This week, I settled down to look at the primary episode of The 100. If you have not seen the present, I will simply level out that it takes place in the close to future (although it ran, on the CW, in the close to previous). For causes that I will not get into, there’s a spacecraft with a bunch of youngsters that’s touring from an area station right down to the floor of the Earth. During the reentry course of, one child needs to point out that he’s the grasp of area journey and that he is superior. So what does he do? He will get out of his seat and floats round as an illustration of his mastery of weightlessness. Another teenager factors out that he is being fairly dumb—and that he’ll get harm very quickly.
OK, that’s sufficient of the outline of the scene in order that we will speak about physics. The level is that there’s one dude “floating” round in the spacecraft throughout reentry.
Before I over-analyze this brief scene, let me add a caveat about my philosophy on science and tales. I’ve talked about this earlier than, so I will simply give a abstract: The primary job for a author of a present is to inform a narrative. If the author distorts science in order to make the plot transfer alongside—so be it. However, if the science could possibly be right with out destroying the plot, then clearly I would choose it.
On to the over-analysis!
What Causes Gravity?
Obviously this scene has to do with gravity, so we must always speak about gravity—proper? In brief, gravity is a elementary interplay between objects with mass. Yes, any two objects which have mass could have a gravitational power pulling them collectively. The magnitude of this gravitational power will depend on the gap between the objects. The additional aside the objects get, the weaker the gravitational power. The magnitude of this power additionally will depend on the lots of the 2 objects. Greater mass means a better power. As an equation, this could be written as:
In this equation, the lots are described by the variables m1 and m2 and the gap between the objects is the variable r. But crucial factor is the fixed G—that is the common gravitational fixed and it has a price of 6.67 x 10-11 Nm2/kg22. That would possibly appear to be it is vital, so let me give an instance that everybody can relate to. Suppose you’re standing someplace and your buddy is correct there with you and also you two are having a dialog. Since you each have mass, there’s a gravitational power pulling the 2 of you collectively. Using tough approximations for distance and mass, I get a pretty power of three x 10-7 Newtons. Just to place that into perspective, this worth is pretty near the power you’d really feel for those who put a grain of salt in your head (sure, I’ve an approximate worth for the mass of 1 grain of salt).
So, the gravitational power is tremendous tiny. The solely method we ever discover this power is that if one of many interacting objects has an excellent enormous mass—one thing just like the mass of the Earth (5.97 x 1024 kg). If you exchange your buddy with the Earth and put the gap between you and your friend-Earth because the radius of the Earth, you then get a gravitational power of one thing like 680 Newtons—and that may be a power you possibly can really feel (and also you do).
Is There Gravity in Space?
Now for the actual query. Why do astronauts float round in area except there isn’t any gravity? It positive looks as if there isn’t any gravity in area—it is even known as “zero gravity.” OK, I’ve answered this earlier than, but it surely’s vital sufficient to revisit the query.
The brief reply is “yes”—there’s gravity in area. Look again on the gravitational equation above. What adjustments in that equation as you progress from the floor of the Earth into area? The solely distinction is the gap between you and the middle of the Earth (the r). So as the gap will increase, the gravitational power decreases—however by how a lot does the gravitational power change? How a few fast estimation?
Let’s use an Earth radius of 6.371 x 106 meters. With this worth, an individual with a mass of 70 kg would have a gravitational power of 686.7 Newtons. Now transferring as much as the orbital peak of the International Space Station, you’d be an additional 400 km farther from the middle. Recalculating with this better distance, I get a weight of 608 Newtons. This is about 88 p.c the worth on the floor of the Earth (you possibly can examine all my calculations right here). But you possibly can see there’s clearly gravity in area.
Oh, right here is a few additional proof. Why does the moon orbit the Earth? The reply: gravity. Why does the Earth orbit the Sun? Yup, it is gravity. In each of those circumstances, there’s a important distance between the 2 interacting objects—however gravity nonetheless “works,” even in area.
