Reentry and Ionized Plasma
Austin Morris, Director of Engineering
6 minute read
In previous columns, we have discussed some common misconceptions regarding orbit, notably the concept that achieving orbit is difficult not because it involves a lot of vertical velocity, but because it involves a lot of horizontal velocity. Specifically, achieving orbit typically takes something like 30 times more energy applied horizontally than vertically. This is an important dichotomy to realize for numerous reasons.
The primary reason you need to understand this energy dichotomy is because you are reading this column and this column is going to walk through the process of how an object reenters the atmosphere and what happens when it does so. In that scenario, it is important to understand that while falling from heights of several hundred kilometers is scary, it’s even more scary to be plowing your way horizontally through an ocean of atmosphere at 7km/s (16,000mph), descending upon a cushion of nuclear flames that has temporarily blocked out your radio communications. All because you had the audacity to go horizontally, you Icarus you.
When an object is travelling horizontally in orbit it is also falling toward the Earth vertically, as objects affected by gravity tend to do. However, because the Earth is round (let’s not get into that discussion right now), the object travels horizontally fast enough that the ground below it continues to slope away, even as the object falls. When an object begins to reenter the atmosphere, this is typically done by slowing its horizontal speed to no longer outpace its vertical descent, allowing the object’s altitude to decrease.
Lots of horizontal and only a little vertical means circles. Quote that sentence in geometry class, I dare you.
As this altitude begins to decrease, the object gradually encounters more and more air friction. An easy way to think of the atmosphere for the purposes of pressure and air friction is to imagine that the Earth has two types of ocean. The one with which most everybody is already familiar is made of water. As you get deeper into that ocean, the pressure gets greater and greater, necessitating things like submarines to survive the pressure. There are fish that swim through this water and there are critters that walk along the bottom of the ocean floor. The second type of ocean in this analogy is that of air. In this ocean of air, birds are the ones that swim through it by flying above our heads, while we crawl or walk along the bottom of our air ocean. The major difference between these two ocean types is that one is a liquid and one is a gas. The water oceans have a roughly consistent density, as water tends to be incompressible. Air, however, is very compressible. This means that as you get higher and higher into the atmosphere and get less and less pressure, the air becomes thinner (and thinner).
As this descending orbital object encounters more air friction, it slows down more, which lowers its altitude, which means it encounters more air friction, and so on until splashdown. This way the atmosphere will help in reducing the speed and altitude of the orbital object, saving on rocket fuel, but there is still the factor of heat to contend with. If you rub your hands together really quickly you build up heat from friction. The same is true for rubbing the outside of your spacecraft against air, and when you are moving at orbital speeds, you tend to build up a lot of heat rather quickly. (As Hamlet said, “Ay, there’s the rub.”) This is why heat shields became necessary in order to safely return capsules to the Earth. Instead of finding a way to avoid this heat, such as slowing down a capsule with rockets firing downward, it is much more practical to simply ignore the problem until it goes away, which I’d like to say strikes me as a very human approach.
So we’ve resolved the manner by which we decrease altitude, we’ve ignored the problem of heat, we should be good to go, right? Obviously not, or else I wouldn’t still be rambling on. One of the most fun (read as “ostensibly terrifying”) things about this kind of reentry is also the exciting term I used as clickbait for the title of this column. When slamming air particles into each other as fast as a reentering object does, the extreme heat from all this friction raises the temperature of the air so high that the gas becomes ionized, forming essentially a cloud of plasma around the outer surface of the craft. In other words, fire.
Except that this isn’t just regular fire. This is fire with passion. This fire is clingy and doesn’t want you talking to anyone else for a few minutes. As such, this ionization causes a communications blackout, lasting anywhere from 30 seconds (such as in the Pathfinder descent to Mars) to 30 minutes (such as the Space Shuttle launches prior to the creation of the Tracking and Data Relay Satellite System). Once you have slowed down enough from all this pressure and temperature (kinda sounds like life, huh?) and stop slamming those poor air particles together, the ionization stops and your communications can be reestablished. Following this, things start to cool down, and are then followed by parachutes, skycranes, or other clever solutions used to continue slowing down to a safe return speed, floating down and gently making contact with the surface of the planet you have decided to greet.
The above is an example of a reentry that is designed to make it back to Earth, such as a crewed capsule or Space Shuttle. If this is a reentry designed to burn up debris however, such as the disposal pallets occasionally released from the ISS or active debris objects that will be removed by companies like KMI, the last part of the reentry process will proceed a little differently. We pick up at the stage of smashing air particles together and generating heat. However, in this case no effort is made to reduce the air friction encountered, in addition to the fact that the object is specifically not designed to resist this heat and as such begins to burn up. Anyone who has ever roasted marshmallows over a fire and has accidentally dropped it a little too low and burned their marshmallow can likely picture the way that it catches fire and begins to shrivel up and turn black and crusty. Anyone who claims to enjoy burned marshmallows is a monster in my eyes, but we can discuss that later. In this scenario, the marshmallow starts to char on the outside, and those charred pieces can easily flake off. The reentry of a debris object can be thought of in much the same way, except that there are a few thousand leaf blowers-worth of air continuously hosing down the outside of the marshmallow/debris object, blowing these flaky pieces off rather quickly. As each chunk flies off, it continues to burn up and turn into more pieces of progressively smaller size, until there is essentially nothing left of the once-mighty garbage pallet or meteoroid. It’s also worth noting that in this scenario, a loss of communications is typically of little concern, as most meteoroids are not equipped with long-range transmitters and wouldn’t have much to say if they were anyway. Regardless, when done properly this process turns the debris into naught but smithereens, leaving nothing to settle down to the planet’s surface.
Now that we've covered all that, it's time to make this column behave like the reentry and slow this baby down to reestablish communication and bring our heads back to Earth safely. Congratulations and welcome home. Here are my final thoughts to wrap up and I'll keep it short because the rest of this was long. The manner in which objects reenter the atmosphere varies depending on the goal of that reentry. When it comes to manned spaceflight, the intent is always to bring everyone home safely, and all the top minds work hard to ensure the best success possible. When it comes to debris removal or disposal, the reentries are designed to ensure that the debris object either dissipates entirely or comes down in a safe location. Regardless of which type of reentry it is, these controlled reentries are absolutely crucial to maintaining the safety, security, and usability of space. I hope you enjoyed today’s descent into reentry and continue to follow KMI in our mission of #KeepingSpaceClearForAll.
Recommended column to read next: Relativity and Perspective