Incredible Star Trails, from Earth and Beyond

esiegel — Fri, 08 Jun 2012 17:02:42 +0000

"Building one space station for everyone was and is insane: we should have built a dozen." -Larry Niven

Here on the solid ground of the Earth, the Sun and Moon rise and set on a daily basis. During the hours where the Sun is invisible, blocked by the solid Earth, the stars twirl overhead in the great canopy of the night sky.

Image credit: Chris Luckhardt at flickr.

In the northern hemisphere, they appear to rotate around the North Star, while in the southern hemisphere, the stars appear to rotate about the South Celestial Pole. The longer you observe -- or for photography, the longer you leave your camera's shutter open -- the longer and more spectacular are the paths that the stars trace out.

Image credit: Roger Groom at www.AstroPhotography.com.au.

This, of course, is because the Earth rotates on its axis. The North and South Celestial Poles are aligned with that axis, and so the stars appear to, with a 24-hour period, rotate about that same axis.

But perhaps, you might think, you wouldn't suffer those same trails from outer space. Not being bound to the surface of the Earth, the stars would appear to be stationary, while the Earth beneath you would be the only thing that rotated. Where would you go to look and test this?

Image credit: NASA Earth Observatory / International Space Station / Johnson Space Flight Center.

To the International Space Station, of course! The relatively new cupola, shown from the outside (above) and the inside (below), allows ISS astronauts to get a prime view of Earth and the 360° horizon, all at once.

Image credit: NASA, of Tracy Caldwell Dyson, via Wayne Hale.

If this cupola-type structure in space looks familiar to you, that's because you've probably seen something very much like it, long ago, in a galaxy far, far away...

Image credit: George Lucas / LucasArts / www.jrbassett.com / #pleasedontsueme.

But I digress. Like practically every one of Earth's satellites, the International Space Station makes a nearly perfect, circular orbit just a few hundred kilometers above our surface. Since it's in low-Earth orbit, it zips around the Earth -- a more than 25,000 mile journey -- in just 90 minutes.

Image credit: Oregon State University / U.S. Navy / Jasmine Nahorniak.

It needs to move at that speed; that's the only speed that will keep a satellite moving in a circular orbit just a small distance above the surface of the Earth!

But this isn't the only motion happening; while this is going on, the Earth is rotating beneath you, with a 24 hour period!

Image credit: Sky & Telescope.

You've got a couple of options as to how you'd run your spacecraft. One option -- and this is what the Hubble Space Telescope does, for example -- is to totally ignore the Earth. Have your spacecraft not rotate at all, point it at a target away from the Sun, one where the Earth will never get in your way, and observe it for as long as you like. Run your satellite like that, and, like Hubble, you'll never see a star trail.

But the ISS wasn't built for looking into space, it was built for looking at the Earth. And if you look down, from the ISS, what would you pass over?

Image credit: Ground Track of the ISS during orbit #60142, via Heavens-Above.com.

Every ninety minutes, you'd pass from the equator up to the northern latitudes, then all the way down towards (but not quite flying over) Antarctica, and back towards the equator. Because the Earth is rotating, the longitude of the place you're passing over changes by about 20° every time the ISS passes.

But because the ISS is designed to look down, rather than -- like Hubble -- not rotating at all, it rotates once every ninety minutes, so that the cupola I showed you earlier always points directly down at the Earth. Which means, if you stuck a camera in the cupola, you'd be able to see something akin to what Alex Rivest has stitched together, below.

As you can see, the stars do, in fact, appear to rotate! This is entirely because of how the ISS rotates in space. Unlike the star trails on Earth, which would take 24 hours to make a complete circle, the trails on the ISS would make a complete loop in a mere 90 minutes, because that's the period of rotation of the ISS around the Earth. (Just like the period of the Moon around the Earth -- both the rotational period and the revolutionary period -- is one lunar month.)

This is by design, of course, and it allows the ISS's cupola to always point at the Earth's surface, perfect for studying our own planet from space. But you don't have to take my word for it, because I now have the photos to show you!

Image credit: NASA / Astronaut Don Pettit / @astro_pettit on twitter.

NASA astronaut and photographer extraordinaire, Expedition 31 flight engineer Don Pettit, took a whole slew of image composites from in cupola of the International Space Station, and stacked them together to produce these fabulous views of star trails from the ISS.