers from the Earth's surface and in LEO (Low Earth Orbit) it is enough drag to eventually slow a satellite down and cause it to gradually lose altitude until it plummets to the Earth. This is why satellites have maneuvering thrusters. It allows them to make course corrections and to counteract this tiny drag over time. But when the satellite runs out of fuel, you get an aborigine in Australia picking up pieces of antenna material and thruster nozzles. You can see an example of the atmosphere here: http://www.astronomycafe.net/qadir/q2...
So getting back to orbits. . . . I mentioned a moment ago that I think the cannon ball would have a ballistic orbit, one shared by ICBMs (InterContinental Ballistic Missiles), which go briefly into orbit and then return to Earth, usually with tragic consequences. This implies that there are different orbits. It's a common mistake in sci-fi to always assume a circular orbit. It's not as common a mistake as thinking an orbit isn't needed at all, but that argument is coming up in a few more paragraphs. Let's get to the heart of the matter shall we?
Everything is in orbit. Everything! Technically you cannot bring a space craft to a complete stop. It wouldn't even make sense. Let me explain. Pretend you are on a merry-go-round on a cruise ship traveling across the pond and you want to stop. If your horse stops moving up and down and you looked at just the floor of the Merry-go-round, you might conclude that you have stopped. But if you looked at the cruise ship, you'd find that you were still traveling in a circle. Stop the merry-go-round and you are still moving on a cruise ship. Stop the cruise ship and you are still spinning with the Earth. Stop the Earth and you are still circling the sun. Let's take a break. Ok, break's over. Stop the sun from circling the galaxy and the galaxy is still circling the local cluster. Ok, I'll stop. Or rather I won't stop because as I've just pointed out all you can really do is stop relative to something else. If you are an author and you have your captain order the ship to come to a stop, everyone will probably understand what you are saying but you might be missing an opportunity to have the captain order the ship to 'match velocities with' or 'orbits with' or 'assume the same orbit as' another ship and thereby with one sentence add a bit of realism to the story.
Returning to the cannon ball, if you wish to put this thing into orbit, whether it's on Earth or some distant moon, you will need a lot of velocity. You will NOT need escape velocity. Please don't use escape velocity in a story unless you are leaving the Earth entirely and even then a lot of times it doesn't make sense. Escape velocity is the velocity needed for the cannon ball to entirely break free from the capturing effect of the planet. The space shuttle did not reach escape velocity. It was still orbiting the Earth. Even the moon is still captured by the Earth, so the Apollo missions, also did not reach escape velocity. Well, maybe they did and slowed down again, I really don't know, but they certainly didn't escape being captured by the Earth. Furthermore, escape velocity varies with the distance from the object. The escape velocity needed for the International Space Station to reach escape velocity is less than the cannon ball on Earth and a cannon ball on the top of Mount Everest would need less velocity to escape Earth's grasp than would a cannon ball on Miami beach. Plus, with unlimited power, a 1 mile an hour ascent is enough to escape the Earth. The cannon ball only has what velocity was imparted to it to begin with, but attach little rockets that can thrust infinitely, it could travel beyond the solar system at a pokey parking lot speed.
Once the cannon ball is in orbit, it will probably have an elliptical orbit. Additional thrust must be given to the cannon ball to circularize it's orbit, assuming you want a circular orbit. Plus, technically a circular orbit is still elliptical, it is just a special case of elliptical where the major and minor axes are equal. Like a square is still a rectangle and a Jacuzzi is still a hot tub. There are advantages to an eccentric elliptical orbit. Spy satellites for instance use them. You see there was this clever guy called Kepler who noticed that over a given time period a planet sweeps out a constant area. That means if you draw a line between say the center of the moon and the center of the Earth and then wait an hour and draw another line between their centers, the area created by those two lines, the curve created by the orbit of the moon and the center of the earth will be the same regardless of which hour in the day you measure it out. A picture is worth a 1000 words so look here, but come back:
https://encrypted-tbn2.gstatic.com/im...
