What “Gravity” Gets Wrong

Let’s establish a few things at the very beginning. I love movies about space. I quite happily go along with improbabilities like warp drives, phasers, Vulcan mind melds and the concept that every extraterrestrial species we encounter will be remarkably humanoid. My very favorite science fiction trope is time travel, as unlikely a physical concept as there is. So I most certainly enjoyed the recent movie Gravity, starring Sandra Bullock and George Clooney, and I would recommend it to anyone as a gripping story and spectacular visual feast. Still, there was this quietly persistent voice in the back of my head sputtering “But, but…you can’t do that!” Will these sorts of things keep you from enjoying the movie? Well, they didn’t keep me from enjoying it, and I am probably the biggest science nerd that most of you know. Without giving away anything that isn’t in the trailer, I’m going to focus on the most egregious of these scientific boo-boos.

When the space shuttle is damaged beyond repair, George Clooney uses his MMU (Manned Maneuvering Unit) to fly him and a tethered Bullock to the International Space Station, where they hope to take an attached Soyuz spaceship back to Earth. In the movie, the shuttle was on a repair mission to the Hubble Space Telescope.  All of these are real. The MMU was last used in 1984; after the Challenger disaster it was deemed too dangerous and all extravehicular activities since then have been carried out with manipulator arms or tethered astronauts.

Once you are in orbit, the capability of your rocket engine is measured in how much change in velocity (ΔV, delta-V) you can produce. This is a function of both the power of the engine and the amount of fuel. The space shuttle orbital maneuvering system had a total ΔV of about 1000 feet per second. The MMU had a total ΔV of about 80 feet per second. Would this be enough to get you from the Hubble Space Telescope’s orbit to that of the International Space Station?
The Hubble and the ISS orbit at different altitudes, the Hubble flying at 347 miles up and the ISS at 230 miles. So you just drop down to it, right? It’s not that simple. Your altitude and your orbital speed are tied together; the closer you are to the surface of the Earth, the faster you must go to stay in orbit. The ISS, in a lower orbit, moves almost 700 feet per second faster than anything orbiting with the Hubble.  But the shuttle doesn’t begin at the Hubble with a full tank.  When the space shuttle flew to the Hubble, it really didn’t have enough fuel left to also fly to the ISS and to slow down enough to re-enter the atmosphere. So if the space shuttle couldn’t have made it, the MMU wouldn’t even come close.

As if that weren’t enough, the two orbit the Earth at different angles, as shown here.

And changing the angle of your orbit (your orbital inclination) requires even more fuel. Get the picture? But I wouldn’t want to see the movie that didn’t give Sandra and George a fighting chance for survival, even if we have to break a few physical laws to make it happen.

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Posted in human spaceflight, Spacecraft

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