# In Gravity, are the orbital mechanics of space debris plausible? [closed]

I Gravity, a satellite somewhere is shot down by a missile, and the debris from that hits other satellites causing a chain reaction of rapidly increasing amount of space debris.

The protagonists are rouglhy in the same orbit with the ISS. The cloud of debris hits the protagonists, after that hitting them again every 90 minutes.

Ignoring the fact whether such a chain reaction is plausible, and ignoring whether the orbits are at correct height and time length[1]:

Can the cloud of debris created from destroyed satellites be on a trajectory to hit the protagonists every 90 minutes?

Shouldn't any mass of debris from destroyed objects follow roughly the same trajectory as the original object? So I would imagine that in order to hit the protagonists, the debris would have to be in the same orbit, but travelling at the opposite direction. Which also means the original objects would have been in the same orbit, travelling in opposite directon. Is this plausible - are there objects around earth travelling in opposite directions in the same orbit? Or am I missing something about the orbital mechanics (I'm not an expert in space orbital physics, as you can see)?

[1] - it seems that the real ISS has an orbit that goes around the earth in 90 minutes, so if there was something in the same orbit in opposite direction, they would actually meet every 45 minutes.

• As an aside, the cascade failure is described by Don Kessler's Kessler Effect. Nov 26, 2013 at 0:17
• not enough for an answer, but note that one of Neil DeGrasse Tyson's Twitter complaints about the movie claims "Nearly all satellites orbit Earth west to east". (Note that the movie even gets the direction wrong!) Dec 6, 2013 at 19:54
• You may find the following helpful: physics.stackexchange.com/a/222298/59023 Feb 11, 2021 at 16:17

It's not very plausible, really. The orbits of the various satellites we have up there are very different. If a satellite were to explode, it would not change its orbit much, so it wouldn't be much of a danger to the other satellites. I remember reading an article after I saw the movie, it's quite interesting.

Here it is.

Anyway, I'll quote what's relevant to your question:

Phil Plait: Debris will move away from an explosion and each little piece will stay more or less on the same orbit. You get a cloud of debris, but over time, it expands. Usually it’s not that bad right away because satellites are specifically put on orbits that are different from other satellites.

Also:

Phil Plait: The three objects — the Hubble, the ISS, the Chinese station — are in three completely different orbits. It’s not just that they’re in different heights, the orbits are in different shapes. It’s tilted, like two hula hoops, one inside the other and then tipped at an angle.
Leroy Chiao: The orbital planes are probably not even co-aligned, so they’re in totally different orbits and totally different altitudes.

However, you wish to ignore this. Well, then, in that case, it is perfectly plausible for two bodies to be in the same orbit with a 'meeting' period of 90 minutes. What determines your orbital height is basically only your velocity. Orbital radius is inversely proportional to the speed squared.

So basically, if two bodies are in the same orbit (opposite angular velocity), they'll have the same speed. Which means they'll meet every half rotation and that their time period of revolution is actually 180 minutes. Anyway, yes, it's plausible.
Don't think we have any bodies in the same orbit in opposite directions right now. That'd be too accident-prone, right? The real scenario is mentioned in the article, though. :)

• Even though I did specifically choose to ignore it, I appreciate addressing those issues. I already suspected they were very failed, but I was interested in whether even the concept would be possible. Thanks for the link, too! Nov 22, 2013 at 9:34
• No problem :) I really loved the movie. I was spellbound by the visuals and gripped by the story. It was only in retrospect that I thought of the obvious physics flaws. However, it wasn't created to be flawless according to Newton. I loved it anyhow. Nov 22, 2013 at 11:43

tl;dr - it's possible, but not with the likely location of current satellites.

You can have two ways to have regular interactions in orbits: different inclinations and different eccentricities.

• Inclination is the vertical angle off of the reference plane:

Picture

http://en.wikipedia.org/wiki/Orbital_inclination

You can see the "ascending node" in this picture - it, and its complimentary "descending node", are the two points at which the orbits are aligned. If the altitudes of the orbits are the same at those nodes, and the station and the debris arrive at the node at the same time, they'll interact regularly. Any orbits that have the same altitudes at those two points will have almost identical altitudes at any point in time, so their orbits will take the same length of time and they'll stay synchronized.