All the blue areas are equal. For this to happen the orbital velocity varies. The satellite, the moon in this case, speeds up as it gets closer to Earth and slows down when it gets farther. A spy satellite might use a highly eccentric orbit so that when the satellite is far from Earth it travel slowly, for a longer period of time. This is called dwell. Having a long dwell time over an unnamed area, like Iraq, would allow cameras more time to take incriminating photos. The trade-off of course is that the cameras are farther away, so some compromises must be made. I stole the picture above from this website which does a great job of explaining Kepler's laws. Hope they don't mind: https://en.wikibooks.org/wiki/General....
The unrealistic travel of spacecraft in Star Wars and many other sci-fi stories is made possible only by having abundant quantities of energy and speed coupled with some kind of time warp and inertial dampers not found in reality. For instance in Star Wars the Millennium Falcon flies by Yavin prime to arrive at the fourth moon in what are presumed to be seconds or minutes. As an example, Neptune and the Earth are about 4.1 light hours apart at their closest. This means even traveling at the speed of light directly at the Earth from Neptune a ship would miss the planet by nearly 442,000 km. That is about 57,000 kilometers beyond the orbit of the moon. Pretty close by galactic standards but a little too far to grab a beer. The point is, even traveling at the speed of light within a solar system would require an orbit of sorts in order to rendezvous with another body in space. The Millennium Falcon flies from planet to moon in seconds, which would require traveling at significant portions of the speed of light, which also means that time dilation effects would be apparent and how do you accelerate to that speed without killing the occupants, etc., etc., etc.
Here's a cool clip showing the Millennium Falcon's journey:
https://www.youtube.com/watch?v=45cev...
In reality to move from one orbit to another, whether it be around the planet or around the sun requires what is called a transfer orbit. A circular orbit close to a planet requires a great deal of velocity to keep from being pulled into that strong gravity well. To overcome that gravity and boost to a higher orbit means increasing speed. All along the way, assuming only one big burst to speed up, the space craft is slowing down as gravity drags it back to Earth. When it gets to the new orbit, it will be moving too slow to maintain that orbit and must again accelerate to whatever speed is needed to maintain it. Unless the new orbit corresponds to a large mass like a planet in which case the pull of the new planet will require slowing down to maintain the new orbit. The nature of this pull is the source for the Oberth and Slingshot effects. Look those up on your own. One of the most efficient transfer orbits is called the Hohmann Transfer Orbit, which I will explain. If you had infinite velocity at your disposal and you wanted to go to Mars from Earth, you would just point and go. A little bit slower and you would have to lead the planet a little so that you and Mars arrive at the same point at the same time. Because you are already orbiting (even if you are on the Earths surface you are moving in a rotating fashion with the earth) your path will not be straight because you essentially have to keep orbiting to prevent falling to Earth as I've explained throughout this blog. So essentially you would open up your orbit, making it more eccentric until it was so big it intersected with the higher orbit of Mars. Calculate how long this will take and leave at the appropriate time (launch window) to meet Mars at your future get together. As it turns out, if you wish to use a low amount of energy to accomplish that the Hohmann transfer orbit will have you leaving on one side of the sun and encountering Mars exactly at the other side of the sun. There are other orbits (see fuzzy orbit) but the Hohmann transfer orbit is one of the most efficient and of course, means it takes a long time, about eight and a half months in the case of Earth and Mars.
All of this orbit stuff can really get confusing. For instance, it is unintuitive that the inner planets must move faster to overcome the increased gravity close to the sun, yet to move to a higher orbit, they would have to speed up. So fast seems to be equal to both a higher and a lower orbit. This is yet another reason how reality is often far more interesting than fiction.
So that concludes my effort to turn the orbit knob. I'm sorry it took so long, but the one thing I want people to take away from this is that spaceships in reality do not move like they do in the movies. There is no reason for a spaceship to do a U-turn. To go the other way, it would simply rotate about it's own axis until it was pointed in the opposite direction it is traveling and hit the thrusters until it 'stops' and goes the other way (neglecting all of the orbit stuff I just told you about). This is well illustrated by the ancient game Asteroids. If you are not familiar with this classic, stop reading this, find and emulator and play it. High resolution graphics aren't the only way to have fun. Anyway, there is no reason for a spaceship to bank or even have wings, unless it goes into the atmosphere to use them. And importantly the ship would not just fly to some other place in straight lines and without orbital maneuvers unless it is abiding by false sci-fi physics. If you are writing sci-fi and don't really care about these things, then there really is no need to do any research or calculating. Just leave the orbital knob alone and make it all up. Hopefully you will be consistent if you do. No one likes to hear how a ship can go ten times as far on one trip than another in less time without some kind of explanation. However, if you are writing hard sci-fi or expect the reader to believe your adventurers are actually traveling through space, do some research on how they get there and start turning some of your own knobs.
