Most spacecraft are launched and orbit prograde -- in the same direction that the Earth rotates -- to give a "speed boost" to get to orbital velocity. One would therefore expect that the trajectories of the debris caused by spacecraft collisions would not be truly random in all directions, but rather biased in prograde directions.

Has this actually been observed by those tracking orbital debris?

Inspired by this comment.

"What is the typical relative impact velocity of orbital debris in low Earth orbit?" is a somewhat similar concept, but the answers take a theoretical approach. I am asking for an evidence-based answer.

I am deliberately not tagging this , as I do not believe the effect depends on the density of orbital debris.

  • $\begingroup$ Are you asking about all "orbital debris", or only the subset "caused by spacecraft collisions"? $\endgroup$
    – BowlOfRed
    May 3, 2019 at 23:32
  • $\begingroup$ @BowlOfRed: Ideally the subset caused by collisions, if data on such is available. If the data on that is not available, then debris in general. $\endgroup$
    – DrSheldon
    May 3, 2019 at 23:36
  • $\begingroup$ Just FYI. I did not look this up recently, but I think a majority of satellites are in polar orbits (or near polar) which do not exactly count as prograde. Of the ones that are polar, some are slight retrograde! $\endgroup$
    – JohnHoltz
    May 4, 2019 at 2:55
  • $\begingroup$ @JohnHoltz - Almost all of those near polar orbits are slightly retrograde. A sun synchronous orbit requires an inclination greater than 90°. $\endgroup$ May 4, 2019 at 7:58
  • $\begingroup$ I would like to see a more evidence based study using actual tracking data as well. Any updates on this? $\endgroup$
    – not2qubit
    Aug 18, 2023 at 9:11

2 Answers 2


Polar and highly inclined orbits mean objects in orbit can shed debris in non-prograde orbits. These objects can also collide at large angles producing big sprays of debris. For example, in 2009 Iridium 33 and Kosmos-2251 collided. They were both in polar orbits but offset almost 90 degrees. They collided with a closing speed of 42,000 km/h.

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After an hour the debris had begun to spread out along their respective orbits.

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Another is debris from anti-satellite weapon tests, such as the one recently conducted by India. Debris from the satellite will continue in its orbit, but debris from the interceptor will continue in the opposite direction creating a sort of double shotgun blast of debris.

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See also...

  • 1
    $\begingroup$ I appreciate your answer. The question is about the the statistical distribution of velocity among all orbital debris, not just selected examples. A few examples may not represent the population as a whole. As such, this post doesn't really answer the question. The UN Report is an interesting read, but it does not provide velocity or inclination distributions. It does mention a model by Nazarenko which uses actual data and considers such parameters. That model probably has the answer. However, I cannot read Russian. $\endgroup$
    – DrSheldon
    May 4, 2019 at 14:01

I am not aware of the data to do this but I am confident that the answer must be yes, there will be a bias.

While there are clearly going to be exceptions (high energy collisions will send bit in all directions) its worth noting things don't stay orbital debris unless they are still in a stable orbit. The mass average west-east bias will remain. Further there is a re-enforcing mechanism: The pieces that are ejected in line with the average velocity have more orbital velocity and are likely to stay, those that are ejected 'backwards' have lower orbital velocity and will return to earth sooner. Its also worth noting that even though the collisions are highly energetic, the very short timescales means that momentum rarely transfers effectively between the impacted and non impacted bits. Hence the majority of the mass just carries on as it was, directly taking up the bias.

Hence the bias for debris is unlikely to be far off the overall bias. Again individually there are exceptions but overall things orbit more 'west-east' than 'east-west'.


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