Astronauts onboard the International Space Station are capturing some amazing photos of the night sky.

by Dr Tony Phillips

It's a weird place for an astronomer. Meteors fly underfoot. Auroras appear just inches in front of your nose. City lights twinkle, but stars don't.

Astronaut Don Pettit loves every minute of it.

"There's always something good to see out the window of the space station," says Pettit, who happens to be an amateur astronomer as well as the science officer of the International Space Station (ISS).

"Lately we've been having some extraordinary auroras," he reports. "They meander like big green amoebas crawling across the sky. Sometimes there is a faint touch of red layered above the green. These lights are constantly changing. They swirl. Bright spots come and go. Green blobs transform into upward-directed rays topped by red feathery structures."

Long before he went to live onboard the space station, Pettit was an avid aurora watcher. "I've taken photos of the Northern Lights from Alaska and Canada," he says. Some of those displays were magnificent, but "the view from the station is even better."

Auroras are caused by electrons and protons from space raining down on Earth's atmosphere. The solar wind, through a set of complex and fascinating interactions with the Earth's magnetic field, is the ultimate source of energy that drives these particles toward our planet. When they hit the top of the atmosphere, they excite atoms and molecules and make the air glow. Reds and greens come from atomic oxygen, blues from nitrogen.

more Astronaut Don Pettit, Expedition Six ISS science officer, takes pictures of Earth through a window in the Destiny laboratory on the International Space Station (ISS)

These colourful lights range in altitude from 80 km to 500 km above Earth's surface. The ISS orbits our planet about 400 km high, so the space station can actually fly through auroras. There's no danger to astronauts, though. The aurora-causing electrons and protons are thousands of times less powerful than potentially hazardous cosmic rays.

"Last January 2002 we flew through an auroral curtain over Canada," recalls Pettit. The station was surrounded by a dimly glowing red fog. Just below were green rivers of light. "It was like I had been shrunk down to some miniature dimension and inserted into a tube of a neon sign. And it was just on the other side of the window pane. I wanted to reach out and touch, but of course I couldn't."

"Afterwards I had to clean my nose print off the window."

Auroras aren't all: "I've seen an occasional meteor while looking down through the Destiny Lab window," he says. Meteors disintegrate in Earth's atmosphere below the space station, so you have to look down to see them! "You can also see space junk orbiting nearby. Sometimes it flickers due to an irregularity catching light as it rotates. And there are satellites, too. A flash of sunlight glinting off an Iridium satellite near the Southern Cross really brought a smile to my face."

Auroras over Canada with the Manicouagan impact crater in the foreground. Clouds and Earth's surface are illuminated by moonlight. "Here in the same picture we have two interesting space phenomena: asteroid impact damage on the surface of Earth and auroras," notes Pettit.

Pettit recently took some lovely pictures of star fields in the southern hemisphere: the Large Magellanic Cloud (a nearby galaxy that orbits our own Milky Way galaxy), the Coal Sack Nebula (an inky-black interstellar cloud), and the Southern Cross.

"These pictures show how wonderfully stable the space station is," says Pettit. "When the camera is mounted to the window, the ISS itself serves as a tripod. Any movement would cause streaks in the star images." But the station's Control Moment Gyros maintain attitude with rock-solid precision. "I don't believe that the ISS was designed for astronomy," adds Pettit, "but it functions very well as a platform for astrophotography."

Credit: NASA and Don Pettit A snapshot of the Large Magellanic Cloud - an irregular galaxy visible from Earth's southern hemisphere.

One of the curious things about sky watching from orbit is the appearance of stars. "They don't twinkle," says Pettit. Twinkling is caused by irregularities in Earth's atmosphere that refract starlight to and fro. But in orbit there is no atmosphere. Stars are remarkably steady and piercing.

City lights, on the other hand, do twinkle. "From the space station we can see city lights when it's night-time on the planet below," explains Pettit. "Shining upwards through the atmosphere, they twinkle like stars. They're beautiful."

When Pettit tried to take pictures of city lights he quickly realized it wasn't as easy as photographing the stars. The station, traveling 17,500 mph, races around Earth in only 90 minutes. Lights on Earth's surface move through the window too quickly for long exposures. Stars, on the other hand, appear nearly motionless because they're so far away. It's like driving down a highway in a fast-moving car: Distant mountains and trees don't appear to move much, but the fringe of the road is a blur.

"I needed something to help me track the city lights, to cancel the orbital motion of the station."

Don Pettit's homemade "barn door tracker" onboard the ISS.

I assembled a 'barn door tracker'," says Pettit. "It's based on the fine gimbal movements in the IMAX camera mount for the Destiny Lab window. I figured out a way to mount a threaded screw and nut (scavenged from a Progress rocket) and drive it with a Makita drill driver." The drill turns the screw, which moves the camera and its spotting scope. "All of these modifications clamp on to the IMAX mount and do not change its original function in any way," notes Pettit.

"I manually compensate for the station's motion by looking through the spotting scope and running the drill at the same time. It takes a bit of practice, but you do learn to track."

Pettit has since photographed cities and towns around the world. "With tracking we can see individual city blocks - no blurring." Some towns are well-organized like checkerboards. Others are more ... organic. London, for example, resembles a glittering luminous spider web splayed across the landscape. "Really nice," says Pettit.

For sheer beauty, though, "my favourite is still auroras," he says. "I can't get enough of them."

more London, England, photographed by Don Pettit from Earth orbit in February, 2003.

Pettit is scheduled to remain onboard the ISS until May 2003. Until then, in his spare time, he plans to continue taking pictures and sending them to Earth. There'll be more auroras, more meteors and star clouds and city lights.

And probably lots more nose prints on the window....