Matters of Gravity


I was just interviewed on the “Jason Rantz” show on KIRO radio (a Seattle station.)  The interview was much more relaxed than most of them have been – Jason was interested in what I was saying, and seemed a lot more relaxed than most of the other radio talk show hosts I’ve been interviewed by.  The interview will be about ten minutes long, although we talked for about twice as long.  Once there’s a podcast I’ll put a link up for it.

We mostly talked about movies, including the recent movie Gravity.  One thing in the discussion which got me thinking was that Jason was surprised that the chain reaction of satellite destruction which drives the main plot is a real idea.  This got me thinking – most people don’t know about it, so it’s a good idea for a blog.  Three points before I start:

1)    There are spoilers for the movie in this blog, so readers beware!

2)    Neal Degrasse Tyson mentioned this in a tweet about the movie a while ago but I want to treat this in more depth; and

3)    I did like the movie despite what I say below.

As far as the third point goes, I like movies even if they get the science wrong.  I repeat the main point of this blog: thinking about the science adds depth to the movies and enjoyment of them, not detract, at least as far as I’m concerned.  If I had to grade the space science in Gravity, I’d probably give it a B-, which is pretty good as far as movies go.  Here, of course, the gold standard for accuracy in depictions of space travel is 2001: A Space Odyssey, made in 1968.  The depictions of how objects move in space are both highly accurate and beautifully depicted, as one would expect from the pairing of Stanley Kubrick and Arthur C. Clarke.  (If you watch the movie, turn it off once you get to the final 20 minutes and imagine your own ending.)

Gravity wasn’t as good, but it did a better job than a lot of others.  There were no banked turns in space, for example.  I found the final scene hard to believe, and there were a few other points which bothered me.  For example, the parachute seemed to billow when Sandra Bullock was maneuvering the Russian space capsule, which wouldn’t happen in the absence of air.  Also, I think (from a short stint at NASA where I consulted on a project related to it) that fires don’t burn the way they were depicted in the capsule scene.  Fires get fresh oxygen through convection, which doesn’t happen in microgravity situations like on the shuttle or in any free-fall orbit. There’s a Youtube video  which shows how a candle flame extinguishes itself in free fall when dropped down a long shaft – this simulates the same physics as a spacecraft in orbit around a planet.  (The theory behind this is originally due to Albert Einstein!)  However, the movie overall was entertaining and pretty intelligent, even if I was able to guess who was going to die and in which order within five minutes.  (The attractive heroine survives, the senior astronaut played by the big-time movie star dies heroically midway through the movie, and the hapless “red shirt” is killed off asap.  Told you there were spoilers!)

The incident driving the plot of the movie is a chain reaction of satellite destruction.  The Russians deliberately destroy an old satellite (presumably so that it won’t endanger other satellites in similar orbits), but this leads to the debris from that satellite destroying others, leading to that debris destroying others, etc.  That is, the Russians create the very situation they were trying to avoid – the planned destruction of one satellite leading to the unplanned destruction of many others.  The consequences include the downing of GPS navigation on Earth and space debris at high velocities coming in to destroy the shuttle the characters are working from.

OK:  two points.  One, the chain reaction is in fact a real concern.  This is known as the “Kessler syndrome” after the space scientist, Donald Kessler, who first predicted it in 1978.  Two, if it does happen, it will not be anything like in the movie.

Issue one:  Low-Earth orbit (LEO), some 160 to 2,000 kilometers above the planet, is filled with debris from older missions.  According to the Union of Concerned Scientists, about half of the thousand or more currently active satellites are in LEO, and there are tens of  thousands of centimeter size or larger fragments of debris from launches or older, inactive satellites.  The potential is that if there is enough material populating these orbits, chain reactions like the one in the movie may start.  Space is big – really big – but there’s a lot of stuff in that particular region of space.  Given enough time, collisions happen.  The collisions take place at very high speed – collisions will take place at relative speeds of something like the orbital speed at that altitude, or 15,000 miles per hour.  It’ll be higher or lower depending on the exact orbital parameters, but whatever it is will be high enough to completely destroy the two colliding objects.

The tricky part is that anything placed into orbit tends to stay there.  It’s not like on Earth, where gravity will cause the debris to fall out of the sky – when two planes collide, the little bits from the collision don’t race off at high speeds to become hazards to other planes for years or centuries to come.  In space, even in a low-Earth orbit, there is barely any atmosphere to cause friction to make the debris fall back down into the atmosphere and burn up there.  The idea behind the Kessler syndrome is that eventually there will be enough junk that the detritus caused by one collision will lead to more, with an ever expanding circle of destruction which will eventually take out most of the satellites at that particular orbital radius.   The mathematics is almost exactly the same as a chain reaction in an above-critical nuclear bomb core:  in that, one neutron’s fission creates more than one neutron, which leads to more fission processes: one leads to two, two create four, four, eight, for example, and eventually there are enough to destroy a city.  It’s harder to do the calculation for space debris, but according to a 2010 paper by Donald Kessler, several regions above 500 km have the potential for a runaway chain reaction.

So that part of the movie reflects real concerns.  However, for dramatic effect, the movie showed the debris ripping through the shuttle, causing mayhem, killing off one of the astronauts, and playing havoc with Earth’s communications satellites and GPS in very short order.  Not going to happen.

First off, most of the satellites are in much higher orbits than LEO.  Much, much higher.  Geosynchronous satellites for satellite TV and communications are at about 35,000 km up.  The GPS satellites, supposedly taken out by the cascade, are up at about 20,000 km.  It takes a whole lot of energy to move a particle from LEO into one of those orbits – most of the particles created by the cascade won’t have enough energy, and the laws of probability dicate that a hit by the small number of the ones which have enough energy is enormously improbable.  So our communications networks are safe.

The bigger problem with the movie is that this is a very slow process.  We think of chain reactions as being fast because of devices like the atomic bomb.  However, space is so sparcely populated that even in a chain reaction situation, the average time between collisions will be months, maybe even years.  The point of the Kessler syndrome isn’t that it’s fast but that one collision will lead to more than one collision afterwards.  The chain reaction wouldn’t have posed a threat to the astronauts up there unless they got incredibly unlucky.  There also wouldn’t have been thousands of these particles just happening to be going in the same direction – the collision would have led to them being spread out in all directions.  (This is not quite true, as the particles would tend to be travelling along the trajectory of the center of mass of the two colliding particles, but it’s close enough.)

I repeat: Gravity was a fun movie.  It’s a good example of the impersonal isolated man or woman against nature film.  If I wasn’t completely bowled over by it, it still made an impact (if you’ll pardon the pun.)