Here’s what ‘Gravity’ gets right and wrong about space

October 21, 2013

Kevin Grazier, the science adviser for Alfonso Cuaron’s new film "Gravity," has a bone to pick with Twitter-famous astrophysicist Neil DeGrasse Tyson.

(Warner Bros.)
(Warner Bros.)

“I have never seen people nitpick such a good film to death,” complained the former NASA engineer and UCLA professor, who has also consulted on series like "Battlestar Galactica."

Tyson made headlines last week with a series of tweets that claimed, in unusually curmudgeonly fashion, that the film mucked up more or less every scientific phenomena it set out to capture — including the one for which it’s named. He made several very good points. It’s a movie, after all. (“No one said it was a documentary,” notes Grazier.) But as Tyson clarified in a later follow-up on Facebook, “Gravity” also got lots of things right. And, perhaps more importantly, it got millions of viewers interested in topics they’d usually groan through in high school physics.

With that said, we decided to try fact-checking the film’s science ourselves, with the help of Grazier and NASA engineer Robert Frost, a man well-known to Q&A site Quora for his lucid explanations of everything from orbital mechanics to airplane design.

Warning: Many, many spoilers (and a lot of physics sorcery) follows. Read this after you’ve seen the movie.

1. The Hubble Space Telescope, the International Space Station and China’s Tiangong-1 are close enough to travel between them.

False. They’re so far apart, in fact, that it’s difficult to visualize. Not only are the three at different altitudes, but they’re also on different orbits — making it very unusual for them to even get within a few hundred miles of each other.

In fact, when Cuaron asked Grazier about that particular issue, Grazier told him the closest the two would ever possibly get was “the distance between here and Mexico.” They were in Hollywood at the time.

2. You can point at an object in space and head toward it, as George Clooney and Sandra Bullock do more or less throughout the whole movie.

False. In space, unlike in the atmosphere, an object’s speed depends on its altitude. That’s really counterintuitive, but it has to do with the forces that keep an object in orbit.

When a satellite — let’s say the Hubble — circles Earth, two carefully balanced forces are making that happen: gravity, which pulls the satellite toward Earth, and centripetal force, a product of gravity that keeps the satellite swinging in an ellipse around it. As you get closer to Earth, gravity’s pull gets stronger — which means that centripetal force must also get stronger to balance it. Translation: A lower altitude means a higher orbital speed, and vice versa.

That makes sense, right? But it also makes travel between, say, a destroyed satellite and the ISS extremely difficult. Every time you speed up or slow down, your altitude changes from that of the object you’re trying to hit.

3. Clooney had to let go to save Bullock.

True — probably. This is a major point of contention among the scientific “nitpickers,” sure to go down in history with Jack’s tragic freezing at the end of the “Titanic.” (For the record, Mythbusters conclusively proved that Jack could have fit on that door with Rose.)

On one hand are people like Tyson, who argue that, since Clooney was in free-fall and thus essentially weightless, Bullock could have pulled him toward her easily. Both Grazier and Frost see it differently, though. Here’s Frost’s explanation:

Isaac Newton tells us that an object in motion will remain in motion unless acted upon by an outside force. Kowalski was unable to arrest his forward movement by grabbing ahold of the ISS, so he goes floating off into space. Other than gravity, which we can ignore for this close contact scene because it is acting upon everything in the same way, there are no forces acting on Kowalski. He is moving away because he was moving in that direction and nothing stopped him. Ryan (Bullock) goes after Kowalski …

This is where I think the scene gets a little hard to interpret. The fact that she just barely grabs him and doesn't continue closing in on him tells us that she is decelerating. She is decelerating because her leg is caught up in the parachute cords from the Soyuz. If we imagine the parachute cords are a rubber band, what would happen? The band would stretch and the energy needed to stretch it would be taken from Ryan. She has a kinetic energy equal to half her mass times her velocity squared. Her mass can't change, so her velocity would go down.

Now, what affect does Kowalski have on the situation? There is no force acting on him. But he too has a kinetic energy equal to half his mass times his velocity squared. So, if the rubber band is to slow Ryan to a stop it also has to slow Kowalski. So now it has to absorb her energy and his energy. Kowalski's interpretation of the situation is that the parachute cords can't absorb that much energy. So, he figures that if he lets go of her hand, the parachute cords, instead of absorbing Ryan's kinetic energy AND his kinetic energy, will only have to absorb Ryan's kinetic energy.

4. Bullock could jet around on a fire extinguisher. True and false. Using the fire extinguisher would certainly move Bullock, but not necessarily in the direction she intended.

In order for the fire extinguisher trick to work, the extinguisher would have needed to sit right at her “center of mass.” Imagine that kind of like the center of a seesaw with equal weights on both seats — it’s the point where, if a pivot were placed there, the object would stay balanced and in place. Spraying a fire extinguisher from this point would push a balanced, upright Bullock in whatever direction she wanted to go.

