# Is ISS ever visible at all on the earth, unaided, by full moon moonlight alone?

Assuming ideal conditions of moonlight and phase angle, is the ISS ever even slightly 'visible' by full moon moonlight alone, that is to say, when the station is not in direct sunlight at all, but only in moonlight from the full moon, perhaps eclipsed by the earth from any direct sunlight?

Although I am researching the amount of light coming off the full moon, compared to direct sunlight, and some other related issues such as phase angle, would I need to have an owl's eyes for even the least hope of seeing the ISS illuminated by moon light alone, well after the normal orbital position required for a pass visible due to directly reflected sunlight?

• Do you mean something like this: reddit.com/r/astrophotography/comments/53zs93/iss_moon_transit
– Paul
Oct 25 '18 at 4:52
• @Paul - I did not read that yet, but by the title, no. I'm almost finished with a pyephem project now for finding sun, moon, or other body ISS transits, for planning observations. During a lunar transit, if the ISS is eclipsed from sunlight by the earth, the sun moon ISS phase angle is basically zero and the eye would be trying to see the 'dark side' of the station, What I am asking here is not concerning a lunar transit, but spotting the ISS 'in the clear', lit by only moonlight reflected off of the ISS. Thank you for the question. And I will read that link later. Oct 25 '18 at 5:19
• You can't have both night time and a full moon illuminating the station at the same time. The moon needs to be below the horizon to reflect light onto the station but then you can't get a full moon without the sun being up the opposite side. Oct 25 '18 at 11:27
• @Vincent- thank you for your interest. Consider: 2am localtime, the full moon is climbing high in the otherwise very sunless night sky. And there is a local ISS pass too. The sun is long down and its light will not reach the ISS on this pass, but the moon is bright enough to ruin my night vision. The moon is to my south, ISS pass to my north. Consider: I stand looking down a road at night. A deer is 50 yards ahead on the side of the road. A car is 150 yards ahead and its lights illuminate the deer 'slightly' because the deer is not 'directly' between me and the car's headlights ... Oct 25 '18 at 14:37
• @Vincent ...(phase angle near but not zero). If instead, the deer is down the road in the other direction from me than the car, now the deer is 'fully' lit to my eye by the headlights behind me, shining full on the deer (phase near max). Me, ISS, full moon. Think of the moon as the headlights. Yes, it is lit by the sun, but the sun is in a position where its direct light can not reach the ISS, but it still reaches the full moon easily. The earth is not in the way of sunlight reaching the moon, as it would be during an eclipse. Is the moonlight alone now enough to light the ISS to my eye? Oct 25 '18 at 14:40

This short answer interprets "visible" as you can see it with your eye.

As seen in the vicinity of the Earth, the visual magnitude (brightness) of the Sun and Moon are -27 and -13, respectively. That's a difference of 14 magnitudes, or $$100^{14/5} \approx$$ 400,000 times different.

Neglecting geometrical effects and unusual reflected "flares" off the flat solar panels, if the ISS illuminated by the Moon were at the limit of visual detectability at magnitude +6, the sunlit ISS would be magnitude -8.

Answers to this Quora question mention that Wikipedia's Apparent magnitude article mentions that Heavens above puts the ISS' maximum apparent brightness at about -6, which is less than a factor of 10 smaller than our simplistic model would require.

The Heavens above creator also has experience with calculating Iridium flares which can max out near -10, and the ISS has way, way bigger (though much less reflective) flat surfaces that make flares possible.

Answer: So it's really unlikely this could be done regularly with the unaided eye, though a good sky camera with a substantial aperture could certainly capture the trail if the Moon were not above the horizon in the location of the camera causing sky brightness issues.

below "View of one of the Main Mission Antenna. The hinged base is on the right side." Cropped. From here.

below: "Scott Kelly fixing a cooling pump during a spacewalk." from Gizomodo's Astronaut Scott Kelly on Liquid Salt, a Stinky Station, and Sleeping in Freefall. Image credit: NASA/Kjell Lindgren

I can't find a public datasheet for the solar cells (if I find one, I'll edit the post), but I can do my best to describe what you're seeing, with the help of this image I found online (rehosted from http://pages.erau.edu/~ericksol/courses/sp300/images/iss_cells.jpg).

The inset image on the lower right is a closeup of the back side of a single cell as seen through the blanket substrate.

below: Screenshot from the Science at NASA YouTube video ScienceCasts: Space Coffee that I just happened to be watching at the moment.

• @uhoh- Thank you for a comprehensive post! Conclusion. A moonlit ISS 'hunt' is possible, hoping for a panel(s) flare. If I use a simple az alt tracker with a one meter dowel, and look along that during a max moonlight phase angle ISS pass, during a full moon, I 'may' get a quick flare off the ISS panel(s) combining with the rest of the structure's reflected moonlight. Such a sighting would be one for the log book! In amateur radio I've also enjoyed what can be done with the lowest power possible. A moonlit ISS sighting. Unlikely, but possible. A challenge! Oct 25 '18 at 14:52
• @always_learning that sounds about right. I don't know how to calculate "max moonlight phase angle" exactly. The movement of the panels is complex since they are actively rotated, but with some difficult to predict algorithm. The question How are the orientations of the ISS' eight independent solar arrays optimized? gets at that, and the current answer, while helpful, is not enough. I'd say put a large aperture camera on a computer-controlled antenna mount, then specify the ISS's orbit and let it run every night with a visible pass predicted.
– uhoh
Oct 25 '18 at 15:06
• @always_learning But the Moon would not have to be up, in fact you'd want a dark sky. As long as it's say between first and last quarter, there will be light. You need lots of opportunity, so use those 14 days whenever it's clear. Unrelated; have a look at these: space.stackexchange.com/search?q=user%3A12102+amateur+radio if you have a minute or three.
– uhoh
Oct 25 '18 at 15:06
• You can look all night because even when the ISS is in eclipse (in the Earth's shadow of the Sun) it can still often receive moonlight. So use a satellite pass calculator that doesn't throw away passes that would be dark due to eclipse.
– uhoh
Oct 25 '18 at 15:09
• @uhoh- Yes. My pass calculator presently does not throw away eclipsed passes, since HAM contacts are still possible even if not 'visual' (at least receiver squelch wide open just in case). I want to know whenever ISS is in my local sky, visible or not. But I will modify an output report table to show when it is eclipsed from sunlight by the earth, and the current phase and position of the moon, phase angle of ISS to moon/observer, etc. That goes on the to do list, and will be a few days of off the clock python time;. Oct 25 '18 at 15:44