See it from space? Surprisingly it should be able to be seen from space. Let’s consider the International Space Station (ISS) which typically passes by at a mere 230 miles (370 km) overhead. Note that the front array of the bike can be reconfigured into a flashlight and pointed upwards.
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The ISS has a nice recently installed viewing cupola (above left) where astronauts can take happy snaps like Las Vegas(above right). This has the reputation of being the world’s brightest city and the brightest lights are probably beams facing upwards highlighting buildings like casinos etc. Individual lights can easily be seen as well as yellow low pressure sodium street light lighting the roads in the “suburbs”. Conclusion: The ISS can see bright lights on the ground, even if the lights are not aimed directly at it. Now, can we calculate whether the flashlight can be seen from space? See the discussion on 4HV forum by Chris Russell for calculations based on a 14 LED array fully driven and with all light fully focussed into a 5 degree beam. My modified version below accounts for a broader beam of 10 degrees and perhaps 50% light loss to “flood” rather than beam. Also there are 15 LEDs although with reduced output of the coloured LEDs. The LEDs are driven to perhaps 80% of rated. Flashlight nominal output: (6000 x 12 + 5000 + 3500 + 1200) = 81,700 lumens nominal. It is driven to perhaps 80% with perhaps 50% flood losses = 32,000 lumens in beam. ISS optimum viewing orbital altitude( >85 degrees elevation) : 370 km (230 miles) Flashlight beam width diameter at 370 km: 2 * tan (5 degrees) * 370 km = 64 km Total illuminated area (assume circle) = 3200 km² = 3,200,000,000 m² Total illuminance (lux) in beam at ISS: (32,000 lumens) / 3,200,000,000 m² = .00001 lux Apparent magnitude: = -[log(50,000,000 * lux/127)]/log(314/125) = – 1.48 This is the same as the brightest star (Sirius)which is also -1.47 For comparison, the Sun (130,000 lux) is App. magnitude = – 26.7 Conclusion: Calculations show that it will be easy to see my light from the ISS with the naked eye. Now let’s compare that with an actual measured reading using a light meter.  Above is the light meter recording 111,000 lux in afternoon sunlight. Meter range is up to 400,000 lux. I have measured flashlight values of 13 lux at 120 m from the Flashlight (200,000 lux at the lens). This gives an illuminance at 370 km of 1.3 x 10^-6 lux = Apparent mag – 0.7 which is the same as the second brightest star, Canopus at – 0.72. There is a discrepancy between calculated and measured (-1.47 vs -0.7 magnitude) likely due to assumptions about amount of loss in flood light. Conclusion: On measurements, the ISS should see my light by the naked eye like a bright star in intensity. But how will you know it is my light when seen from space? Easy. Just modulate it on and off.   Above is a model I made with a lot of LEDs to demonstrate the concept. The still photo gives no indication which is “my” light. The moving photo image, however, makes it very obvious and shows a row of dashes as my light is turned on and off electronically. In addition, the modulation can be smarter than simply on and off. How about the world’s first flashlight to satellite messaging using Morse code? Morse code is a largely obsolete messaging system using long “dashes” and short “dots”. The best Morse code users are faster than SMS texting. Conclusion: My light can be differentiated from other adjacent lights by on-off modulation.

 Above shows a PIC microcontroller programmed as a Morse code transmitter. It uses a PICAXE 08M and details are here. Above shows the preprogrammed message “PITTSBURGH” named after a long running Morse beacon there. I have modified the program to give a 2 second delay then a dot then the message after a further 0.5 s. The Morse speed was doubled from the default of 10 WPM but needs to be faster. This is a 6 second exposure hence message is about 4 seconds. Needs to be a lot faster. “TDU to ISS” in Morse dots and dashes would be: – -.. ..- – — .. … … Or in more detail including spaces: ===…===.=.=…=.=.===…….===…===.===.===…….=.=…=.=.=…=.=.= ” Remember SOS is dit, dit, dit, dah, dah, dah, dit, dit, dit  I took the above photo of the International Space Station in a 4 second exposure on 4th March 09. The moon has a lens flare and there is a lot of extra light from the moon and the time around sunset. The ISS image passes about 3 moon diameters (3 * 0.5 degree = 1.5 degrees) in 4 seconds although I didn’t record the elevation which was not immediately overhead. Apparent motion is from west to east – opposite to the sun. Now they were using a 400 mm lens on the ISS so the field of view at full magnification will be restricted to 3 x 5 degrees. With their camera not tracking, I will pass through their 3 degree minimum camera field in 2.5 seconds. This assumes it is 90 degrees (vertically overhead) travelling at a velocity of 7.8 km/s at 370 km.  