But spraying from any other point would throw her off balance and spin her around — kind of like pushing someone on an ice rink in the shoulder, Frost says. This spinning would be faster and more disorienting than on Earth, though, because there’s no resistance in space. (For what it's worth, other NASA alums have also complained that American spacecraft don't carry this style of extinguisher.)

5. A satellite, once destroyed, can form a catastrophic cloud of space debris. True. It wouldn’t happen as fast as in the movie, Frost says, and it wouldn’t impact quite so many satellites (more on that in #6), but a space debris event could definitely come with huge consequences.

In fact, it’s happened before. In January 2007, the Chinese unleashed more than 1,600 pieces of debris into the atmosphere when they destroyed one of their own satellites with a missile. The impact of that missile strike sent the debris into orbits much different from the original satellite’s, creating, in Frost’s words, a cloud of debris “enveloping the Earth and [continuing] to threaten any spacecraft between those two altitudes.”

Here’s one very scary thing about space debris that “Gravity” got wrong: Frost says that it can move so fast that astronauts wouldn’t even see it. Holes would just mysteriously appear in the equipment around them.


A 2011 NASA map of space debris. (NASA)

6. Space debris from a low-flying satellite could knock out communications satellites. False — mostly. The space debris in “Gravity” somehow knocks out both the Hubble and the communications satellites NASA uses to communicate with spacecraft, which means Clooney and Bullock can’t reach Earth.

But those communications satellites, which go by the acronym TDRSS (Tracking and Data Relay Satellite System), orbit 35,900 kilometers above Earth — about 100 times higher than Hubble, per a Tyson tweet. The same cloud of space debris wouldn’t have knocked out both ... unless, Grazier argues, we’re talking about a “cascade” of debris from one of Russia’s special high-tilt orbit satellites, which operate at a different latitude than most.

“I understand that’s a teachable moment,” Grazier said of Tyson’s tweets, “but I think a more interesting teachable question might be, ‘how might we get that debris up there?’” His answer: Eccentricity.

7. Space stations fall out of orbit.

True. Remember how orbits result from two big forces? When you add a third force — atmospheric drag — it can throw the whole equation off.

Satellites like the Hubble, which fly pretty low, encounter that kind of drag and eventually fall out of orbit. It happens over a period of years, though, not nearly as quickly as what we see in the film.

8. Bodies freeze instantly when exposed to space.

False. Space is a vacuum. It’s counterintuitive, but there’s more or less nothing out there, including temperature or air. A human body exposed to space would radiate away all its heat, but it wouldn’t freeze-dry instantly, like the gruesome corpses in “Gravity.” The writer and astronomer Phil Plait explains this over at Slate.

You would lose consciousness within seconds, however: The low pressure of space makes the gas in the lungs expand and forces it out of the body. Because there’s no air in space, you can’t breathe in again. According to astronaut Michael Barratt, that leaves only 12 seconds before hypoxia, or lack of oxygen, causes you to black out.

9. Tears float off your face in space.

False. Canadian astronaut Chris Hadfield demonstrated this pretty wonderfully in a video last April.

What explains that phenomenon? Surface tension, a powerful force that bonds the molecules in liquids together and accounts for things like bugs that can walk on water. Surface tension in space, however, is way stronger than surface tension on Earth, because those molecular forces don’t have to fight gravity so much. Astronaut Scott Parazynski actually told Vulture that surface tension is “the strongest force up there.”

10. A week in space can damage your muscles.

True-ish. NASA reports that astronauts can lose up to 20 percent of their muscle mass during space flights as short as five to 11 days.

That’s not for lack of exercise: Astronauts work out more than two-and-a-half hours each day. But even intense exercise can’t make up for the fact that, without gravity, the “antigravity” muscles (calves, quadriceps, much of the the back and neck) just aren’t working very hard and tend to atrophy. That explains why Bullock struggles to get out of the water at the end of the movie, though she seems unusually affected for just a few days in space.

Also, as many a snarky online commentator has noted, it seems somehow improbable that Bullock could kick her way out of a sinking space capsule … but lack the muscle tone to stand upright. (In fact, this is an issue NASA worries about: “The loss of muscle mass means a loss of strength that can be potentially dangerous if an astronaut must perform a strenuous emergency procedure upon re-entry into the Earth's gravitational field,” one agency fact sheet warns.)

None of this is to imply, of course, that “Gravity” isn’t a good movie. It’s an amazing movie. But both Frost and Grazier want to make sure that, despite that ultra-realistic CGI, people understand that this is, ultimately, fiction.

“I appreciate that sometimes we have to just let movies be movies,” Frost wrote in a message. “[But] as a space educator, it is a little frustrating for me to know that because of the stunningly realistic appearance of this film, people will absorb some of the things they see and allow those to manipulate their understanding of space.”

Caitlin Dewey runs The Intersect blog, writing about digital and Internet culture. Before joining the Post, she was an associate online editor at Kiplinger’s Personal Finance.
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