However, even if this were to happen to the ISS, the timing is off, as the ISS orbits Earth every ~ 93 minutes, so the debris collisions would happen every 46 minutes (once at each node).

• Eccentricity is how circular or elliptical an orbit is. Orbits with eccentricity close to 0 are very circular, orbits with eccentricity less than one are somewhat elliptical, and orbits with eccentricity >= 1 are either parabolic or hyperbolic, meaning the orbiting body will escape and never return.

http://en.wikipedia.org/wiki/Orbital_eccentricity

If two orbits align at some point, and one's eccentricity causes its orbital period to be a multiple (let's call it `n`) of the other's orbital period, then they'll run into each other every `n` orbits. For example, the ISS is on a very circular orbit with an orbit period of ~ 93 minutes. If the debris were on a smaller, eccentric orbit with a period of (for example), 1/2 that of the ISS, they would run into each other every 93 minutes, because every second orbit of the debris would put them in the same place at the same time as the ISS.

HOWEVER, all of these situations are highly improbable. Orbits - their altitude, inclination, and eccentricity - are chosen very carefully based on their purpose. The purpose of the ISS is to be serviceable from as many space-faring countries as possible, requiring it to be low enough to move fast to give everyone lots of launch windows. Most satellites are trying to take very precise pictures or measurements, and so will be at a much higher altitude orbit in order to have a slower radial velocity (if they're looking down) or be more isolated from our atmosphere and therefore last longer (good in general). It's very unlikely that there would be a station close enough to the ISS to cause the effects seen in the movie, but it is possible.

I've been pondering on this and reading up also. I've found it useful to compare different scenarios. 1st, the "facts" as laid out in the movie:

1. The debris is hitting the Hubble/Shuttle site, ISS and Tiangong ("the three orbiters" from here on), in sequence (my impression from the camera angles is that they are hit in that order too)
2. It's hitting every 90 minutes.
3. The Hubble/Shuttle, ISS and Tiangong are on the same orbital path and seem practically stationary in relation to one another.

Ok? Scenarios:

1. The debris is following the same orbital path as the three orbiters, but faster. [Impossible: If orbiting faster it would be at a higher orbit, end of. Same for slower orbit - lower]
2. The debris is (somehow) stationary i.e. not orbiting and the three orbiters are coming upon the debris field each orbit. [Impossible: if not orbiting, the debris would be pulled toward earth at about 1G and would be long gone by the next trip around by the three orbiters]
3. The debris is orbiting the same path, but in the opposite direction than the three orbiters. [Mental: it would be a bit bonkers to do that in the first place, assuming the debris field has roughly the same momentum as the satellite which spawned it. That aside, the ISS has a orbital period of about 90 minutes, so the field would hit every 45 minutes in this scenario. It would also have a closing speed of about 32,000 mph - they literally wouldn't know what hit them. They certainly wouldn't be "seeing" the debris field approach]
4. One semi-plausible version of events I can get to is that the debris is on a slightly esoteric, elliptical orbit, which is tuned just so that its perigee intersects with the three orbiters. At perigee it would be moving at its fastest, so would be able to "catch up" with them and do the violencing of their boats.
5. Otherwise, a canted orbit which intersects with theirs, or this in combination with a slight elliptic in order to avoid the problem of meeting twice each orbit. The problem with this is the fact that it hits all three orbiters, thus would have to be really wide, yet dense enough to look like it did and wreak havoc.

Gravity.

The scenario in the movie Gravity is not plausible. Any object struck by an explosive missile would spread out both vertically and horizontally very quickly. The momentum and explosive force of the missile would cause this within a few seconds of impact. Also orbital physics would predict that the effectively random amounts of moment imparted to each piece of debris by the missile would cause these pieces to radically change there orbit from the original orbit of the destroyed satellite. The movie depicts the debris travelling in the same orbit as the space station in a close formation, and this must be in the exact opposite direction for the physics to work. If something was destroyed by explosive on the earths surface and the pieces all flew of in the exact same direction at the exact same speed it would be surprising to say the least.

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