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This night I've seen two SL-16 R/B on the sky with my binoculars. These where part of some Zenit rockets that launched satellites in the 90s. They travel at altitudes that double the ISS, uncontrolled.

I tried to understand what part of the Zenit rocket are they exactly but I couldn't find an answer. I guess they are not the first stage since that shouldn't get to orbit, is it then the second stage?

My questions are:

1) What part of the Zenit rockets are they?

2) Is there any model or picture to see how they look?

3) What size does it have?

4) How long is expected for these objects to be in orbit? One of those I saw this night was there since 1995 so I guess there is little drag at that altitude.

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According to Heavens Above, it's the upper (2nd) stage of a Zenit-2 launcher, as you guessed. SL-16 was the identifier used by Western intelligence agencies during the cold war for the Zenit. They're cylindrical, about 4m diameter and 12m in length.

Here's an artist's rendering of such a stage in orbit:

enter image description here

It's hard to predict the orbital lifetime of objects much above 400km because the process is chaotic and the variables include the effect of variable solar activity on Earth's atmosphere, but starting from a circular 800km orbit you can expect them to remain for hundreds of years at least.

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  • $\begingroup$ Perfect answer. I'm sorry if I sound a bit rude but I would like to have also a source for the claim it is the second stage indeed. I couldn't find it so it would be appreciated so I can point to it. $\endgroup$ – Swike Jul 11 at 12:38
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    $\begingroup$ Added a source. $\endgroup$ – Russell Borogove Jul 11 at 15:53
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    $\begingroup$ I don't have a clear source for that, but it can't be anything else. The first stage doesn't go orbital; Zenit-3 does have a third stage but it would be smaller and in a higher orbit, much harder to see. $\endgroup$ – Russell Borogove Jul 12 at 21:47
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    $\begingroup$ If a random tweet exchange counts: twitter.com/planet4589/status/922419446794334208 $\endgroup$ – Russell Borogove Jul 12 at 21:52
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    $\begingroup$ Here we go, NASA's Orbital Debris Quarterly Newsletter: "SL-16 R/Bs (Zenit second stages, 4 m diameter by 12 m length; 8900 kg dry mass)" $\endgroup$ – Russell Borogove Jul 12 at 22:02

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