2015-05-26

Space and Orbit

I followed a link to my site back to this spiffy post from a like-interested fellow.  In it, he discusses the energy that a Star Destroyer would have to dump when de-orbiting to crash as in the new film trailer.

I rather liked the cut of his intellectual jib, but the flaw of his calculation was pointed out by a commenter.  The simple fact is that Star Wars ships are rarely seen in what we would call an orbit.  Sure, they're often in space above a planet at orbital altitudes, but that's not the same thing.



The space station orbits at around 400 kilometers.  We often see unrealistic but amazeballs time-lapses like this:


But in reality, the orbit is much slower, though still helluva-fast, and there is no soothing Solaris music, or even whatever this music is:

 

The lower you are, the faster you have to be going to 'outpace' gravity's pull so you keep falling around the planet.  Low Earth orbit (LEO) is usually quoted as requiring a velocity of 7.8 kilometers per second.  The much higher geostationary orbit, around 35,000 kilometers up, only requires a bit over 3 kilometers per second.

Meanwhile, consider the opening of ANH.  The battle between the Tantive IV and the Devastator (whoops, sorry, that's Disney universe) Star Destroyer happens in low orbit over Tatooine.  The planet looks stationary beneath.  Further, per the novelization, Luke watches the battle in binoculars, goes to town, and then shows the folks he meets there.  The ships are still in the sky above.

That's not orbital velocity.  An ISS pass overhead is complete in a couple of minutes from horizon to horizon.

But now ponder the opening battle of Revenge of the Sith high in the atmosphere of Coruscant, similarly watched from the surface by Coruscanti citizens.   When the repulsorlift and any onboard grav systems partially fail after a fierce broadside cannonade by a Republic Attack Cruiser, the Invisible Hand starts to fall nose-first.  The occupants are subjected to the gravity of the planet, just as they should be if the ship were stationary or otherwise not at freefall velocity.

So, no, that is not orbital velocity, either.  But even Phil Plait gets this one wrong by assuming it is, too

Of course, the ship also burns on the way down the second time, seemingly, but not necessarily, as if undergoing re-entry ... but that will be the topic of another, more calc-heavy post, in which we'll discuss the fact that the flying wreck of the Hand had a probable terminal velocity of 2.7 km/s, among other interesting number-crunchy nerdy stuff.

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