“Gravity”

I’ve been a delinquent spacecraft engineer, and didn’t see “Gravity” until today.

In short: it was awesome. It’s a tremendous story about courage, fear, perseverance, the human spirit, our ability to solve the most insurmountable problems, and triumph in the face of adversity. It’s also visually, sonically, musically, and generally aesthetically breathtaking. The integration of the stunning visuals, physically accurate sound, camera movement through space and spacecraft, and music was extraordinarily well integrated into a complete artistic whole.

And, although the events depicted in the movie would not (or could not) play out exactly as shown, they are all plausible from a physics standpoint.

Everyone should go see it. And, yes, see it in 3D – because this is the first movie I have ever seen in which the 3D adds to the visuals and the drama.

Don't let go.

Don’t let go.

Before I read any other physicists’ reviews, I’m going to go through some of the concepts and sequences in the movie, make a few points about the physics involved, and then explain why I am totally fine about it all.

Spoilers ho!

The debris cascade

As depicted: The Russians conduct a missile test, destroying a satellite. While at first this seems like no big deal, the debris causes a catastrophic chain reaction with other spacecraft – and generates a cloud of hypervelocity missiles that tries to shred our heroes every 90 minutes.

Any problem? Several years ago, the Chinese military conducted a similar test, which caused a big stir in the international space community as other nations worried about the resulting debris cloud. On top of that, some researchers predict that after the amount of space debris reaches a critical level, a cascade will occur where the debris crashes into more debris, creating even more debris, which could demolish any operational satellites in orbit – it’s called the Kessler Syndrome.

The debris cloud shown in the movie even bears a striking resemblance to simulations of the debris from the 2009 collision of Iridium 33 and Kosmos 2251, another scary space debris event from real life.

However, there is little chance that the debris from a collision would become catastrophic within minutes of the event. Kessler Syndrome relies on orbit dynamics, and the chain reaction would take many orbits to build up. The debris also would not encounter the Space Station every 90 minutes (once per ISS orbit) except if it’s in certain specific orbits, so that particular item is unlikely. (At least, it’s unlikely immediately following the cascade.)

Why that’s okay: The debris cascade is a real fear in the space community. NASA even conducts “debris avoidance maneuvers” to push manned spacecraft out of the way of potential impacts. The timeline in the movie is accelerated…but this is something we spacecraft engineers worry might happen.

I’ve got to let go

As depicted: Two astronauts are flung away from the Space Station – caught only by an improvised tether. They strain for a moment at the end of the line, until one astronaut realizes he has to let go to release the tension in the cables. He sacrifices himself to let the other survive.

Any problem? Yes. As depicted, it appears that astronauts Stone and Kowalsky should have behaved exactly as Stone’s Soyuz capsule does later in the film: when they reach the end of the tether, it should snap them both back to the station. The only reason for both astronauts to be pulled consistently outward is if there is a force acting on them. This is potentially an error in the physics, and it involves a crucial plot point.

Why it’s okay: There are conceivable situations when one astronaut might have to let go of the tether to let the other survive. For example, if the entire system was rotating, centrifugal force would pull both astronauts outward. That’s just one obvious example: maybe one of the astronauts instead gets irretrievably stuck or something.

I’m willing to give the director the benefit of the doubt here in favor of getting a deeper human drama.

Do you copy?

As depicted: The Kessler Syndrome cascade eliminates all contact between the astronauts and Houston’s Johnson Space Center (where Gene Kranz Ed Harris is once again in Mission Control).

Any problem? A Kessler debris cascade would be limited in orbital space – that is, a low Earth orbit cascade would generate debris in low Earth orbit. Communications between NASA spacecraft and mission control are maintained by the TDRS spacecraft, which are geostationary. Those spacecraft will be fine, and so will communications.

Even if the satellite communications system is destroyed entirely, the astronauts still make ground passes over Mission Control. They’ll be able to talk to the ground during those passes, just like in the old Apollo days.

