EVERYONE KNOWS THE INTERNATIONAL SPACE STATION. This means it has been in low Earth orbit for over 20 years. This means that it is about 400 kilometers above the Earth's surface and is moving at a speed of 7.66 kilometers per second. (For the record, it's very fast.) At this speed, the ISS takes about 90 minutes to complete one orbit. With 16 orbits per day for more than two decades, this represents more than 100,000 orbits around the world. If you are in the right place, you can see it with the naked eye or with your smartphone.
But things don't last forever, not even space stations. NASA announces that the ISS will be de-orbited in 2031. This means that they will intentionally dump them in the ocean.
It seems like a waste to throw away an absolutely awesome space station. Wouldn't it be great to have the ISS in a museum set up for everyday people to walk through something that has been in space for so long? We could all feel like astronauts.
Let's see what it would take to save the ISS.
Can't We Just Leave It in Orbit?
It seems the best place for the ISS is space. There's a catch though: it won't sit there without occasionally bumping into each other. Without one, it will eventually crash to earth. Deliberately knocking it out of orbit is a way to ensure it lands in an empty ocean and not on top of someone's house.
Low Earth Orbit or LEO is only a temporary location. In an ideal orbit, such as the moon's orbit around our planet, the object moves solely due to its gravitational interaction with the Earth. This creates a force on the object that pulls it towards the center of the earth when it moves in a direction perpendicular to the force. If the object has the correct speed, it will move in a circle. It's like swinging a ball on a string in a circle around your head, except in this case the string replaces the force of gravity.
But for an object like a satellite or a space station in LEO around the planet, there is another force: an interaction with the atmosphere. You have probably heard that there is no air in space. This is largely correct. As you move away from the Earth's surface, the atmosphere thins, which means its density decreases. But atmospheric density does not magically cancel out at any given altitude. Instead, it just fades.
This means that at an altitude of 400 km (in LEO, where the ISS orbits), there is not much air, but there is. The very fast moving space station collides with this small amount of air to create a very small drag force pushing in the opposite direction to the speed of the space station. This slowdown will eventually cause the ISS to move to lower altitudes where there is even more air and even more drag. Things get pretty messy with orbital mechanics, but this tug would eventually crash the space station to earth. This is exactly what happened to the Chinese space station Tiangong-1.
To keep the ISS in orbit until 2031, the space agencies that maintain it must take regular measures to counter this drag. The ISS doesn't have its own rocket engines, so it needs a boost or nudge from a supply ship. An impulse pushes the space station and increases its speed. (Here's a bonus: my analysis of what it's like to be an astronaut in the ISS during a reboot, published on the European Space Agency blog.)
Would the ISS Burn Up on Reentry?
While re-entry can be a violent event and can completely destroy many objects, it's entirely possible that something the size of the ISS will survive, at least in part. For example, parts of Skylab passed through the atmosphere on re-entry in 1979 and landed on Earth as debris.
But anything that falls into the atmosphere gets very hot. Objects in orbit move very quickly and when they start moving through the atmosphere they push air in front of them because that air is in their path. Some of that air is pushed to the side, but a lot of it is pushed forward. This is a problem because there is already air in it. When more air is forced into the same space, compression occurs. You may have noticed while inflating a bicycle tire that the tire gets hotter the more air you inflate; This is because you are compressing the air that is already in the tube. The same thing happens when an object is moving rapidly through the atmosphere: the compressed air in front of it heats up and the object itself heats up. As "things melt" heat levels.
Some spacecraft, like the Space Shuttle or SpaceX's Crew Dragon, have a heat shield, a material that insulates the rest of the craft from all that hot air. But the ISS has no heat shield. So at least some parts of it would burn on re-entry.
The debris that remains might make it a museum exhibit, but not one you can walk through.
Could We Get the ISS Down Without a Normal Reentry?
There is a difference between re-entry and simply falling out of space. Simply picking up an object 400 kilometers away and dropping it is very different from re-entry. Keep in mind that objects in LEO move very quickly, while a "fallen" object would start moving at zero meters per second. Yes, the ejected object would accelerate and heat up, but not as hot as an object re-entering orbit.
So think about this: what if we used a few rockets to stop the ISS in orbit and then bring it straight back down to avoid the whole "burn-up on re-entry" problem?
Let's see what happens with some simple calculations. We can start with Newton's second law. This gives a relationship between a net force on an object and the acceleration of that object. In one dimension, it looks like this:
Yes, the m in this equation is the mass, and the mass of the ISS is 444,615 kilograms, but let's just call it 450,000. The a is the acceleration or rate of change of velocity.
So if we assume that the ISS is decelerating at a constant rate, then the acceleration would be:
where v2 is the terminal velocity (which would be zero m/s) and v1 is the initial velocity (orbital velocity of 7.66 x 103 m/s).
