Physics in the Gravity Trailer | by RHETT ALLAIN (Wired)
Let’s take a look at this trailer for the upcoming movie Gravity. I don’t know much, but it seems like it is about two astronauts dealing with some problems on the International Space Station. After watching this, my physics alert system went off. It might be a false alarm, so I will take a closer look.
If you look at the Wikipedia page on the ISS, it lists the orbit altitude at 250 to 263 miles above the Earth. Here is a shot from the trailer.
Ok. I don’t know for certain this is the ISS in the trailer. I assume it is, but it could be some new space station. However, at “372 miles above the Earth” the ISS is much higher than it normally operates. I guess that’s ok – I mean this is a fictional story. I don’t see any reason why it couldn’t be this high. I just don’t see why they would significantly change the altitude. Maybe there is some plot element regarding the altitude – but if not, this is just sad. Really, it isn’t difficult to look up the orbital characteristics of the ISS.
Well, there is one big difference with an increased altitude. The orbital period would be longer. Maybe that is important in the plot. Maybe.
Air Resistance in Orbit
This is a short trailer with short clips of things. This means that it’s not quite clear what’s going on in each scene. Let’s look at a few.
This shows part of the ISS exploding for some reason. I think this short clip is ok in that it has the debris expanding in all directions.
In this scene, there is stuff that is clearly getting pushed back by air. At a 372 mile altitude, there is still air – but very little. Even at the ISS’s current orbit, there is air resistance – and this is why the space station occasionally needs a reboost. But at that height, the air resistance wouldn’t do anything like this. The next scene shows material leaving trails, so perhaps the space station got much closer to the Earth.
Here is an astronaut hanging on to the ISS.
If you are just flying past the space station and you grab on to stop yourself, that would be it. You would stop. You wouldn’t keep getting pulled back. If the space station was low in the orbit with significant air resistance, then that could happen. However, I have a feeling that you would have to be pretty low to get some serious forces as depicted in this scene.
Clearly, I don’t have any definite answers here. So, instead I will give homework questions. That’s what I do.
Estimate the drag force on the ISS at its current orbit altitude. Here is a hint. If you increased the altitude to 327 miles, how would the air drag force change?
What would the density of air have to be for an astronaut to experience an air drag force of about half the astronaut’s weight on the surface of the Earth?
Suppose re-entry starts at an altitude of 200 miles (I just guessed). What is the change in energy (both kinetic and gravitational potential) for an ISS going from an altitude of 372 miles to 200 miles? (sorry for using miles – but that’s what it lists in the video).
Let me point out one more thing. Why did I even start this post in the first place? After my first pass of the trailer, I was afraid that there was a very wrong misconception. The common idea is that when you knock something off of a fast moving object, that knocked off thing will slow down. This is indeed what happens to a fast moving air plane. The debris falls back due to an interaction with the air. However, in high orbits the air drag is quite small. This means that if you knock something off the ISS, it will basically just stay there.
After examining the trailer again, I’m not sure this problem is in the short clip. It seems that all of the objects moving past the ISS are due to some type of re-entry. I guess I will have to wait for the movie or another trailer to really find out what is going on.