<p>There are orbits with 90min period. The difficult point is that if you are not in these orbits, the debris will not impact you, and if you are, you have their same speed, so they will not impact you but float around you.</p>

CC BY-SA 3.0
2013-12-12T23:14:14.190Z
2013-12-12T23:14:14.190Z
Envite
<p>If I understand you correctly (and I haven't seen the movie ^_^), you want to know if it is possible for two different objects in orbit to come close to each other periodically, with roughly the same period as one of the object's orbital period. </p>

<p>Yes, that is possible, but not probable.</p>

<ul>
<li>Obviously, if the objects are in the <em>same</em> orbit, they are always close to each other </li>
<li>If they are in <em>slightly</em> different orbits, one of the objects will seem to move about the other object in regular or irregular patterns, depending on orbit specifics</li>
<li><p>If they are in completely different orbits, the necessary conditions are </p>

<ol>
<li>the orbital period of both is equal</li>
<li>they have at most 1 point in common (or at least very closely approach each other)</li>
<li>the phasing of both objects is such that the objects are in the close approach region at the same time.</li>
</ol></li>
</ul>

<p>In ordinary two-body Keplerian celestial mechanics, an orbit's semi-major axis is the only orbital element that determines the orbital period. Therefore, one orbit may be circular and the other elliptic and inclined in all sorts of ways, but as long as the semi major axis of both is the same, they will have the same period. </p>

<p>However, this whole scenario is completely unstable for any real-world celestial body. The (relatively small) asphericity of the Earth, as well as the presence of the Moon for example, causes all orbits around it to drift in some way or another. If you put two objects in orbits like you describe, they will not likely meet up very often before never seeing each other again for centuries. </p>

CC BY-SA 3.0
2013-12-13T17:07:45.213Z
2013-12-13T17:07:45.213Z
Rody Oldenhuis
<p><em>[some spoilers]</em></p>

<p>If you were in the ISS and orbiting the Earth roughly every 90 minutes, would it be possible for debris to be orbiting such that it passes you every 90 minutes?</p>

<p>While Neil DeGrass Tyson pointed out scientific <a href="http://gawker.com/neil-degrasse-tyson-thinks-gravity-is-scientifically-i-1441842497">flaws</a> in the movie on <a href="https://twitter.com/neiltyson">twitter</a>, he <a href="http://www.theverge.com/2013/10/10/4823078/neil-degrasse-tyson-gravity-movie-high-quality">praised</a> many things the movie got right in interviews, including "The 90-minute orbital time for objects at that altitude."  I think I recall an interview on a podcast that I can't remember where he specifically said that referring to the debris recurring every 90 minutes.</p>

<p>In reality wouldn't the debris <em>have</em> to be in a different orbit?  If the ISS orbits every 90 minutes (92.87 really but I'm saying 90) then after 90 minutes they would be in the same relative position.  The Earth would have rotated in the mean time so they wouldn't be over the same point on Earth, but they would be in the same position in orbit about the Earth.  Is there an orbit that would allow the same debris to cross the same spot in their orbit every 90 minutes?  Would it be nearly the same orbit, just inclined slightly?  Would that explain why the debris was moving so slowly relative to the ISS as well?</p>

<p><em><strong>note:</em></strong></p>

<p>There are problems with the orbits and location in the movie, I simplified my question to ask about the particular orbit of the ISS.</p>

CC BY-SA 3.0
2013-12-12T21:03:51.130Z
Could you please provide a detailed explanation of both orbits you describe for people like me who haven't seen the movie?
Eduardo Serra
CC BY-SA 3.0
2013-12-12T23:53:31.690Z
Are you sure the movie tries to depict same orbital debris intersecting the orbit of the space station protagonists occupy or are in its vicinity? It was my impression that the movie tries to depict [Kessler Syndrome](https://en.wikipedia.org/wiki/Kessler_syndrome) and the second wave of incoming debris could be something else than the first one. But technically, what you're asking is possible if somehow the debris in question was retrograde to your own, intersecting yours every _n_ minutes at different spots over Earth. It's just not feasible, since most would have prograde orbit like yours.
TildalWave
CC BY-SA 3.0
2013-12-13T00:24:22.877Z
I think my explanation is over-complicating it and ignoring obvious errors in location in the movie.  In the movie the astronaut sets a timer because the debris will be back in 90 minutes.  The only explanation is that they themselves would complete an orbit in 90 minutes.  This is done before they move to the ISS and not re-done before moving to the chinese station.  My question is if you were in the orbit say of the ISS, would there be a possible orbit debris would be in so that it would strike the ISS every 90 minutes.
Jason Goemaat
CC BY-SA 3.0
2013-12-13T02:09:31.277Z
Well yes, technically, but unrealistic in reality. Since the orbital period of the ISS is roughly 90 minutes, that would mean the the debris would have to be in either slightly elliptical equatorial orbit and all around it for ISS to intersect it over the equator on each new pass, or in slightly inclined retrograde circular polar orbit for ISS to intersect it every a bit under 90 minutes over more or less the same datum. This is however as unrealistic as it gets, and an astronaut during EVA (or anyone else) wouldn't have any way to infer orbits of chain-reacting debris or next intersect time.
TildalWave
<orbit>
Is there an orbit that could cause regular debris showers like in the movie Gravity?
Jason Goemaat
# Is there an orbit that could cause regular debris showers like in the movie Gravity?

