Southern lights greet ISS and Atlantis

Bad Astronomy
The entire universe in blog form
July 18 2011 10:30 AM

Southern lights greet ISS and Atlantis

On July 16, an astronaut on the International Space Station captured this eerie and cool picture of Atlantis and the ISS with the aurora australis in the background:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


[Click to embiggen.]

You can see Atlantis on the right and one of the station's solar panels on the left. In the middle is the Orbiter's robot arm hanging down (as much as "down" makes any sense in space). That light near the top of the arm is not a star but an actual light, to help illuminate shadowed areas being worked on.

The aurora australis, or southern lights, are the counterpart to the aurora borealis (northern lights). The actual phenomenon is quite complicated, but in essence subatomic particles from the Sun are captured by the Earth's magnetic field. They're channeled down to the magnetic poles, which are very near the Earth's physical poles. The particles slam into the atmosphere, stripping electrons off of air molecules. When the electrons recombine with the molecules, they give off light exciting the electrons in atoms high in the air, and when the electrons give up that energy the atoms glow. The color depends on the atom or molecule involved; oxygen emits strongly in the green, while nitrogen is preferentially red. In reality most substances emit at several different colors, but the strengths change; oxygen emits in the red as well but much more weakly than green. When you see red in an aurora, it's usually mostly nitrogen you're seeing.

That thin brownish arc is real too! That's a layer of aerosol haze, particles suspended high in the atmosphere. When we look up from the ground we see right through it, but seen from nearly edge-on it becomes visible. You can spot it in a lot of photos of the Earth's limb taken at night from space.

I'll admit, when I first saw this picture it momentarily threw me. How could the clouds be so bright (like it's daytime) and yet the aurora be visible? Then I remembered that the Moon was just past full on July 16, when this picture was taken. Even though this is a night scene, the Moon was bright enough to light up the clouds. The exposure time was several seconds (you can see the stars are slightly trailed as the Orbiter moves around the Earth), plenty of time for the Moon to illuminate the clouds. It also lit up the cowling over the Orbiter's engines as well.

Today, Monday, July 17, the astronauts from Atlantis moved from the ISS back to the Orbiter and closed the hatches. Tonight at 02:28 Eastern (US) time (06:28 GMT), Atlantis is scheduled to undock from the station, and on July 21st it will return to Earth for the final time, marking the end of the Shuttle era for NASA.

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