But why do astronauts float round in area? Well, they float round when in orbit—if there was an excellent tall tower reaching into area, they would not float round. The “weightless” atmosphere is attributable to the orbital movement of the folks inside a spacecraft or area station. Here is the actual deal. If the one power performing on a human is the gravitational power, that human feels weightless. Standing on a tall tower would consequence in two forces (gravity flattening and the tower pushing up). In orbit, there’s solely the gravitational power—resulting in that feeling of weightlessness.
Actually, you do not even must be in orbit to really feel weightless. You could be weightless by having the gravitational power as the one factor performing on you. Here is a state of affairs so that you can contemplate. Suppose you’re standing in a stationary elevator on the high of a constructing. Since you’re at relaxation, the full power have to be zero—which means the downward gravitational power flattening is balanced by the upward pushing power from the ground. Now take away the power from the ground. Yes, that is troublesome however it may be achieved. Just have the elevator speed up down with the identical acceleration as a free falling object. Now you may be falling inside an elevator. The solely power is gravity and you may be weightless.
Some folks suppose this falling elevator is enjoyable. That’s why many amusement parks have a journey like The Tower of Terror. Basically, you get in a automobile that drops off a tower. During the autumn, you’re feeling weightless—however you do not crash on the backside. Instead, the automobile is on a observe that in some way slows down extra steadily than if it smashed into the bottom. They have one in all a lot of these rides on the NASA heart in Huntsville. went on this with my children—it was truly scarier than I had imagined.
How about one other instance? If you’re in an airplane and the aircraft flies with a downward acceleration, everybody inside will probably be weightless. Even a canine. Check it out.
In the top, there appears to be enormous misunderstanding about gravity. I consider the reasoning follows like so: Astronauts are weightless in area. There is not any air in area. Therefore, if there isn’t any air, there isn’t any gravity. This no-air/no-gravity thought pops up on a regular basis in motion pictures (incorrectly so).
Here’s how you may see it: Some dude is floating round in area (that is OK) after which he enters the airlock of a spacecraft, nonetheless floating. The airlock door shuts and air is pumped into the chamber and increase—he falls to the bottom as a result of now there’s gravity.
Here is what it ought to appear to be—from the epic film 2001: A Space Odyssey. SPOILER ALERT: Hal is loopy and will not open the pod-bay doorways. Not even for Dave.
Wow. That scene is just about excellent. They even don’t have any sound till the air comes in.
What Happens During Reentry?
Now again to the occasions in The 100. The scene does not happen in orbit, it happens throughout reentry. This is the half the place the spacecraft enters again into the ambiance and encounters an air resistance power (as a result of there’s air). Let me begin with a easy power diagram exhibiting the spacecraft sooner or later throughout this movement.
Clearly, this not weightless. Yes, there’s a gravitational power performing on every thing—however there’s additionally that air drag power that can make the spacecraft decelerate because it strikes down. If the human goes to remain contained in the spacecraft, there should even be an additional power on that human (from the ground). So, not weightless—in reality, the human would really feel extra than regular gravity due to the acceleration. You already know this, although, as a result of the very same factor occurs to you in an elevator. As the elevator is transferring down and coming to a cease, it is usually slowing down. During this time, you’d really feel somewhat bit heavier due to the power from the ground pushing on you. You aren’t actually heavier, you simply really feel that method due to the acceleration.
Again, there’s one other film instance the place somebody will get this reentry physics proper. It’s from Apollo 13. Check it out.
Notice the water falling from the ceiling. In this case, the capsule is transferring downward at an angle. However, the air resistance power is pushing in the wrong way of movement inflicting the spacecraft to decelerate. But what slows down the water? The water does cling to the floor somewhat bit—however the acceleration is an excessive amount of to maintain it there and it “falls” in direction of the astronaut. Note that “falling” right here doesn’t suggest straight in direction of the floor of the Earth however somewhat simply in the wrong way because the acceleration.
Looking again on the scene from The 100, this is how they may repair the scene—and it is fairly easy. Have the daring floating man transfer round earlier than they get to reentry. Then the opposite guys fall as quickly because the spacecraft begins to work together with the ambiance. That would not even change the plot—and it might be extra scientifically correct.