(Asked by: username_0 on 2013-12-12)

*[some spoilers]*

If you were in the ISS and orbiting the Earth roughly every 90 minutes, would it be possible for debris to be orbiting such that it passes you every 90 minutes?

While Neil DeGrass Tyson pointed out scientific [flaws](http://gawker.com/neil-degrasse-tyson-thinks-gravity-is-scientifically-i-1441842497) in the movie on [twitter](https://twitter.com/neiltyson), he [praised](http://www.theverge.com/2013/10/10/4823078/neil-degrasse-tyson-gravity-movie-high-quality) many things the movie got right in interviews, including "The 90-minute orbital time for objects at that altitude." I think I recall an interview on a podcast that I can't remember where he specifically said that referring to the debris recurring every 90 minutes.

In reality wouldn't the debris *have* to be in a different orbit? If the ISS orbits every 90 minutes (92.87 really but I'm saying 90) then after 90 minutes they would be in the same relative position. The Earth would have rotated in the mean time so they wouldn't be over the same point on Earth, but they would be in the same position in orbit about the Earth. Is there an orbit that would allow the same debris to cross the same spot in their orbit every 90 minutes? Would it be nearly the same orbit, just inclined slightly? Would that explain why the debris was moving so slowly relative to the ISS as well?

***note:***

There are problems with the orbits and location in the movie, I simplified my question to ask about the particular orbit of the ISS.

**Tags:** orbit
> **Comments:**
> **username_1**: Could you please provide a detailed explanation of both orbits you describe for people like me who haven't seen the movie?  
> **username_2**: Are you sure the movie tries to depict same orbital debris intersecting the orbit of the space station protagonists occupy or are in its vicinity? It was my impression that the movie tries to depict [Kessler Syndrome](https://en.wikipedia.org/wiki/Kessler_syndrome) and the second wave of incoming debris could be something else than the first one. But technically, what you're asking is possible if somehow the debris in question was retrograde to your own, intersecting yours every _n_ minutes at different spots over Earth. It's just not feasible, since most would have prograde orbit like yours.  
> **username_0**: I think my explanation is over-complicating it and ignoring obvious errors in location in the movie.  In the movie the astronaut sets a timer because the debris will be back in 90 minutes.  The only explanation is that they themselves would complete an orbit in 90 minutes.  This is done before they move to the ISS and not re-done before moving to the chinese station.  My question is if you were in the orbit say of the ISS, would there be a possible orbit debris would be in so that it would strike the ISS every 90 minutes.  
> **username_2**: Well yes, technically, but unrealistic in reality. Since the orbital period of the ISS is roughly 90 minutes, that would mean the the debris would have to be in either slightly elliptical equatorial orbit and all around it for ISS to intersect it over the equator on each new pass, or in slightly inclined retrograde circular polar orbit for ISS to intersect it every a bit under 90 minutes over more or less the same datum. This is however as unrealistic as it gets, and an astronaut during EVA (or anyone else) wouldn't have any way to infer orbits of chain-reacting debris or next intersect time.  

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## 2 answers

### Answer by username_4 on 2013-12-13. Score: 5 | Accepted answer

If I understand you correctly (and I haven't seen the movie ^\_^), you want to know if it is possible for two different objects in orbit to come close to each other periodically, with roughly the same period as one of the object's orbital period.

Yes, that is possible, but not probable.

* Obviously, if the objects are in the *same* orbit, they are always close to each other
* If they are in *slightly* different orbits, one of the objects will seem to move about the other object in regular or irregular patterns, depending on orbit specifics
* If they are in completely different orbits, the necessary conditions are

  1. the orbital period of both is equal
  2. they have at most 1 point in common (or at least very closely approach each other)
  3. the phasing of both objects is such that the objects are in the close approach region at the same time.

In ordinary two-body Keplerian celestial mechanics, an orbit's semi-major axis is the only orbital element that determines the orbital period. Therefore, one orbit may be circular and the other elliptic and inclined in all sorts of ways, but as long as the semi major axis of both is the same, they will have the same period.

However, this whole scenario is completely unstable for any real-world celestial body. The (relatively small) asphericity of the Earth, as well as the presence of the Moon for example, causes all orbits around it to drift in some way or another. If you put two objects in orbits like you describe, they will not likely meet up very often before never seeing each other again for centuries.


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### Answer by username_3 on 2013-12-12. Score: 1

There are orbits with 90min period. The difficult point is that if you are not in these orbits, the debris will not impact you, and if you are, you have their same speed, so they will not impact you but float around you.


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