Why that’s okay: There are more effects that could disrupt communications than destruction of the TDRS system. If the astronauts suffered impacts on their transceivers or antennae, that would certainly do it. It’s also possible that the debris cloud simply creates a lot of scattering sources co-orbiting with the astronauts, making the radio link difficult to maintain.

But, in the end, this is a story about being alone and reaching out for human contact. It’s about being alone and reaching into oneself to find a reservoir of strength. It’s a good story, and the loss of communication is a device to keep it going. Infrequent, garbled communications with the ground might have added to the realism, but would not add very much to the movie as a whole.

Sunday driver

As depicted: Dr. Stone uses one of the International Space Station’s docked Soyuz capsules to move to the Chinese space station Tiangong.

Any problem? Well…the ISS, Tiangong, and Hubble Space Telescope are all in different orbits and would not likely be near one another all at once. They would certainly not all be in visual range for the time periods shown in the movie.

Why that’s okay: It’s not inconceivable to get from ISS to Tiangong in a Soyuz. Both stations are at the same orbit altitude, which means that the Soyuz wouldn’t have to add or subtract any energy from its orbit to get from one to the other – so not much delta v is necessarily involved (though I’m not sure whether the Soyuz landing rockets provide enough, and those stations are at different orbital inclinations). [Updated 14 Oct 13: A friend more familiar with Tiangong’s orbital parameters than I am tells me that it would take several kilometers per second of delta v…so, no, the Soyuz landing system could not do this.] What it would definitely take is time, especially depending on the orbit phasing between them. Again, the movie compressed the plausible timeline in favor of a more dramatic story; although it stretches the limits of plausibility, the situation is not necessarily impossible. [With a big enough rocket.]

Shaky station

As depicted: The Tiangong space station is losing altitude and entering the atmosphere. Its solar arrays shiver in the wind of the exosphere, and the modules vibrate against one another dramatically from the forces of drag.

Any problem? Not really. Space stations are subject to atmospheric drag, and we typically have to “reboost” them to keep them in space. However, the movie implies – since Tiangong is losing altitude shortly after the ISS was struck – that Tiangong is falling due to the Kessler event. If a debris impact caused any explosions on Tiangong, there could have been enough momentum transfer to knock the station Earthward.

…Except that if the station was vibrating that much, there had to be a significant amount of force on the station. Such forces would only happen deep enough in the atmosphere that the station would already be lost. There would be no time for Dr. Stone to rendezvous and get aboard.

Why that’s okay: The vibration is a device used to convey urgency to the viewer. Dr. Stone drops a line about how Tiangong is losing altitude, but there would be no visual or auditory cues to the audience to relate how dangerous the situation is. It’s an enhancer, not a critical point.

Coming down

As depicted: The Shenzou spacecraft is tumbling wildly as it enters the upper atmosphere. In a few moments, it straightens out and falls heat-shield-forward until parachutes deploy.

Any problem? Actually – no! One of the advantages of a re-entry capsule design is that it’s passively stable when falling heat shield first, meaning that the capsule probably will straighten out. [updated 14 Oct 13] maybe not! My fiancee, who is way the heck more knowledgeable about aerodynamics than I am, informs me that Soyuz capsules are passively stable for small perturbations from their nominal reentry orientation, but not for large perturbations or tumbles. For the capsule to survive, it has to get into the proper orientation using its thrusters. The thruster operation could happen automatically. Whether that happens in time is a good question, though. Certainly, what’s shown in the movie is at the very edge of (or probably beyond) the normal operational parameters of the Shenzou vehicle!

All in all, I’d say Dr. Ryan Stone’s triumphant attitude when she stands on solid ground is entirely well deserved.

Because, in the end, this is a tremendous movie and a good story. The physics issues are few and far between, and they don’t detract from the drama or characters. And there’s so much the movie gets right, from its depictions of the Earth, to the behavior of tethers, to the spacecraft hardware, to the masterful use of realistic sound.

Gravity is how you do a space movie.

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