But what about the time interval Δt? Assuming we can slow the ISS down for one orbit, that would be 90 minutes or 5400 seconds. With these values we can calculate the acceleration. Multiply that by the mass of the ISS and you get the average thrust it would take for a rocket to bring this orbiting space station to a halt.
Plugging in the numbers gives a rocket thrust of 6.31 x 105 Newtons. That's about half the total thrust of a Boeing 747. Of course, you can't use a 747 engine because it needs air, and there isn't enough air in low earth orbit to make it work.
I guess that means we need a rocket. How about a Merlin 1D vacuum motor? These are the types used in SpaceX's Falcon Heavy second stage. Rocket engines generate thrust by ejecting mass (fuel) from a nozzle. You can get more thrust by increasing the rate of fuel consumption or by increasing the speed of material as it exits the engines. The Merlin 1D can generate thrust of up to 981,000 Newtons. When you decrease fuel flow, you also decrease thrust, but this increases the life of your fuel.
One way to describe rocket performance is in terms of specific impulse. If you take the average thrust of the rocket and multiply it by the time interval the rocket fires, you get the thrust.
Dividing the momentum by the weight of the rocket gives the specific impulse. The Merlin 1D has a specific impulse of 348 seconds:
where g is the gravitational field at the earth's surface (9.8 Nk/kg).
Knowing the thrust and the time interval, I can use that to calculate the total mass needed to stop the ISS in orbit. This results in a mass of nearly 1 million kilograms. If fuel had the same density as water, it would fill about half of an Olympic swimming pool. Yes, that's a lot of fuel. In addition, the rocket would have to be sent into space, which would require even more fuel.
Well, maybe you can understand why spaceships don't use rockets to get out of orbit. It would consume too much fuel. Using a heat shield and the earth's atmosphere to slow you down is free, and no one wants to deny it.
But unless it's possible to stop the ISS before it's shot through the atmosphere, there's really no hope of it returning to Earth in one piece.
So if we are not happy with the other two options, leave it in LEO and reboot once in a while or let it come back and crash into the ocean, there is only one option left. We could put it in a higher orbit where there is virtually no drag and it could stay there undisturbed. Of course, it would take more energy to get there and provide that thrust, so you would need a bigger rocket. And you don't want it to become high-flying space junk that could endanger other ships.
Personally, I prefer the latter variant. It would be like turning the ISS into a time capsule. And once we finally understand commercial space travel, it would make a great "floating" museum exhibit in space.
Read Also : What are 10 ways to stay fit?
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EVERYONE KNOWS THE INTERNATIONAL SPACE STATION. This means it has been in low Earth orbit for over 20 years. This means that it is about 400 kilometers above the Earth's surface and is moving at a speed of 7.66 kilometers per second. (For the record, it's very fast.) At this speed, the ISS takes about 90 minutes to complete one orbit. With 16 orbits per day for more than two decades, this represents more than 100,000 orbits around the world. If you are in the right place, you can see it with the naked eye or with your smartphone.
But things don't last forever, not even space stations. NASA announces that the ISS will be de-orbited in 2031. This means that they will intentionally dump them in the ocean.
It seems like a waste to throw away an absolutely awesome space station. Wouldn't it be great to have the ISS in a museum set up for everyday people to walk through something that has been in space for so long? We could all feel like astronauts.
Let's see what it would take to save the ISS.
Can't We Just Leave It in Orbit?
It seems the best place for the ISS is space. There's a catch though: it won't sit there without occasionally bumping into each other. Without one, it will eventually crash to earth. Deliberately knocking it out of orbit is a way to ensure it lands in an empty ocean and not on top of someone's house.
Low Earth Orbit or LEO is only a temporary location. In an ideal orbit, such as the moon's orbit around our planet, the object moves solely due to its gravitational interaction with the Earth. This creates a force on the object that pulls it towards the center of the earth when it moves in a direction perpendicular to the force. If the object has the correct speed, it will move in a circle. It's like swinging a ball on a string in a circle around your head, except in this case the string replaces the force of gravity.
But for an object like a satellite or a space station in LEO around the planet, there is another force: an interaction with the atmosphere. You have probably heard that there is no air in space. This is largely correct. As you move away from the Earth's surface, the atmosphere thins, which means its density decreases. But atmospheric density does not magically cancel out at any given altitude. Instead, it just fades.
This means that at an altitude of 400 km (in LEO, where the ISS orbits), there is not much air, but there is. The very fast moving space station collides with this small amount of air to create a very small drag force pushing in the opposite direction to the speed of the space station. This slowdown will eventually cause the ISS to move to lower altitudes where there is even more air and even more drag. Things get pretty messy with orbital mechanics, but this tug would eventually crash the space station to earth. This is exactly what happened to the Chinese space station Tiangong-1.
