From this question we know that there are quite a few satellites orbiting Earth. But how do ESA and NASA avoid their rockets colliding with these satellites (is there some kind of database of (man-made) satellites)? And is it possible that countries have secretly sent up spy satellites which Space organisations cannot track?

I am also interested if there have ever been collisions between rockets and satellites before...

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    $\begingroup$ All satellites are trackable. Secret satellites' purpose is a secret, but their launch is impossible to hide, and once launched they are tracked. $\endgroup$
    – called2voyage
    Dec 7, 2016 at 16:31
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    $\begingroup$ Space is big, satellites and rockets are small. You would have to aim very precisely to hit anything. $\endgroup$
    – jkavalik
    Dec 7, 2016 at 17:12
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    $\begingroup$ @jkavalik I understand that, but just because the chance to hit one is small, doesn't mean Space agencies don't take them into account :) $\endgroup$ Dec 7, 2016 at 17:20
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    $\begingroup$ Of interest: Space Surveillance Network $\endgroup$
    – called2voyage
    Dec 7, 2016 at 17:25
  • $\begingroup$ Also related Can I get higher with Space Junk? $\endgroup$ Dec 7, 2016 at 19:09

2 Answers 2


Space agencies actively track not only satellites but debris in orbit using radar, and they do have a database of these objects and powerful computer which can calculate where these objects will be based on their parameters. When a launch is planned these objects are taken into account and the trajectory planned to avoid them.

It is possible (although unlikely) that stealthy satellites exist which are not tracked. In this case the controlling agency of that satellite will almost certainly do their own calculations to make sure their asset remains clear, and if not the sheer volume of space would make the probability of any collision very very low.

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    $\begingroup$ I would doubt the "unlikely" part. A few US agencies are experimenting with payloads using the cubesat form factor, which is right at the limit of observability when no effort is given to obscure them. Applying techniques to reduce their observability (e.g., radar absorbing coatings, etc) would make them all but impossible for third parties to track. I have no direct knowledge of any of these in existence, but I could see very good reasons to do so, so I would peg this as "more likely than you might guess." $\endgroup$
    – Tristan
    Dec 8, 2016 at 16:50

Nasa have records of data about debris and satellite orbiting the earth which it share with other agencies out there.

Tracking Debris

The Department of Defense maintains a highly accurate satellite catalog on objects in Earth orbit that are larger than a softball.

NASA and the DoD cooperate and share responsibilities for characterizing the satellite (including orbital debris) environment. DoD’s Space Surveillance Network tracks discrete objects as small as 2 inches (5 centimeters) in diameter in low Earth orbit and about 1 yard (1 meter) in geosynchronous orbit. Currently, about 15,000 officially cataloged objects are still in orbit. The total number of tracked objects exceeds 21,000. Using special ground-based sensors and inspections of returned satellite surfaces, NASA statistically determines the extent of the population for objects less than 4 inches (10 centimeters) in diameter.

Collision risks are divided into three categories depending upon size of threat. For objects 4 inches (10 centimeters) and larger, conjunction assessments and collision avoidance maneuvers are effective in countering objects which can be tracked by the Space Surveillance Network. Objects smaller than this usually are too small to track and too large to shield against. Debris shields can be effective in withstanding impacts of particles smaller than half an inch (1 centimeter)

Space Debris and Human Spacecraft

Isro uses the data complied by Nasa with a Radar specifically design to track multiple objects at same time called MOTR right now.



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