An astronaut on board the International Space Station (ISS) took this wide-angle photograph of Earth’s night lights while looking out over the remote reaches of the central equatorial Pacific Ocean. At the time, the ISS was passing over the island nation of Kiribati about 1,600 miles (2600 kilometers) south of Hawaii. In this view, we look in the direction of the center of the Milky Way galaxy. Between us and the center, dark patches of dust shed by earlier generations of stars create the blotchy, dark patches blotting out stars here and there.
The curvature of the Earth crosses the center of the image and is illuminated by a variety of airglow layers in orange, green, and red. Setting stars are visible even through the dense orange-green airglow. Airglow occurs when gases in the atmosphere get charged up by the sun’s ultraviolet light during the day and release colored light as they return to their previous relaxed states. Lightning also flashes in the lower right corner of the frame, reflecting off the space stations shiny solar arrays.
As you might guess, the photo was taken when the ISS orbited over the nightside of the planet. Astronauts get to spend about 45 minutes 16 times a day over the nightside, just enough time for their eyes to become fully dark adapted before the sun pops up again along the Earth’s curvature. In darkness, about as many stars are visible from orbit as you’d see from a dark rural sky.
On the ground, the atmosphere is juxtaposed between us and the stars, and yes, it does absorb some starlight. But assuming no fire smoke or thick haze, its effect is minimal except near the horizon, where we peer through hundreds more miles of air laden with dust and other aerosols. Stars close to the horizon are dimmed several magnitudes, an effect astronomers call “extinction”.
Once the ISS has crossed back into sunlight, its intense glare coupled with the bright reflections of water and clouds 250 miles below, make seeing stars in the daytime a nearly impossible task for the astronauts. That said, the very brightest are still visible. Right now, the planet Mars shines even brighter than Sirius, the brightest star. Back in 2003, when Mars was even closer to Earth than it will be this weekend, astronaut Edward Lu wrote:
“Mars … is bright enough that even when we are on the lit side of the Earth, and with all the lights on inside, it is clearly visible against the black background of space.”
So yes, it is possible to see the brightest stars and planets in daylight from the space station. Likewise for the moon where glare from the sun, the lunar surface and Earth connive to make for a nearly star-free sky. The Apollo astronauts all landed in the daytime when the sun was up and reported that the brighter stars were visible when they stood in the shadow of the lunar module and raised their protective outer visors.
From the moon, Earth appears about four times larger than a full moon and shines at magnitude –16 or about 15.6 times brighter. The reason you never see stars in photos taken either on the lunar surface or in the vicinity of the sunlit Earth from the ISS is because it’s too doggone bright! To capture a sunny scene the camera’s shutter must be set to only a fraction of a second (typically 1/500″) — much too short to reveal dim, stellar points.
Sun-strewn landscapes are BRIGHT and require only a brief exposure. To record stars, you need both a dark, nighttime sky and exposures of many seconds because they’re just so faint compared to the sun. And forget about capturing stars with long exposures in daylight. All you’ll get is an overexposed and washed photo of the scene.
In the end, we really don’t have it too bad after all just standing on the ground and looking up at night. Then again, orbiting 250 miles up does have its perks. Here’s more from Lu:
“There is something really cool about floating in a spaceship looking at all the stars! It’s fun to watch stars as they rise and set through the atmosphere as we circle Earth. They start to twinkle as the light rays bend while passing through the uneven density of the atmosphere. They, as they get closer to the actual horizon, they start to look orange and then red before blinking out. Sometimes they even turn green briefly. This is just the same effect that makes sunsets look orange and red. “