To keep the ISS in orbit until 2031, the space agencies that maintain it must take regular measures to counter this drag. The ISS doesn't have its own rocket engines, so it needs a boost or nudge from a supply ship. An impulse pushes the space station and increases its speed. (Here's a bonus: my analysis of what it's like to be an astronaut in the ISS during a reboot, published on the European Space Agency blog.)
Would the ISS Burn Up on Reentry?
While re-entry can be a violent event and can completely destroy many objects, it's entirely possible that something the size of the ISS will survive, at least in part. For example, parts of Skylab passed through the atmosphere on re-entry in 1979 and landed on Earth as debris.
But anything that falls into the atmosphere gets very hot. Objects in orbit move very quickly and when they start moving through the atmosphere they push air in front of them because that air is in their path. Some of that air is pushed to the side, but a lot of it is pushed forward. This is a problem because there is already air in it. When more air is forced into the same space, compression occurs. You may have noticed while inflating a bicycle tire that the tire gets hotter the more air you inflate; This is because you are compressing the air that is already in the tube. The same thing happens when an object is moving rapidly through the atmosphere: the compressed air in front of it heats up and the object itself heats up. As "things melt" heat levels.
Some spacecraft, like the Space Shuttle or SpaceX's Crew Dragon, have a heat shield, a material that insulates the rest of the craft from all that hot air. But the ISS has no heat shield. So at least some parts of it would burn on re-entry.
The debris that remains might make it a museum exhibit, but not one you can walk through.
Could We Get the ISS Down Without a Normal Reentry?
There is a difference between re-entry and simply falling out of space. Simply picking up an object 400 kilometers away and dropping it is very different from re-entry. Keep in mind that objects in LEO move very quickly, while a "fallen" object would start moving at zero meters per second. Yes, the ejected object would accelerate and heat up, but not as hot as an object re-entering orbit.
So think about this: what if we used a few rockets to stop the ISS in orbit and then bring it straight back down to avoid the whole "burn-up on re-entry" problem?
Let's see what happens with some simple calculations. We can start with Newton's second law. This gives a relationship between a net force on an object and the acceleration of that object. In one dimension, it looks like this:
Yes, the m in this equation is the mass, and the mass of the ISS is 444,615 kilograms, but let's just call it 450,000. The a is the acceleration or rate of change of velocity.
So if we assume that the ISS is decelerating at a constant rate, then the acceleration would be:
where v2 is the terminal velocity (which would be zero m/s) and v1 is the initial velocity (orbital velocity of 7.66 x 103 m/s).
But what about the time interval Δt? Assuming we can slow the ISS down for one orbit, that would be 90 minutes or 5400 seconds. With these values we can calculate the acceleration. Multiply that by the mass of the ISS and you get the average thrust it would take for a rocket to bring this orbiting space station to a halt.
Plugging in the numbers gives a rocket thrust of 6.31 x 105 Newtons. That's about half the total thrust of a Boeing 747. Of course, you can't use a 747 engine because it needs air, and there isn't enough air in low earth orbit to make it work.
I guess that means we need a rocket. How about a Merlin 1D vacuum motor? These are the types used in SpaceX's Falcon Heavy second stage. Rocket engines generate thrust by ejecting mass (fuel) from a nozzle. You can get more thrust by increasing the rate of fuel consumption or by increasing the speed of material as it exits the engines. The Merlin 1D can generate thrust of up to 981,000 Newtons. When you decrease fuel flow, you also decrease thrust, but this increases the life of your fuel.
One way to describe rocket performance is in terms of specific impulse. If you take the average thrust of the rocket and multiply it by the time interval the rocket fires, you get the thrust.
Dividing the momentum by the weight of the rocket gives the specific impulse. The Merlin 1D has a specific impulse of 348 seconds:
where g is the gravitational field at the earth's surface (9.8 Nk/kg).
Knowing the thrust and the time interval, I can use that to calculate the total mass needed to stop the ISS in orbit. This results in a mass of nearly 1 million kilograms. If fuel had the same density as water, it would fill about half of an Olympic swimming pool. Yes, that's a lot of fuel. In addition, the rocket would have to be sent into space, which would require even more fuel.
Well, maybe you can understand why spaceships don't use rockets to get out of orbit. It would consume too much fuel. Using a heat shield and the earth's atmosphere to slow you down is free, and no one wants to deny it.
Read Also : What are 10 ways to stay fit?But unless it's possible to stop the ISS before it's shot through the atmosphere, there's really no hope of it returning to Earth in one piece.
So if we are not happy with the other two options, leave it in LEO and reboot once in a while or let it come back and crash into the ocean, there is only one option left. We could put it in a higher orbit where there is virtually no drag and it could stay there undisturbed. Of course, it would take more energy to get there and provide that thrust, so you would need a bigger rocket. And you don't want it to become high-flying space junk that could endanger other ships.
Personally, I prefer the latter variant. It would be like turning the ISS into a time capsule. And once we finally understand commercial space travel, it would make a great "floating" museum exhibit in space.