It's time to mess with another control on the old blog control panel. It's been a while since I've posted and some new info needs to be imparted. Today I'm going to turn the orbit knob. Let's begin shall we?
Zero gravity doesn't exist in the strictest sense and here's why. I think it was Newton who explained things this way. Imagine firing a cannon ball, tangential to the Earth's surface, from a cannon. It flies a certain distance and then hits the ground. If you fire it faster, it flies further, then hits the ground. If you could fire it with enough velocity, it would try to hit the ground, but the curvature of the Earth would match the cannon ball's downward path and the cannon ball would essentially fall off of the Earth. This theoretical cannon ball would be in orbit. I apologize if you've already heard this much used example.
If you were the cannon ball, you would feel as if there were no gravity. In fact, what you would be feeling is a constant state of free fall, like skydivers experience when they jump from a plane. Gravity is very much effecting you, so it's not zero g. Orbiting is essentially a constant state of free fall. Even in deep space there is always some amount of gravity from distant planets and stars, even if it is immeasurable. Incidentally, I think it is just fine to use the term 'zero g' because there really is no such thing as zero g we might as well define it as the experience of 'near zero g' or of constant free fall, right? It's essentially the 'feeling' of weightlessness. And here's a fun fact. In a vacuum a cannon ball and a feather do NOT fall at the same rate. I'm always surprised at how many shows like to point out that wind resistance is the only thing that keeps the feather from falling at the same rate as the cannon ball. The fact is that the force of attraction between two bodies depends on the mass of BOTH bodies. So unless the feather is as massive as the cannon ball, its attraction to the Earth will be smaller. The difference is so minute that it is completely discarded by scientists in their examples, but if you want to get technical, the cannon ball is faster!
A big difference in free fall experienced by a sky diver and an astronaut is the effect of wind resistance. If you actually fired this theoretical cannon ball from the Earth's surface, it would be subject to drag caused by the wind. Drag is kind of a big deal, too. It increases with the square of the velocity. Double the velocity, quadruple the drag. It would be hard to launch a cannon ball into orbit. First you would have to be above the highest object in its path, to keep it from hitting a mountain or other structure, then it would have to have enough velocity to overcome wind resistance but not so much that it destroyed the cannon ball due to heating and compression from the increased wind pressure. Finally, I'm pretty sure its orbit would be a ballistic orbit, meaning that it would intersect with the Earth after going into space. I'll leave it as an exercise for the reader to figure it out. But if you actually try the experiment, contact me. I want to see it. That said, in the world of sci-fi, you could realistically place an object into orbit at an altitude of 10 or 20 feet if you had the right circumstances. What if you were on a very round, flat planet, or better yet, an asteroid, that had a smaller gravitational pull and no atmosphere? This has already been postulated by sci-fi authors, but the idea of a satellite whipping by, just over your head every 30 minutes or so, is really cool, isn't it? Oh and just because the astronaut is in outer space does not mean he isn't subject to atmospheric drag. As it turns out the majority of the Earth's atmosphere ends at about 100 km above the Earth, but there is still a decreasingly faint wisp of atmosphere that extends out thousands of kilometers from the Earth's surface and in LEO (Low Earth Orbit) it is enough drag to eventually slow a satellite down and cause it to gradually lose altitude until it plummets to the Earth. This is why satellites have maneuvering thrusters. It allows them to make course corrections and to counteract this tiny drag over time. But when the satellite runs out of fuel, you get an aborigine in Australia picking up pieces of antenna material and thruster nozzles. You can see an example of the atmosphere here: http://www.astronomycafe.net/qadir/q2...
