In 2011, a person from Raytheon called me up out of the blue and asked if I wanted to be involved in a project to help eliminate space debris. I was interested.
A little background (the first): The majority of space is not really a vacuum. There are particles pretty much everywhere in space. The question really is what is the density of those particles – or how many particles are there in a given volume. The atmosphere of Earth is a relatively narrow shell around the Earth. We know that if we climb Mt. Everest there is not much air up there. But, to us, “not much air” is still a whole lot of air.
We think of space as having no air at all. Astronauts have to wear space suits to protect them from the “vacuum of space” after all. There are a number of very interesting things about this, but it should really be a totally separate post. Let’s simply leave it that there is, indeed, atmosphere up in space where the International Space Station (ISS) orbits.
Since there is air up there, objects such as satellites and the ISS feel a drag force, as we have discussed in a few other posts. The density in the atmosphere up there is very very small, but the objects in orbit are moving extremely fast. This means that the drag force that the objects feel is not really large, but it can strongly affect their orbital motion.
As a rule of thumb, objects that are orbiting below about 200 km (120 miles) will deorbit extremely quickly, in a matter of days to weeks, because of the atmospheric drag. The atmosphere is so thick there that the objects just can’t stay aloft and they ultimately decrease their altitude rapidly and (hopefully) burn up in the atmosphere around 70 km altitude.
Satellites above about 700 km (420 miles) or so will basically never deorbit because of atmospheric drag. The atmospheric density there is so low that drag is not an important force on objects in orbit. Don’t get me wrong – there is still an atmosphere, but it is so tenuous that orbiting objects don’t care about it.
Between these two altitudes, drag is important. In fact, the ISS, which orbits at around 400 km (240 miles) altitude has to be moved up every couple of weeks, because atmospheric drag pulls it down. The ISS is constantly being pulled back towards the Earth due to drag, and we lift it back up to 400+ km using propulsion to keep it in orbit. If we didn’t do this, the ISS would fall back to Earth and burn up in the atmosphere in 6-12 months.
A little background (the second): Space is filled with a lot of crap. There are about 20,000 objects in orbit around the Earth that are about the size of a softball or larger. This stuff is pretty dangerous, since it is moving at speeds of 7,600 m/s (17,000 MPH). It can cause a lot of damage (Gravity, anyone?). Here is a picture of the space shuttle window after being hit with a piece of debris that was significantly less than a millimeter in size:
There is a realization that we need to do something about all of this space debris. One of the worst things that has happened because of the amount of debris is that an Iridium satellite was destroyed due to a collision with a retired Russian spy satellite. This caused the creation of thousands more pieces of debris, which could then impact other objects. There is real concern.
I was called by this guy from Raytheon to see if I wanted to help solve this problem. I said, sure, that would be great. We wrote a proposal to NASA to see if we could get money for it. They gave us some seed money to see if the idea could pan out. The project was called “Space Debris Elimination (SpaDE)”.
The general idea of the project was to see if we could increase the atmospheric drag in front of an object in orbit around the Earth enough to cause it to deorbit. Here is a graphic to illustrate the idea:
At first, this idea seems very interesting and plausible. My main profession is a modeler of the upper atmosphere. A long time ago, I wrote a large-scale model of the atmosphere that looks at what happens when large amounts of energy is added to the atmosphere in the form of the aurora (i.e., the northern/southern lights). I continue to work on this model today, and have a bunch of graduate students who use the model to do research. For project SpaDE, I took the model and made it so we could run it over a very small area (like a couple of hundred kilometers by a couple of hundred kilometers) and inject a huge amount of energy quite low in the atmosphere. The model then simulated what would occur.
You might ask, what is “a huge amount of energy”? Well, that is a good question. The model can handle almost any amount of energy, but for this project we were looking at roughly nuclear bomb types of energies. The general idea would be to take a large explosive device to the stratosphere, or about 30 km, and explode it there. We would use some sort of device to direct the majority of the energy upwards, creating a very large density perturbation that would propagate upwards to the upper atmosphere where a piece of debris would travel through the density enhancement and deorbit.
