The ISS does leak some light at night but we don't see it. For instance, it will leak light when someone is in the Cupola with shutters open, or when there are lights on the Canada arm, or when the visiting spacecraft are docked with positional lights.
But at least normally, we don't see any of these lights on the ground. I don't know if anyone has ever observed these lights - does anyone know? I found some discussion here: Why does the International Space Station have a downward facing light?
At any rate,it seems that you need quite a bright light to be shining in the ISS to see anything from the ground. Surely brighter than a 60 watt bulb.
Also there's the matter of Airglow. Even on the darkest night, far away from any city, if there are no clouds, the sky has a faint glow to it.
So how bright a light can we see from the ground?
The experiment has been done the other way. It was found they could easily spot a 1 watt laser beamed in their direction as a bright star. So that definitely is possible. It was overkill, it was as bright as the brightest stars or brighter, so they can surely see much dimmer lights than that. But that is a laser so to a large extent, counteracts inverse square law.
If someone on the ISS had a one watt laser light and shone it towards the Earth then you would see it easily so long as the light was shining directly towards you. Just as the amateur astronomers could signal to the ISS, surely someone on the ISS with careful positioning of a laser light could signal to the Earth.
But for the more general case we need to look into stellar magnitudes.
The faintest star visible to the naked eye is about sixth magnitude, some can see seventh magnitude.
A candle at 10 km is as bright as a sixth magnitude star (Cosmology, page 404)
Now we can use Robert Frost's calculation, but this time use it for stellar magnitudes. I'll use all the same numbers he did so we can do a direct comparison with the ideal situation he described.
So, a candle is 12 lumens. A 60-watt lightbulb is 840 lumens. 840/12 = 70.
The ISS at 370,000 meters is 37 times farther away than our candle standing in for a sixth magnitude star. By inverse square law that makes it 37^2 times fainter. Or 1369 times fainter.
To make it so it's as bright as a candle at 10 kilometers, we need 12*1369 = 16428 lumens.
That's 16428/480 or 34 of the 60 watt light bulbs.
So, a cluster of 34 of the 60 watt bulbs it would be a just visible sixth magnitude star from remote dark sky locations when dark adapted - perhaps easier to see because it is moving.
From suburbs, for fourth magnitude, need to multiply by 2.512^2 or about 6. (It's 5 magnitudes to multiply by 100 in brightness so about 2.512 for each magnitude).
So that's 2.512^2 *16428/480 or about 216 of those 60 watt light bulbs. Or about
60*2.512^2 *16428/480 = 13,000 watts (to two sig. figs).
So then you could see it from suburbs in ideal conditions.
It does seem possible that the bright external lights for EVA and robotics work could be observed, especially from dark sites, e.g. Hawaii and Atacama observatories and would be interesting to hear if they have been :).
As for the experiment Robert Frost mentions, that's Selig Heicht's experiment here How Far Can the Human Eye See?
He aimed 510 nanometer light at the most sensitive peripheral region of the eye and found that humans can detect pulses of between 54 and 148 photons. The candle at 48 km is a theoretical calculation from that data.
So - it's not taking account of airglow, also doesn't take any account of absorption in the atmosphere. Seems that perhaps it would give a reasonable estimate of what you could see in the vacuum of space, if the candle was set against a completely dark backdrop. For instance perhaps during lunar night, looking at a candle set against the lunar landscape, one might be able to see it 48 km away.
But at least normally, we don't see any of these lights on the ground. I don't know if anyone has ever observed these lights - does anyone know? I found some discussion here: Why does the International Space Station have a downward facing light?
At any rate,it seems that you need quite a bright light to be shining in the ISS to see anything from the ground. Surely brighter than a 60 watt bulb.
Also there's the matter of Airglow. Even on the darkest night, far away from any city, if there are no clouds, the sky has a faint glow to it.
So how bright a light can we see from the ground?
The experiment has been done the other way. It was found they could easily spot a 1 watt laser beamed in their direction as a bright star. So that definitely is possible. It was overkill, it was as bright as the brightest stars or brighter, so they can surely see much dimmer lights than that. But that is a laser so to a large extent, counteracts inverse square law.
Amateur astronomers flash the space station
If someone on the ISS had a one watt laser light and shone it towards the Earth then you would see it easily so long as the light was shining directly towards you. Just as the amateur astronomers could signal to the ISS, surely someone on the ISS with careful positioning of a laser light could signal to the Earth.
But for the more general case we need to look into stellar magnitudes.
The faintest star visible to the naked eye is about sixth magnitude, some can see seventh magnitude.
A candle at 10 km is as bright as a sixth magnitude star (Cosmology, page 404)
Now we can use Robert Frost's calculation, but this time use it for stellar magnitudes. I'll use all the same numbers he did so we can do a direct comparison with the ideal situation he described.
So, a candle is 12 lumens. A 60-watt lightbulb is 840 lumens. 840/12 = 70.
The ISS at 370,000 meters is 37 times farther away than our candle standing in for a sixth magnitude star. By inverse square law that makes it 37^2 times fainter. Or 1369 times fainter.
To make it so it's as bright as a candle at 10 kilometers, we need 12*1369 = 16428 lumens.
That's 16428/480 or 34 of the 60 watt light bulbs.
So, a cluster of 34 of the 60 watt bulbs it would be a just visible sixth magnitude star from remote dark sky locations when dark adapted - perhaps easier to see because it is moving.
From suburbs, for fourth magnitude, need to multiply by 2.512^2 or about 6. (It's 5 magnitudes to multiply by 100 in brightness so about 2.512 for each magnitude).
So that's 2.512^2 *16428/480 or about 216 of those 60 watt light bulbs. Or about
60*2.512^2 *16428/480 = 13,000 watts (to two sig. figs).
So then you could see it from suburbs in ideal conditions.
It does seem possible that the bright external lights for EVA and robotics work could be observed, especially from dark sites, e.g. Hawaii and Atacama observatories and would be interesting to hear if they have been :).
As for the experiment Robert Frost mentions, that's Selig Heicht's experiment here How Far Can the Human Eye See?
He aimed 510 nanometer light at the most sensitive peripheral region of the eye and found that humans can detect pulses of between 54 and 148 photons. The candle at 48 km is a theoretical calculation from that data.
So - it's not taking account of airglow, also doesn't take any account of absorption in the atmosphere. Seems that perhaps it would give a reasonable estimate of what you could see in the vacuum of space, if the candle was set against a completely dark backdrop. For instance perhaps during lunar night, looking at a candle set against the lunar landscape, one might be able to see it 48 km away.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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