orbituary

I watched Gravity. In 3D. It is super rad. I found myself ducking debris at least twice. There have been a whole host of people that have praised and criticized the science in this film. I mostly got hung up on one thing, inter-vehicle travel.

Before I begin, a caveat. As others before me have also pointed out, the science in this film is good enough to earn criticism. One doesn’t reasonably criticize the science of The Hulk or Cloudy with a Chance of Meatballs. Gravity is really good, especially in 3D. That it warrants criticism is in itself a compliment.

To aid with the plot, the Hubble Space Telescope and the International Space Station are (incorrectly) stated to be in the same orbit and then Sandra Bullock travels between them. That travel is what caught my attention.

Let’s say you want to “catch up” to something that shares your orbit but is ahead of you. Let’s also keep the orbits circular to simplify things. The first instinct for 2D beings that are used to driving, bicycling, or walking might be to think you can just aim your nose at the target and hit the gas. Sandra Bullock does that and it works great for her. Sadly this would actually get you further away. Specifically, you will end up above (higher orbit) and further behind it (greater linear distance away). Here is a doodle I made in mspaint. You are now also orbiting slower than your target (higher orbits are by definition slower).

Accelerating toward your target pushes you to a higher orbit

Accelerating toward your target pushes you to a higher orbit

You and I (and Sandra) are not the first people to be confused by this. Gemini 4 failed to achieve their secondary mission objective of “first ever orbital rendezvous” because of this trickiness. Fortunately for NASA, Buzz Aldrin published published a PhD thesis in 1963 titled “Line-of-sight guidance techniques for manned orbital rendezvous.” This document served the flight engineers well and they got a handle on orbital rendezvous in time for Gemini 6.

The trick in this case is to slow down so your orbit drops, this then causes you to orbit faster than the object above you. Once you get ahead of and are below your target, you can push back up to meet it!

Moving to a lower orbit will allow you to get ahead of your target, putting you in position to move back up for a rendezvous!

Moving to a lower orbit will allow you to get ahead of your target, putting you in position to move back up for a rendezvous!

Suffice it to say, orbital mechanics are tricky. I will let both Sandra Bullock and James McDivitt off the hook.

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One Response to orbituary

  1. Pingback: getting high on EVA | Eat Logical Sentences

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