So getting back to orbits. . . . I mentioned a moment ago that I think the cannon ball would have a ballistic orbit, one shared by ICBMs (InterContinental Ballistic Missiles), which go briefly into orbit and then return to Earth, usually with tragic consequences. This implies that there are different orbits. It's a common mistake in sci-fi to always assume a circular orbit. It's not as common a mistake as thinking an orbit isn't needed at all, but that argument is coming up in a few more paragraphs. Let's get to the heart of the matter shall we?
Everything is in orbit. Everything! Technically you cannot bring a space craft to a complete stop. It wouldn't even make sense. Let me explain. Pretend you are on a merry-go-round on a cruise ship traveling across the pond and you want to stop. If your horse stops moving up and down and you looked at just the floor of the Merry-go-round, you might conclude that you have stopped. But if you looked at the cruise ship, you'd find that you were still traveling in a circle. Stop the merry-go-round and you are still moving on a cruise ship. Stop the cruise ship and you are still spinning with the Earth. Stop the Earth and you are still circling the sun. Let's take a break. Ok, break's over. Stop the sun from circling the galaxy and the galaxy is still circling the local cluster. Ok, I'll stop. Or rather I won't stop because as I've just pointed out all you can really do is stop relative to something else. If you are an author and you have your captain order the ship to come to a stop, everyone will probably understand what you are saying but you might be missing an opportunity to have the captain order the ship to 'match velocities with' or 'orbits with' or 'assume the same orbit as' another ship and thereby with one sentence add a bit of realism to the story.
Returning to the cannon ball, if you wish to put this thing into orbit, whether it's on Earth or some distant moon, you will need a lot of velocity. You will NOT need escape velocity. Please don't use escape velocity in a story unless you are leaving the Earth entirely and even then a lot of times it doesn't make sense. Escape velocity is the velocity needed for the cannon ball to entirely break free from the capturing effect of the planet. The space shuttle did not reach escape velocity. It was still orbiting the Earth. Even the moon is still captured by the Earth, so the Apollo missions, also did not reach escape velocity. Well, maybe they did and slowed down again, I really don't know, but they certainly didn't escape being captured by the Earth. Furthermore, escape velocity varies with the distance from the object. The escape velocity needed for the International Space Station to reach escape velocity is less than the cannon ball on Earth and a cannon ball on the top of Mount Everest would need less velocity to escape Earth's grasp than would a cannon ball on Miami beach. Plus, with unlimited power, a 1 mile an hour ascent is enough to escape the Earth. The cannon ball only has what velocity was imparted to it to begin with, but attach little rockets that can thrust infinitely, it could travel beyond the solar system at a pokey parking lot speed.
Once the cannon ball is in orbit, it will probably have an elliptical orbit. Additional thrust must be given to the cannon ball to circularize it's orbit, assuming you want a circular orbit. Plus, technically a circular orbit is still elliptical, it is just a special case of elliptical where the major and minor axes are equal. Like a square is still a rectangle and a Jacuzzi is still a hot tub. There are advantages to an eccentric elliptical orbit. Spy satellites for instance use them. You see there was this clever guy called Kepler who noticed that over a given time period a planet sweeps out a constant area. That means if you draw a line between say the center of the moon and the center of the Earth and then wait an hour and draw another line between their centers, the area created by those two lines, the curve created by the orbit of the moon and the center of the earth will be the same regardless of which hour in the day you measure it out. A picture is worth a 1000 words so look here, but come back:
https://encrypted-tbn2.gstatic.com/im...
All the blue areas are equal. For this to happen the orbital velocity varies. The satellite, the moon in this case, speeds up as it gets closer to Earth and slows down when it gets farther. A spy satellite might use a highly eccentric orbit so that when the satellite is far from Earth it travel slowly, for a longer period of time. This is called dwell. Having a long dwell time over an unnamed area, like Iraq, would allow cameras more time to take incriminating photos. The trade-off of course is that the cameras are farther away, so some compromises must be made. I stole the picture above from this website which does a great job of explaining Kepler's laws. Hope they don't mind: https://en.wikibooks.org/wiki/General....
