Anonymous asked: why the sky is black from orbit?
I presume you mean why does the sky appear black in pictures we see from orbit, such as images like this, where we can clearly see the Earth, the space shuttle and the ISS, but no stars or other planets:
Or even why you can’t see other stars if you’re taking pictures on the surface of the Moon, like this picture from Apollo 11:
The quick, simple answer is because our cameras suck.
Here’s a better explanation.
Generally, when astronauts take pictures in space, they’re taking pictures of something near them: the Earth, a space shuttle, the ISS, the Moon or themselves. The reflected light of these objects is much brighter than that of stars thousands, hundreds or even dozens of light years away.
Cameras can only pick up a certain amount of information and tend to focus on the things providing the most information. As a result, they focus on the brightest objects, those reflected by the Sun, Earth and Moon. This brightness causes cameras to take pictures at faster shutter speeds, so as to capture the bright object, while not overexposing the photo.
If you alter the camera’s settings to allow for longer exposures, however, you allow cameras to take in and process more information.
Obviously I am not in orbit right now, but, as an example, here is a series of pictures I took last night of Cassiopeia, at exposure times of one-half, one, five, ten, fifteen and twenty seconds.
As you can see, the longer the exposure time, the more information is received by the camera and the more stars become visible. (Also more light pollution seeps into the picture.) If I were out in the desert, instead of in the heart of a city, many more stars would be visible (and ideally much less light pollution).
The logical question, then, is why don’t astronauts take longer exposure pictures? Well, sometimes they do.
Astronaut Don Pettit did this during Expedition 31:
“My star trail images are made by taking a time exposure of about 10 to 15 minutes. However, with modern digital cameras, 30 seconds is about the longest exposure possible, due to electronic detector noise effectively snowing out the image. To achieve the longer exposures I do what many amateur astronomers do. I take multiple 30-second exposures, then ‘stack’ them using imaging software, thus producing the longer exposure.”
Using this method, Pettit created these images.