Q&A: Polar Satellites

Question:  Sometimes I see these tiny dots of light moving really fast across the sky. They don’t blink, so they can’t be airplanes, right? Any ideas? — CF, Glendale, AZ

Answer:  You’re extremely observant to even notice these tiny points of light. What you most likely saw was a satellite, either weather or military. They’re about as bright as the dimmest stars, shine with more or less steady light, and speed across the sky along a line running approximately north-south. If you spot one, it’ll be gone in a few minutes.

The most commonly seen satellites are those in polar orbits, as shown above. Note that the orbit in the graphic is drawn much larger than correct scale for clarity. Satellites in polar orbits go around once every 2-3 hours, and will eventually pass over every point on Earth (which takes 24 hours for one rotation). Look at the graphic and visualize the motions. Polar orbits have some obvious advantages for both weather and military imaging applications, since they can repeatedly examine every point on the globe.

You are correct that they don’t have lights like aircraft. You can see them because they are usually smooth, shiny, metallic objects that reflect sunlight very well. If you do see one blink, or change in its brightness, that’s caused by its rotation, and the fact that some of its parts are shinier than others.

If you’re wondering how satellites can reflect sunlight at night, look again at the graphic. Polar satellites are so high up, typically around 500 miles, that they can be seen from the night side of Earth while being illuminated by the same sunlight that’s hitting the day side of Earth.

Q&A: The Total Number of Stars
Q&A: Rain that Doesn't