The unrealistic travel of spacecraft in Star Wars and many other sci-fi stories is made possible only by having abundant quantities of energy and speed coupled with some kind of time warp and inertial dampers not found in reality. For instance in Star Wars the Millennium Falcon flies by Yavin prime to arrive at the fourth moon in what are presumed to be seconds or minutes. As an example, Neptune and the Earth are about 4.1 light hours apart at their closest. This means even traveling at the speed of light directly at the Earth from Neptune a ship would miss the planet by nearly 442,000 km. That is about 57,000 kilometers beyond the orbit of the moon. Pretty close by galactic standards but a little too far to grab a beer. The point is, even traveling at the speed of light within a solar system would require an orbit of sorts in order to rendezvous with another body in space. The Millennium Falcon flies from planet to moon in seconds, which would require traveling at significant portions of the speed of light, which also means that time dilation effects would be apparent and how do you accelerate to that speed without killing the occupants, etc., etc., etc.
Here's a cool clip showing the Millennium Falcon's journey:
https://www.youtube.com/watch?v=45cev...
In reality to move from one orbit to another, whether it be around the planet or around the sun requires what is called a transfer orbit. A circular orbit close to a planet requires a great deal of velocity to keep from being pulled into that strong gravity well. To overcome that gravity and boost to a higher orbit means increasing speed. All along the way, assuming only one big burst to speed up, the space craft is slowing down as gravity drags it back to Earth. When it gets to the new orbit, it will be moving too slow to maintain that orbit and must again accelerate to whatever speed is needed to maintain it. Unless the new orbit corresponds to a large mass like a planet in which case the pull of the new planet will require slowing down to maintain the new orbit. The nature of this pull is the source for the Oberth and Slingshot effects. Look those up on your own. One of the most efficient transfer orbits is called the Hohmann Transfer Orbit, which I will explain. If you had infinite velocity at your disposal and you wanted to go to Mars from Earth, you would just point and go. A little bit slower and you would have to lead the planet a little so that you and Mars arrive at the same point at the same time. Because you are already orbiting (even if you are on the Earths surface you are moving in a rotating fashion with the earth) your path will not be straight because you essentially have to keep orbiting to prevent falling to Earth as I've explained throughout this blog. So essentially you would open up your orbit, making it more eccentric until it was so big it intersected with the higher orbit of Mars. Calculate how long this will take and leave at the appropriate time (launch window) to meet Mars at your future get together. As it turns out, if you wish to use a low amount of energy to accomplish that the Hohmann transfer orbit will have you leaving on one side of the sun and encountering Mars exactly at the other side of the sun. There are other orbits (see fuzzy orbit) but the Hohmann transfer orbit is one of the most efficient and of course, means it takes a long time, about eight and a half months in the case of Earth and Mars.
All of this orbit stuff can really get confusing. For instance, it is unintuitive that the inner planets must move faster to overcome the increased gravity close to the sun, yet to move to a higher orbit, they would have to speed up. So fast seems to be equal to both a higher and a lower orbit. This is yet another reason how reality is often far more interesting than fiction.
So that concludes my effort to turn the orbit knob. I'm sorry it took so long, but the one thing I want people to take away from this is that spaceships in reality do not move like they do in the movies. There is no reason for a spaceship to do a U-turn. To go the other way, it would simply rotate about it's own axis until it was pointed in the opposite direction it is traveling and hit the thrusters until it 'stops' and goes the other way (neglecting all of the orbit stuff I just told you about). This is well illustrated by the ancient game Asteroids. If you are not familiar with this classic, stop reading this, find and emulator and play it. High resolution graphics aren't the only way to have fun. Anyway, there is no reason for a spaceship to bank or even have wings, unless it goes into the atmosphere to use them. And importantly the ship would not just fly to some other place in straight lines and without orbital maneuvers unless it is abiding by false sci-fi physics. If you are writing sci-fi and don't really care about these things, then there really is no need to do any research or calculating. Just leave the orbital knob alone and make it all up. Hopefully you will be consistent if you do. No one likes to hear how a ship can go ten times as far on one trip than another in less time without some kind of explanation. However, if you are writing hard sci-fi or expect the reader to believe your adventurers are actually traveling through space, do some research on how they get there and start turning some of your own knobs.
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