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Are there any satellites in LEO that we can see with naked eyes in the daytime, or any special situations where they might become visible during the day?

If there is, how high, and at what orientation would those orbits be?

If not, how high can we see in the daylight?

Thanks!

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  • $\begingroup$ This is a really good question! While usually not, there are many interesting exceptions, so I expect there will be more than one answer. $\endgroup$ – uhoh Sep 7 '18 at 9:53
  • $\begingroup$ 'how high can we see in the daylight' - we sometimes see the moon, so at least 300.000km , and Halleys Comet (millions of km) was also visible during daytime... as were some supernovas (trillions of km) - what is it you are actually interested in - please refine your question. $\endgroup$ – bukwyrm Sep 7 '18 at 10:04
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    $\begingroup$ @bukwyrm Also, we can see the sun. $\endgroup$ – gerrit Sep 7 '18 at 10:05
  • $\begingroup$ @gerrit i did indeed miss that object - though it is even more special: we can only see it in daylight :-) $\endgroup$ – bukwyrm Sep 7 '18 at 10:10
  • $\begingroup$ I mean how height for the artificial object (especially satellties) in the sky can be seen by us in the daytime. As gerrit said, we can see the sun. $\endgroup$ – alex Sep 7 '18 at 10:18
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While @gerrit may very much doubt it the periodic reflections from the original Iridium satellites, known as Iridium flares can be as bright as magnitude -8 and certainly can be as bright as Venus more frequently than that.

Venus can be seen in the daytime if you know where to look of course.

And of course if a satellite's orbit goes low enough to begin reentry, it will generate a fireball which could potentially be viewable in the day, if it was large (e.g. Skylab, ISS, Tiangong-2, Mir...) Of course then it will cease to be a satellite, and cease to be in orbit.

It's hard to state a particular height when an object will be visible during the day. A 747 jumbo jet at 40,000 feet (about 12 km) something definitely not in orbit is already difficult if not impossible to spot unless it is making condensation trails or it has some peculiar reflection.

An extremely tiny dot of light on a black background is visible even if unresolved, but an unresolved dot of black on a bright background is just not detectable with human vision. This is a combination of the nonlinear way the human eye and visual system works, and shot noise (or whatever the biological equivalent is), but that's a different question.

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  • $\begingroup$ HhHh, that's a abnormal case for observing the LEO man-made objects:-) Is there any guess for the height, up to which mankind can see a moving object in the very clear sky with naked eyes? I think Gerrit provided a good way to get the height for seeing. $\endgroup$ – alex Sep 7 '18 at 11:06
  • $\begingroup$ @alex its possible more answers will show up. I've added some more about height. Really it's not a question of height though, it's a question of brightness and resolution. $\endgroup$ – uhoh Sep 7 '18 at 11:13
  • $\begingroup$ @alex it entirely depends on the size of the object. $\endgroup$ – Tom.Bowen89 Sep 7 '18 at 13:09
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    $\begingroup$ It's probably both. The brighter it is, the easier it is to see. The closer it is, the easier it is to see. $\endgroup$ – Mooing Duck Sep 7 '18 at 17:57
  • $\begingroup$ @MooingDuck that's a good point! And color, and texture and orientation and shape and... $\endgroup$ – uhoh Sep 7 '18 at 18:09
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I've been part of stratospheric balloon launches. Those balloons go up to 30 km and inflate to huge spheres up there with dozens of metre in diameter. They are barely visible with the naked eye, you won't see them if not looking very carefully in the right direction.

Satellites are much (factor 10 in each dimension) smaller and much (factor 20) higher up. They don't normally reflect enough sunlight to be visible against the blue background sky. Maybe you could see one flashing if specular reflection reflected a sunray straight to the observer, but I very much doubt it.

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  • $\begingroup$ Thanks. that's one of the real observations. Could you give a guess of the height of the situation when the balloons are barely visible with your naked eyes? $\endgroup$ – alex Sep 7 '18 at 10:52

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