This Echo is READ ONLY !   NO Un-Authorized Messages Please!
 ~~~~~~~~~~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Space Station Science

Picture of the Day

May 13, 2003

Space Station Star Trails
Photo credit: ISS Expedition 6 science officer Don Pettit, NASA
with assistance from astronomers Robert Reeves and Rob Matson

May 13, 2003: Mount your camera on a tripod. Point it toward the
North Star (Polaris) at night. Open the shutter for a while--a few
tens of seconds or even a few hours--then develop the film. The
picture you get will look like a vortex with Polaris in the middle.
Astronomers call the concentric arcs "star trails." 

Star trails are caused by the rotation of our planet. The sky turns
circles above our heads, streaking the images of stars across film.
Only Polaris at the North Celestial Pole seems stationary. (There's a
South Celestial Pole, too, but no bright star lies at the stationary
point.) 

On April 30, 2003, International Space Station (ISS) science officer
Don Pettit took this picture of star trails from the ISS. The
exposure was brief, only about 30 seconds, so the star trails are
stubby and subtle, but they are there. The telltale vortex hangs in
the black sky above the limb of the earth, which is lit by red and
green auroras. 

The vortex itself looks like thousands of others photographed by
amateur astronomers on Earth. But there's a difference: it's not
centered on either of Earth's celestial poles. 

Rob Suggs of the NASA/Marshall Space Flight Center explains: "The ISS
usually orbits our planet in a level attitude--belly-down like an
airplane. The station therefore flips relative to the stars once per
trip around Earth." This flipping motion is what gives the space
station its star-trailing spin. The spin of the ISS is not aligned
with the spin of Earth; the station's celestial poles are therefore
different.

The space station was above the south Pacific Ocean with its spin
axis tilted toward the constellation Grus when Don took the picture.
The stars are swirling around ... nothing particular. 

"The station's celestial poles wobble around the sky once every 70
days," says Suggs. They move because the gravity of Earth's
equatorial bulge tugs on the space station and causes its orbital
plane to rotate. "Earth's polar axis wobbles, too," he notes "but one
wobble of Earth takes 26,000 years rather than 70 days."

So while Earth's celestial poles seem constant, the space station's
are in constant motion. Every day there's a new batch of star trails
and maybe a new North Star. What more could an astrophotographer
want?

Bonus: One of the streaks in this image isn't like the others. Can
you find it? Rather than circling the celestial pole, it points
almost directly away from it. The oddball is an Iridium flare.
Predicted by orbital dynamicist Rob Matson of SAIC, that flare is the
reason Don Pettit went to the window in the first place. The star
trails and the auroras were a welcome bonus. Stay tuned to the
Picture of the Day for more Iridium flares later this week. (Click
here to view a labeled version of today's image.)

 - END OF FILE -
==========

@Message posted automagically by IMTHINGS POST 1.30
---