Note that if the ISS view is only 3 degrees wide, then there is no requirement for my 10 degree beam to move at all. Interesting. However, that assumes the ISS camera is aimed exactly vertically but this may not be possible in what has usually been a handheld camera. In reality the camera would be pressed up against one of the cupola windows which would be unlikely to be at 90 degrees. In fact they would have to aim it carefully to make sure the path includes my position and they have only seconds to be sure. So I would be most likely to get best results with tracking it after all. It will be easier for the ISS to have a wide field of view and for me to have a narrower tracking beam. Bear in mind that the main cupola window is 31 inches wide and the cupola was designed for two astronauts. Hence even though the camera view at full magnification would be brief, it may well be possible to have one or two astronauts actually looking directly for the flashing light with a potential visibility for perhaps a minute. Tracking may well be appropriate in that case. The high speed morse code may be too fast for visible flashing so may have to add longer on/off periods outside the main code transmission. My beam will be naked eye visible in a 40 mile path ie 8 seconds in the ISS. Conclusion: The beam width and relative passage times have been calculated. Tracking by me is preferred but not essential to prolong the ISS view and reduce the requirement for ISS aiming accuracy. Just imagine that I am on the ISS. Would I be able to see the light or a streak with flashes of Morse code with a 400mm lens?   Here is Alpha Centauri which is one of the Pointers near the Southern Cross and the third brightest star at +0.01 apparent magnitude. The left photo shows clear images of the star and others with my 180mm lens and a Nikon D300 with a 5 second exposure. The right photo shows the streak of manually swinging the camera (faster than ISS transit time). The star remains clearly visible although it has a lot of “twinkle” as it was not that far off the horizon. It would be even better with a 400mm lens and slower transit and the actual flashlight should be 0.7 magnitude brighter anyway than Alpha Centauri. So, yes, point the ISS camera in the right direction and they will see it with magnitudes to spare provided the cupola is in the right viewing orientation. Conclusion: The ISS should be able to record the light from the flashlight with a long exposure from a fixed camera rather than tracking one. Also getting the “TDU to ISS” message across needs to be done twice in 2.5 seconds to ensure that at least one full sequence is received. That’s too fast for human generated Morse code so it will be electronically generated with a small computer chip (PIC microcontroller) and ideally will be seen in full across one picture of duration of just over 2.5 seconds. Conclusion: I can send a Morse code message to the ISS. Where is the ISS? At 15 orbits per day, the ISS often comes close to my home town of Bunbury in Western Australia in any given 24 h even if not visible. Using NASA Skywatch, one can plot passover times and tracking data. How can I be sure of the best ISS view. The brightest view will occur when the ISS is immediately above and closest. Realtime views on Google maps are here for example. If I travelled 125 km to Arthur River (33.396 S, 117.035 E) in dark rural countryside and faced the light exactly vertically upwards on Jan 12, 2010 at 22:43:20 hours (14:43:20 GMT) then the ISS would be directly above. I could flash it a few times to say “Hi”. Easy as that. But a combined Morse code and prolonged visual contact with tracking would be better and have a photographic proof of the contact. Note that I have to make sure this is wake time for the astronauts which is 06:00 to 22:00 if I recall. They could pop over to the cupola after lunch at 14:00 GMT. Conclusion: I can travel to get the best view for the ISS hopefully at exactly overhead at exactly the right time and know tracking details in advance. But what could go wrong? Lots. Clouds, for example, but in summer here November to March are not that common – perhaps less than 10%. Weather forecasting and satellite imagery can give enough time to get a message to ISS to cancel and “rebook”. ISS camera aiming. I have no control over that and I am concerned that a 400 mm lens has too narrow a field for a handheld camera to be accurate with a margin for error. That will be a matter for discussion. Backing off to 200 mm might be safer. My mechanical aiming error. With 10 degrees, there is a big margin of error. The mechanical tracking just needs to pass through the vertical at an appropriate speed perhaps from 30 degrees either side with known directions from and to. I won’t need to set astronomical coordinates. I could have an astronomical colleague confirm accuracy as needed in test runs perhaps using a laser guide on my flashlight. The cupola orientation might not be optimal, but I don’t have that information. Time to complete electronics and mechanicals. No doubt I will be energised if given the go ahead. Conclusion: The predictable potential threats to success are hopefully small. Overall this project seems viable and would likely generate some positive publicity for the ISS (ahem.. and me). The simplicity of “seeing a flashlight from space” is one that the public can understand. I’ll drop Mr NASA a line….. Christmas day 2012. I have just found out that the ISS has taken arranged shots of a blue laser.|