This photo made astronaut Scott Kelly do a double take

It takes a lot to make an astronaut say, "Wow." And this sight did it.

About a quarter of the way into his yearlong stay on the International Space Station, NASA's Scott Kelly has seen a lot of amazing things outside his window. But this latest one was a first.

Monday afternoon, while floating somewhere over Russia, Kelly tweeted a picture of a breathtaking red aurora borealis, exclaiming, "I've never seen this before- red #aurora. Spectacular!"

The aurora borealis, aka the northern lights, occurs around the north magnetic pole (at the south magnetic pole it's called an aurora australis) when highly charged electrons from solar winds interact with elements in the earth's atmosphere, such as oxygen and nitrogen. The color of the aurora depends on which atom is struck, and the altitude of the meeting. The most common color, yellow-green, typically comes from a reaction with oxygen at altitudes around 60 miles above the earth.

Red auroras are more rare and are produced by high-altitude oxygen, at heights of up to 200 miles. When this photo was taken, ISS was hovering about 251 miles above the ground.

Kelly has been a font of great Twitter fodder over the last three months, sending inspirational messages, awe-inspiring "Earth art" images and even an intimate invitation into his minuscule bedroom.

In:
• International Space Station
• NASA