What It Would Take to Bring the ISS Back to Earth in One Piece

Everyone knows about the International Space Station. I mean, it's been in low Earth orbit for over 20 years. That means it's about 400 kilometers above the surface of the Earth traveling at a speed of 7.66 kilometers per second. (For the record: That’s very fast.) At this speed, it takes about 90 minutes for the ISS to complete one orbit. With 16 orbits a day for over two decades, that's more than 100,000 trips around the planet. If you are in the right spot, you can see it pass over with your naked eye, or with your smartphone.

But stuff doesn't last forever—even space stations. NASA says that the ISS will be deorbited in 2031. That means they are going to intentionally crash it into the ocean.

It seems like a waste to throw away a perfectly awesome space station. Wouldn't it be great to have the ISS in a museum, set up so that ordinary people could walk through something that spent so much time in space? It could make us all feel like astronauts.

So let's see what it would take to save the ISS.

Can't We Just Leave It in Orbit?

It might seem like the best place to keep the ISS is in space. However, there's a problem: It won't stay there without an occasional push. Without one, it will eventually crash back to Earth. Deorbiting it on purpose is one way to make sure it falls into an empty ocean, and not on top of anyone’s house.

Low Earth orbit, or LEO, is just a temporary location. In an ideal orbit, like the moon’s orbit around our planet, the object has a motion due only to its gravitational interaction with the Earth. This produces a force on the object that pulls it towards the center of the Earth while it moves in a direction perpendicular to the force. If the object has just the right speed, it will move in a circle. It's just like swinging a ball on a string in a circle around your head—except in this case the string is standing in for the gravitational force.

But for an object like a satellite or space station in LEO around the planet, there is another force—an interaction with the atmosphere. You’ve probably heard that there's no air in outer space. That's mostly correct. As you get farther away from the Earth's surface, the atmosphere gets thinner, meaning it decreases in density. But the atmospheric density doesn't just magically hit zero at some particular height. Instead, it just sort of fades away.

This means that at an altitude of 400 km (in LEO, where the ISS orbits) there isn't much air—but there is some. The very fast-moving space station collides with this very small bit of air to produce a very slight drag force pushing in the opposite direction of the space station's velocity. This decrease in velocity will eventually cause the ISS to move to lower altitudes where there is even more air and even more atmospheric drag. Stuff gets pretty complicated with orbital mechanics, but this pull would eventually make the space station crash into the Earth. This is exactly what happened to the Chinese space station Tiangong-1.

To keep the ISS orbiting until 2031, the space agencies that maintain it need to periodically do something to counteract this drag force. The ISS doesn’t have its own rocket engines, so it needs a reboost, or a push from a resupply craft. A reboost nudges the space station and increases its velocity. (Here is a bonus: My analysis of what it’s like to be an astronaut inside the ISS during a reboost, posted on the European Space Agency’s blog.)

Would the ISS Burn Up on Reentry?

Although reentry can be a violent event and completely destroy many objects, it’s quite possible that something the size of the ISS would at least partially survive. As an example, pieces of Skylab made it through the atmosphere upon reentry in 1979 and hit the Earth as debris.

But anything that falls through the atmosphere gets super hot. Orbital objects are going really fast, and when they start to move through the atmosphere, they push the air in front of them, because that air gets in their way. Some of this air gets pushed to the side, but much of it is pushed forward. This is a problem—because there is already air there. Pressing more air into the same space causes a compression. You might have noticed while pumping up a bike tire that the tire gets hot as you pump more air in; it’s because it’s compressing the air already in the tube. The same thing happens as an object moves quickly through the atmosphere: The compressed air in front of it heats up, and the object itself gets hot. Like, “melt stuff” levels of hot.

Some spacecraft, like the Space Shuttle or the SpaceX Crew Dragon, have a heat shield, material that insulates the rest of the craft from all that hot air. But the ISS doesn’t have a heat shield. So at the very least, parts of it would burn up on reentry.

The remaining debris might make it to a museum exhibit, but not one you could walk through.

Could We Get the ISS Down Without a Normal Reentry?

There's a difference between reentry and simply falling from space. If you just take an object up to an altitude of 400 kilometers and drop it, that’s significantly different than reentry. Remember, objects in LEO are moving super fast, while a "dropped" object would start with a velocity of zero meters per second. Yes, the dropped object would speed up and get hot—but not nearly as hot as an object reentering from orbit.

So consider this: What if we used some rockets to stop the ISS in its orbit, and then brought it straight down in an effort to avoid the whole "burning up on reentry" 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 that object's acceleration. In one dimension, it looks like this:

Illustration: Rhett Allain

Yes, the m in that 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 the rate of change of velocity.

So, if we assume that the ISS decreases speed at a constant rate, then the acceleration would be:

Illustration: Rhett Allain

Here, v₂ is the final velocity (that would be zero m/s) and v₁ is the starting velocity (orbital velocity of 7.66 x 10³ m/s).

But what about the time interval, Δt? Let's just make the assumption that we can slow down the ISS during one orbit—so that would be 90 minutes, or 5,400 seconds. With those values, we can calculate the acceleration. Multiply that by the mass of the ISS and you will have the average thrust force a rocket would need to stop this space station in its orbit.

Plugging in the numbers gives a rocket thrust of 6.31 x 10⁵ Newtons. That's about half the total thrust from a Boeing 747. Of course, you couldn't actually use a 747 engine because that requires air, and there's not nearly enough air in low Earth orbit for that to work.

I guess that means we need a rocket. How about a Merlin 1D Vacuum engine? These are the kind used in the SpaceX Falcon Heavy second stage. Rocket engines produce thrust by expelling mass (fuel) out of a nozzle. You can get more thrust by increasing the rate of fuel use or by increasing the velocity of the material as it leaves the engines. The Merlin 1D can produce a thrust of up to 981,000 Newtons. If you reduce the fuel rate, you will also decrease the thrust, but that will increase the time that the fuel will last.

One way to describe the performance of a rocket is with the specific impulse. If you take the average rocket thrust and multiply this by the time interval that the rocket fires, that would give you the impulse.

Dividing the impulse by the weight of the rocket gives the specific impulse. The Merlin 1D has a specific impulse of 348 seconds:

Illustration: Rhett Allain

In this case, g is the gravitational field on the surface of the Earth (9.8 Nk/kg).

Since I know the thrust force and the time interval, I can use this to calculate the total mass required to stop the ISS in its orbit. This gives a mass of just under 1 million kilograms. If the fuel had the same density as water, it would fill up about half of an Olympic size pool. Yes, that is a lot of fuel. Also, you’d have to get the rocket into space, and that would take even more fuel.

OK, maybe you can see why spacecraft don't use rockets to de-orbit. It would just take too much fuel. Using a heat shield and the Earth's atmosphere to slow down is free—and no one wants to turn down free.

But if it's not possible to stop the ISS before bringing it down through the atmosphere, there's really no hope of getting it back to Earth in one piece.

So if we’re not happy with the other two options—leaving it in LEO and reboosting it from time to time, or allowing it to re-enter and crash into the ocean—there’s only one possibility left. We could push it to a higher orbit where there's essentially no air drag and it could stay there unbothered. Of course, it would take more energy to get there to provide that push—so you would need a bigger rocket. And you wouldn’t want it to become high-flying space junk that could endanger other crafts.

Personally, I sort of like the last option. It would be like turning the ISS into a time capsule. And once we finally figure out commercial space travel, it would make for a great “float through” museum exhibit—in space.