February 24, 2012
Earth Photography: It’s Harder Than It Looks
From my orbital perspective, I am sitting still and Earth is moving. I sit above the grandest of all globes spinning below my feet, and watch the world speed by at an amazing eight kilometers per second (288 miles per minute, or 17,300 miles per hour).
This makes Earth photography complicated.
Even with a shutter speed of 1/1000th of a second, eight meters (26 feet) of motion occurs during the exposure. Our 400-millimeter telephoto lens has a resolution of less than three meters on the ground. Simply pointing at a target and squeezing the shutter always yields a less-than-perfect image, and precise manual tracking must be done to capture truly sharp pictures. It usually takes a new space station crewmember a month of on-orbit practice to use the full capability of this telephoto lens.
That's Canada's Manicouagan Crater near the center. Click on the photo to see it full-size.
Another surprisingly difficult aspect of Earth photography is capturing a specific target. If I want to take a picture of Silverton, Oregon, my hometown, I have about 10 to 15 seconds of prime nadir (the point directly below us) viewing time to take the picture. If the image is taken off the nadir, a distorted, squashed projection is obtained. If I float up to the window and see my target, it’s too late to take a picture. If the camera has the wrong lens, the memory card is full, the battery depleted, or the camera is on some non-standard setting enabled by its myriad buttons and knobs, the opportunity will be over by the time the situation is corrected. And some targets like my hometown, sitting in the middle of farmland, are low-contrast and difficult to find. If more than a few seconds are needed to spot the target, again the moment is lost. All of us have missed the chance to take that “good one.” Fortunately, when in orbit, what goes around comes around, and in a few days there will be another chance.
It takes 90 minutes to circle the Earth, with about 60 minutes in daylight and 30 minutes in darkness. The globe is equally divided into day and night by the shadow line, but being 400 kilometers up, we travel a significant distance over the nighttime earth while the station remains in full sunlight. During those times, as viewed from Earth, we are brightly lit against a dark sky. This is a special period that makes it possible for people on the ground to observe space station pass overhead as a large, bright, moving point of light. This condition lasts for only about seven minutes; after that we are still overhead, but are unlit and so cannot be readily observed.
Ironically, when earthlings can see us, we cannot see them. The glare from the full sun effectively turns our windows into mirrors that return our own ghostly reflection. This often plays out when friends want to flash space station from the ground as it travels overhead. They shine green lasers, xenon strobes, and halogen spotlights at us as we sprint across the sky. These well-wishers don’t know that we cannot see a thing during this time. The best time to try this is during a dark pass when orbital calculations show that we are passing overhead. This becomes complicated when highly collimated light from lasers are used, since the beam diameter at our orbital distance is about one kilometer, and this spot has to be tracking us while in the dark. And of course we have to be looking. As often happens, technical details complicate what seems like a simple observation. So far, all attempts at flashing the space station have failed.
Mr. Pettit, please remind people to never shine lasers at aircraft.
Comment by Z.T. — February 24, 2012 @ 7:52 pm
I followed your information and I saw your light from Spain. I hope you get the picture of your home. luck
Comment by Bego — February 27, 2012 @ 2:35 pm
Dear Mr. Pettit,
your pictures of the Earth surface are actually more useful than it might look. Astronauts on an earlier mission took a nighttime picture of the area between Venice and Padua in Northern Italy. You can’t imagine how useful that picture was for detecting and mapping light pollution in the area covered. Its resolution permitted to almost identify single luminaries in severely polluted areas within the image (see for example: http://www.esa.int/images/5309015350_8cf8630dd1_o.jpg). Now researchers, professional and amateur astronomers are here cooperating the Regional Government to reduce light pollution and we are wondering if there is any way for personnel on board the ISS to periodically repeat images on the same area at approximately the same angle. This would enable us to check if the improvements in the regulations and their application can determine significant changes and convince the stakeholders that what we are doing is worth the efforts. One of your single pictures, though difficult to take, is far more effective than years-long terrestrial surveys. Satellite images are seldom taken at nighttime and are normally set at much less resolution than pictures like the one obtained by the ISS camera with tracker. We know you have a lot more important things to do up there, but one minute of your time would be a great step for us down here. We hope you can help us again with new images of the same and neighboring areas. Thanks a lot for your work. Have a good time on board the ISS!
Dr. Luca Zaggia
National Research Council of Italy, Institute of Marine Sciences
Member of the Permanent Observatory on Light Pollution of The Regional Government of Veneto
Comment by Luca Zaggia — March 3, 2012 @ 8:36 am
[...] sounds deceptively easy. In an earlier post I wrote about the technical requirements. But like so many other tasks, it becomes much more involved in the execution than in the [...]
Pingback by A Flashing Success | Letters to Earth — March 5, 2012 @ 11:02 am
Mr. Pettit,
Thank your for a interesting article on your perspective. Could you please tell us the circumstances and settings used in the now classic photograph of the last space shuttle decent in which its streak is seen below in what looks like a several second exposure. Fascinating picture.
Drew C. Wilson
Comment by Drew C. Wilson — March 5, 2012 @ 8:06 pm
[...] sounds deceptively easy. In an earlier post I wrote about the technical requirements. But like so many other tasks, it becomes much more involved in the execution than in the [...]
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[...] Don Pettit, who maintains a blog about his experiences, writes that taking pictures of Earth is harder than it looks: Even with a shutter speed of 1/1000th of a second, eight meters (26 feet) of motion occurs during [...]
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[...] Pettit, who maintains a blog about his experiences, writes that taking pictures of Earth is harder than it looks: Even with a shutter speed of 1/1000th of a second, eight meters (26 feet) of motion occurs during [...]
Pingback by Auroras, Meteors, and Photography from the International Space Station — March 22, 2012 @ 3:02 pm
[...] sounds deceptively easy. In an earlier post I wrote about the technical requirements. But like so many other tasks, it becomes much more involved in the execution than in the [...]
Pingback by » A Flashing Success for International Space Station (ISS) » — March 27, 2012 @ 8:33 pm
[...] Welcome on board spaceship earth, http://bfi.org/about-bucky might have said. And http://www.jsc.nasa.gov/Bios/htmlbios/pettit.html, on board the International Space Station, could reply: “Ironically, when earthlings can see us, we cannot see them. The glare from the full sun effectively turns our windows into mirrors that http://blogs.airspacemag.com/pettit/2012/02/earth-photography-it%e2%80%99s-harder-than-it-looks/.” [...]
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