When people find out about this project, they always ask: why did you stop working on it? Well, this is really the point of this post – to describe why this type of debris mitigation strategy is unlikely to work.
Problem, the first: You have to take an extremely large explosive device up on a balloon and explode it in the atmosphere. That is unlikely to be ok with pretty much anyone. Also, directing the blast so that the majority of energy would go upwards could be rather heavy, making the balloon quite big. Basically, there would be a lot of logistical problems. You could envision that each balloon launch could run upwards of several hundreds of thousands of dollars or more. For comparison, if you wanted to do something like capture debris with a satellite and deorbit it, it might cost 10s of millions of dollars. So, a balloon with a large bomb is much cheaper, but a logistical nightmare.
Problem, the second: This is somewhat more technical. The atmosphere breathes. When it warms up, it expands, and when it gets cooler, it contracts. The upper atmosphere absorbs a bunch of energy from the aurora. The aurora deposits about 40 Giga-Watts of energy into the upper atmosphere continuously. During extremely disturbed times, it can deposit over 500 GW of energy for several hours. Those are big numbers, so what does it mean? Let’s say we have an extremely disturbed aurora (500 GW) for 6 hours. That is about 1e16 Joules of energy, which is 2.6 megatons of TNT, which is roughly 10 times LESS energy than a very large nuclear bomb. With the aurora, it is distributed over a very large region, and will cause an increase in density at around 400 km altitude of about a factor of 10. That is a pretty big increase in density. A coworker noted to me that a hurricane is caused by a density change of about 10%. This is a 1000% change in the thermosphere due to the aurora. The thermosphere is a pretty interesting place!
A nuclear bomb will cause a much larger change in the density, clearly. But, the problem is that it will change is only in a very small volume. You get a mushroom cloud that goes up into the atmosphere and causes a (lets say) 100 times increase. By the time the mushroom cloud gets up to orbital altitudes, it will be about 200-ish km across. A satellite will pass through this mushroom cloud in about 25 seconds. Then the density is back to normal. For a large auroral event, the entire atmosphere is increased by a factor of 10 for 6 hours, meaning that the satellite goes through 6 hours of 10 times larger drag. This is enough to change the satellite’s orbit. Definitely. But, it will really only change the altitude of the satellite by a meter or two. Not much in the grand scheme of things. So, a 100 times increase in the density for 26 seconds doesn’t change the orbit very much at all. Simplistically, you would need about a 1 million times increase in the density at satellite altitudes over the 26s in order to get the same effect as a 10 times increase for the storm over six hours. That would be a very large bomb indeed. And it would only cause something like a meter or two altitude change. Not deorbiting. For that you would need on the order of a hundred or so of these events. That is a lot of bombs.
Hopefully that was not too technical. Sorry if it was.
Problem, the third: In order to actually effect the single piece of debris, you would have to launch the balloon several hours before hand to allow it to get to a high enough altitude, and be over the pretty much the exact spot in which an object will be orbiting. Then you have to time the explosion perfectly, so that the orbiting object passes directly through the shock front of the blast. This is probably the easiest of the three problems, since with a model, you can pretty easily determine how quickly the blast wave will move through the atmosphere in all directions. Controlling the path of the balloon is more of a challenge, but I have actually written software that will allow you to figure out the trajectory of high altitude balloons, since this is a sort of a hobby or mine. (I will say that I have never exploded a nuclear device on one of our balloons!) Check out a video of one of our balloon launches where we launched two balloons at the same time.
What basically happened with Project SpaDE is that we did not get selected for funding for round two. This was not too surprising, since the concept is just not very scalable. You could actually see this working if you wanted to slightly nudge a small object that was going to hit a major satellite or something. If you perturbed the orbit a couple of days before the collision was supposed to occur, it could actually change things. Possibly. But, it would not be able to deorbit anything.
Sorry that this post was so long. Hopefully it was still entertaining.