Hacker News Reader

Comment by talldan | Hacker News Reader

I think Gravity has a few moments of dodgy orbital mechanics.

Not sure if that was the point!

Yeah, it had some, but it's still a separate league from what came before. I don't expect every movie to be hard sci-fi (wouldn't mind it, though). But they should start getting the very basics right.

2001 space odissey came quite early as well as space 1999

Going from ISS to Tiangong in anything like a real-world spacecraft is, like, super impossible.

It's perfectly possible.

If you land and launch again.

That's a movie stage setup problem though, not a physics issue.

How so? The movie shows them going from A to B, physics says going from A to B takes far more delta-v than the spacecraft would have, therefore it's a physics issue. It would be like a WWII movie showing a bomber taking off from NYC and bombing Berlin.

The Tiangong in the movie isn't a real space station so if the Chinese were to build it they could put it in any orbit they like, even one hypothetically 10m/s of delta-v away from the ISS and there might even be realistic reasons for them want to be energetically close to the ISS. There are reasons not to do it but if there's a good reason like the Chinese want to keep the option of physically linking to the ISS open then there's nothing actually preventing it.

So it isn't two known and far distant locations like New York and Berlin, its more like New York and an unnamed city.

That wouldn't make much sense, but even if we grant that, just switch the complaint to Hubble and ISS. We know where both of those are.

When that movie came out, I was studying a basic astronomy course at the university and we were going through some simple orbital mechanics. We had a long discussion about can the debris of Hubble hit the ISS or not.

The conclusion was probably not, but under perfect storm circumstances (and especially timing) a fragment or two might not be out of the question.

> The Tiangong in the movie isn't a real space station

It's a real space station name. The one they use is much bigger than the real station with the name, but it might be intended to be a future development, or it could be a fictional station with the same name (a city called New York, but with a population of 200 million and located in central France, to borrow your analogy), or they could have just intended the existing station and not bothered to research anything about it besides it's name and nationality.

>It would be like a WWII movie showing a bomber taking off from NYC and bombing Berlin.

NYC to Berlin is well within the originally required flight range of the B-29 (5,333mi at 400mph with a 2k lbs load). It wouldn't be an intelligent thing to do, but it isn't physically impossible.

I was thinking of a round-trip mission, but I always appreciate nitpickery. (And I learned just how much carrying a heavy bomb load affected the range of the B-29!)

You can assume that they just repositioned any three of Hubble, the Russian satellite, ISS, and Tiangong into inaccurate (for the things that are real, like Hubble and ISS, and maybe Tiangong, if it's a further development of the existing station of the same name) or improbable (for those that are fictions, like the Russian satellite or Tiangong, if you assume it's a complete fiction that shares the name of a a real station) orbits, or you can assume they got physics wrong. I don't think the movie provides enough information to make clear where the error is.

If I remember, momentum and orbit matching in Gravity were pretty shady. Still way better than most predecessors, though.

Reply on news.ycombinator.com