What’s up in the air this month? Actually, we are going to look only a bit above the air – above Earth’s atmosphere – to see what mankind has put up there, and how you can see those things yourself.
There’s a lot of stuff up there: U.S. Space Command is tracking about 8,000 objects baseball-size or larger. That’s not really crowded, though. Spread out over a region about 500 miles up, there are only a few over the equivalent of the airspace over the U.S., according to Space Command. With all due respect to the movie “Gravity,” collisions are very unlikely.
The International Space Station, the Hubble Space Telescope, and a number of other satellites skim the top of the Earth’s atmosphere a few hundred miles above the surface, at so-called low-Earth orbit (LEO). At low orbit, it takes these objects about 90 minutes to circle the globe.
I remember an Intro Astronomy lab night when this was nicely demonstrated. I had started the students observing in the early lab session outdoors and was back inside when some students came in to report having seen a bright object cross the sky, blinking on and off. An hour and a half later, in the late lab, I was back inside again when some students reported that a bright meteorlike object had crossed the sky.
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The first group had seen a satellite about to plunge to its death, flashing light from the twilight sun as it tumbled in the upper atmosphere. The second group, one orbit later, saw it finally burn up. You may remember when we had the shuttle, which went up to LEO, the launch or return trips were often delayed by some multiple of 90 minutes.
You can get notification emails about Space Station passes visible over your location by registering at http://spotthestation.nasa.gov. It looks like a bright star taking a couple of minutes to cross the sky. Also, the Iridium communication satellites are visible as bright flares, from twilight glinted off their large antennas.
Another sweet spot for orbits is up about 22,000 miles from the surface, where an object’s orbit takes 24 hours. Satellites placed there orbit the Earth synchronized with the Earth’s day, staying above the same place on Earth. If you have satellite TV, that is why your dish is permanently aimed to one place.
Other satellites have different periods and there are a lot that you can watch cross the sky. Register at www.heavens-above.com for a viewing guide, which includes ISS, Iridium and other visible events.
Happy satellite hunting!
Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State. Email: email@example.com. More on this column is at: www.upintheair.info.