Gravity

[Guest]

The other night we watched the film Gravity with George Clooney and Sandra Bullock.  The film wasn't bad but I think some of the physics was a bit off.  What do you think?

The film starts with Russia shooting down a satellite and the wreckage from that caused a chain reaction which took out a number of satellites causing a huge amount of debris  which rushed around the orbit eventually destroying the space shuttle and hubble telescope it was servicing.  George sets his timer to 90 minutes which is the orbital period for the next time the debris comes around. But wouldn't that debris have the same orbital speed as the space shuttle and any debris which has a different speed would move into a different orbit anyway.  Actually the guy from mission control screams something like "It will be coming at you with the speed of a high velocity bullet" which I don't think he would have said even if it was true.

Another thing was at one point Sandra has to travel from the international space station to a Chinese spacecraft in an escape module by firing it's engines. "It's only a hundred miles" says George. " Just a sunday drive". So I thought that must be harder than it looked in the film and I ended looking up Hohmann Transfer orbits which are quite interesting.  I don't think you can just point and shoot even if the two craft are in the same orbit. I don't think it works like that.

I found something interesting about Hohmann transfer orbits. Suppose you want to go from a low orbit to a high orbit. The speed for the high orbit is lower than the speed for the low orbit but in order to go from low to high you have to increase your speed twice. 

I know I'm nit picking and it was a good film and quite exciting but it did get me thinking.

I bought a book called "Fundamentals of astrodynamics" which is quite good but all the units are things like ft/sec and nautical miles.  The units for G are in dynes cm2 /gm2

Cheers

Steve

 

[JamesF]

18 minutes ago, woodblock said:

Another thing was at one point Sandra has to travel from the international space station to a Chinese spacecraft in an escape module by firing it's engines. "It's only a hundred miles" says George. " Just a sunday drive". So I thought that must be harder than it looked in the film and I ended looking up Hohmann Transfer orbits which are quite interesting.  I don't think you can just point and shoot even if the two craft are in the same orbit. I don't think it works like that.

I found something interesting about Hohmann transfer orbits. Suppose you want to go from a low orbit to a high orbit. The speed for the high orbit is lower than the speed for the low orbit but in order to go from low to high you have to increase your speed twice. 

I've just read "Carrying  the Fire" by Michael Collins (the one who got to stay behind when Armstrong and Aldrin went down to the Moon's surface).  He talks about this, because obviously they had to practice docking as part of the preparation for the trip.  For Gemini 10 they had to dock with an Agena booster that had already been put into orbit and use that to boost themselves up to higher orbit and "catch up with" a second Agena booster from Gemini 8(?) where he performed an EVA.

His description suggests that it is certainly not a simple process to meet up with another object in orbit and you certainly can't just "point and shoot".

James

[Guest]

I've got the Michael Collins book as well and excellent it is too. Sadly he died last month. I think a large part of the Gemini project was to practice those kind of manoeuvres. 

I think that whole thing about navigating is space is interesting. How do you actually measure your velocity and position accurately? By the stars I assume.

Steve

 

[andrew s]

The debris might just be on an intersecting orbit with high eccentricity and or inclination. However, I doubt he could have eyeballed the calculation. 

Guess that's why it's called science fiction. 

Regards Andrew 

[DarkAntimatter]

A satellite blowing up seemed to cause a lot of carnage.  Imagine something like that blowing up on the surface of the earth.  Bombs larger than that (unfortunately) go off frequently and the damage area is small compared with the earths surface.  The region a few hundred of miles above the earth is even larger than the earth's surface. Granted there is no atmosphere to stop pieces from orbiting.

Also, I didn't understand why the Chinese station was all of a sudden falling into the atmosphere.

[blameTECHIE]

If it had been Chuck Norris instead of George Clooney then I'd have believed it, but then Chuck would have been fine without a suit and most likely just performed his own re-entry and landed on the White House lawn

[StevieDvd]

If you watched the film with George and Sandra, did you ask them any of those questions?

😀 sorry just saying what we are all probably thinking.

[Carbon Brush]

I saw the film a few years back. On a small screen, back of an aircraft seat. Long flight to somewhere nice. Remember those?
Without the spectacular views provided by a cinema experience, or wall size TV, it was a cure for insomnia.

Hollywood and reality rarely have significant overlap. We watch for fun.

Changing orbit height by a few metres to dock, is a very counter-intuitive procedure. Never mind going for many KM changes.

[Lariliss]

Starlink launches are said to be producing 50% of collisions nowadays.

Having 20k+ junk pieces already on the orbit for all the time, it looks inevitable as a result of Starlink, sending 15k and preparing twice more.

I assume, there must be mitigation points behind:

- Where the collision numbers become critical to hold on new launches.

- Automatic manoeuvring systems continuous calibration.

- Emergency actions for the critical collisions that might happen.

Every stargazer/photographer might shoot theory-proof film of collisions’ impact and become famous.