Published: 2014-08-19 17:04:00
Updated: 2014-08-19 17:10:24
Posted August 19, 2014
By Tony Rice
If you are a Twitter user and are not following astronaut Reid Wiseman, you should. Wiseman is one of six Expedition 40 astronauts aboard the International Space Station (ISS). The photos and short videos he shares are a science, and often geography, lesson in 140 characters.
Monday morning Wiseman tweeted an image of “airglow,” faint bands of light enveloping Earth. There have been many images of airglow taken from the ISS but most show only the brightest yellowish-green band. Wiseman’s tweet beautifully shows the chemical changes going in layers of the upper atmosphere, each produces a different wavelength of light.
The reddish-orange band in the photos is the thinnest part of the atmosphere, 150 - 300 km up. Here oxygen and nitrogen atoms along with molecules of hydrogen and oxygen recombine each night after being broken down by solar radiation throughout the day. Energy released during that recombination produces a faint red glow.
The emerald green layer is atomic oxygen at about 90 km. Swedish physicist Anders Angstrom first discovered this in the mid-1800s. This is the brightest layer and appears often in images from the ISS. It can be photographed from Earth under the right conditions but is difficult to see with the naked eye and requires very dark conditions.
The yellow light in the next layer comes from sodium atoms left by meteors as they vaporize entering the atmosphere.
Lower blue layers are likely moonlight scattered by moisture in the atmosphere.
Long objects in the center of the photo are some of the station’s solar panels, seen from the side.
This photo also helps explain why most images taken from space are free of stars. Astronauts can see stars in space, but their photos rarely demonstrate it. Contrast between dim stars and any significant light source (like the lunar surface or even moonlight reflected off Earth) makes capturing space as astronauts see it difficult. Long exposure times are required. Wiseman left the shutter open for three minutes in this photo, bringing out not only a rainbow of airglow colors but also the stars in our Milky Way.
Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on twitter @rtphokie.