I always enjoy the return of the International Space Station (ISS) to the evening sky. It’s fun to look up and know a half dozen people 250 miles up are flying at more than 17,500 mph over my house. For many locations in the northern hemisphere it’s now easy to spot the ISS at dusk as a brilliant, pale yellow “star” moving from west to east. Most passes happen during twilight and last about 5 minutes. Times and links for looking can be found at the end of this blog.
Since the station’s orbit is tilted 51.6 degrees to the Earth’s equator, it swings over the southern tip of South America (51.6 degrees south) back up to 51.6 degrees north across Canada and northern Europe. If you live north or south of 51.6 degrees latitude, no problem. While the ISS will never pass overhead from your high latitude, you can still see it well north and south of its orbital limits because it’s 250 miles high and visible far and wide.
U.S. spacecraft, like the Hubble Space Telescope, are normally launched into orbits inclined 28.5 degrees, the same as Cape Canaveral’s latitude, to take advantage of the Earth’s speed of rotation. Our planet rotates fastest at the equator and slowest at the poles. When you launch a rocket it, gets a free ride in the west to east direction courtesy of our spinning planet. If you want to send a craft into an orbit with an inclination different from the latitude it was launched, you have to burn more fuel. That costs money.
So why not save money by sending the ISS into an orbit equal to Cape Canaveral’s latitude? Sure, but we’re not the only ones running the space station operation. Russia shuttles astronauts and cargo to and from the space station with its Progress and Soyuz spacecraft. The U.S. worked with Russia to pick the best orbital tilt. Since Russia launches Progress and Soyuz from Baikonur (latitude 46 degrees N), a high inclination orbit made economic sense. It also lets astronauts study more of the Earth’s surface compared to an orbit closer to the equator. 75 percent of the planet and 95 percent of its inhabited lands are open to view.
OK great. So why isn’t the orbit inclined 46 degrees? Baikonur’s not too far from the Chinese border as the rocket flies. Any booster stages falling back to Earth after launch would land in China, not the most desirable situation. Launching at the steeper trajectory of 51.6 ensures the boosters remain in Russian with the spacecraft well on its way into space when it passes over China.
With its lower inclination orbit of 28.5 degrees, the Hubble Space Telescope can’t be seen from the northern U.S. and Canada. Had we not cooperated with the Russians on ISS missions, it’s possible that the space station would have been launched into a 28.5 degree orbit and been invisible to skywatchers across large stretches of the globe. Including my town of Duluth!
I’d like to tell you what the astronauts are up to this week, but the government shutdown has shuttered most NASA websites including those connected with the space station and its status. Strangely enough, NASA’s Spotthestation site, which will e-mail you with predictions of where and when to see the ISS, is still up and running.
Below I’ve included times when the station is visible from the Duluth, Minn. region. You can always get predictions for your town from Spaceweather’s Satellite Flyby page or times and handy maps from Heavens Above.
All times CDT:
* Tonight Oct. 10 beginning at 7:30 p.m. Maximum height: 32 degrees. Appears in the south and disappears in the east.
* Fri. Oct. 11 at 8:18 p.m. Max. height: 76 degrees. Appears in the west and disappears nearly overhead
* Sat. Oct. 12 at 8:19 p.m. Max. height: 44 degrees. Appears in the west and disappears in the north.
* Sun. Oct. 14 at 7:31 p.m. Max. height: 57 degrees. Appears in the west and disappears in the northeast