Orbital Mechanics

Earlier, I was watching NASA TV's coverage of tonight's press conference on the shuttle. I discovered later that I'd left the connection active, meaning that my media player was still playing NASA TV.

There, on the screen, was a lovely sight . . . a video feed from the shuttle, looking down on Earth from her orbital distance of 310km (195mi) or so as of this writing. The image showed the horizon at the very bottom, with wispy clouds visible and, soon enough, the terminator appearing. It's really quite remarkable how fast the shuttle is going . . . I guess it's been too long since I saw that view . . . but the planet spins below in a rather top-like manner.

(Well, okay, the lady just said it was a replay of video recorded earlier as Discovery passed over the United States (not that I could tell). So, it wasn't live video, but it was only a few minutes prior. In any case, I digress . . . )

And that's when it occurred to me . . . and I could've slapped myself for not thinking of it sooner . . . but here we had evidence that the RoTS space battle was not a moving fight.

The battlefield was in Coruscant orbit, and the orbital height was not that great. Assuming an Earth-size Coruscant, then the orbital height ought to have been significantly less than that of ISS (which averages 356km (220mi) . . . currently 353km), judging from eyeball estimation.

However, the vessels were not moving at an expected orbital speed, one which would show motion relative to the planet. If anything, they were in a very low stationary orbit, presumably over the area of the Senate and Chancellor's location.

This has some intriguing implications. For instance, I've noted previously that:

A ballistic re-entry from standard orbital velocities (i.e. somewhere in the 25,000km/hr range or less) usually produces temperatures in the 3000ºC range.

However, if this wasn't a ballistic re-entry from orbital speed, then the temperature ought to have been significantly less. I mean, sure, the ship was de-orbiting, but it's not like she was hitting the atmosphere at 25,000km/hr (15,500mph) as she would've had she de-orbited while at orbital velocity.

I'm curious to know just what an object dropping like a brick from the sky would end up at, speed-wise. If the battle occurred at 100 - 200km above the surface, then the force of gravity at that height would be about 95% of standard surface gravity, so the ship could definitely have accelerated. I'll get around to doing the math later.

(Incidentally, while many have complained about the shifting angle of gravity in the film, it actually made a fair bit of sense. However, the ship would've had to have been at some sort of reverse thrust status when her nose pitched down to cause everything to fall forward . . . had she been in freefall, the apparent gravity on board the ship would've been zero, and Invisible Hand would've been Vomit Comet.)

(Also, some orbit video from the space station is visible here.)


Anonymous said...

Well, there's going to be quite a bit of a fudge factor involving the depth of Coruscant's atmosphere/altitude of the landing pad, but until the ship dropped to about 30,000 meters, drag would be essentially nothing for a ship heading for Earth. Maybe a peak speed of 1-2 km/sec would be my guess.

IIRC, though, we saw some air resistance in play blowing off drones, which would put re-entry down in the already-lots-of-drag region and re-entry peak speed as plain old terminal velocity.

Anonymous said...

Well, I watched the view from "Discovery". It is clear that Earth is moving FAST below, and this on 400 km distance. It is clear ships in ROTS aren't ORBITING - they hang over Coruscant through repulsorlifts. The maximal Velocity for such deorbiting is 2 km/s - less then modern fighters. IH glowing means REAL bad hull.