What's the brightest magnitude that the ISS can appear from Earth's surface?

Heavens-above indicates that this evening, it will fly almost over my head (79°) with a sizzling magnitude of -3.9. What is the maximum brightness it can have?

Is there a formula to calculate its lowest magnitude, theoretically?

• If you're in the right place at the right time, you could catch a flare from the solar panels, which would increase its brightness many times over. astroengine.com/2009/05/01/space-station-flare-captured-on-film Jul 12, 2022 at 3:45
• Ah yes, you mention makes me realize that I did see an ISS glare before. A minus -2 or more subtraction to the predicted brightness in totally achievable. Jul 12, 2022 at 11:38

The ISS maximum brightness is reported at about -5.9, sometimes -6. I guess you could technically estimate it analytically by considering how much light it receives, but I imagine that would be messy to get accurate.

Here you can see an archived version of the original fact sheet by heavens-above, where the maximum is given as -5.7: https://web.archive.org/web/20090705134152/http://heavens-above.com/satinfo.aspx?SatID=25544

I believe whoever came up with the -5.9 value simply observed the ISS when it's best lit by the sun and the Earth is in perihelion (closest to the sun in its orbit).

Personally, I've only seen the ISS once, the night of 29/04/22 when it shined at -2.4.

• Could you lead me with a link to that -6 claim? How did the person prove it's -6? Hell, how did they even know what's -6 when watching the flying station? Jul 12, 2022 at 11:39
• Here you can see an archived version of the original fact sheet by heavens-above: web.archive.org/web/20090705134152/http://heavens-above.com/… Jul 12, 2022 at 20:41
• @longtry it's a very good question! Wikipedia says it's about -4 Looking at heavens-above FAQ and finding "How do you estimate the brightness of a satellite?" The paragraph answer ends "...For this reason, our magnitude estimates should only be treated as a rough guide, and the actual brightness you see could be considerably more or less than this."
– uhoh
Jul 12, 2022 at 21:23
• Personally, I've been noting every Heavens Above prediction for the ISS for my location since April 3, 2018. At about 41.6° N, the brightest pass peak magnitude prediction in my records is -4.0, which happened three times in 2018. Jul 14, 2022 at 14:57