Western Slope Skies - Spotting Earth Satellites

Apr 21, 2017

Credit Joyce Tanihara

Have you ever seen a star-like object moving across the night sky over several minutes?  You may have seen an artificial satellite. 

The first of these was Sputnik 1, launched by the Soviet Union in 1957.  Currently, the U.S. Strategic Command tracks more than 8,000 artificial objects orbiting Earth, including hundreds of active satellites.

On any clear, dark night, we can see an average of several dozen Earth satellites crossing our sky.  If the object you see is blinking and has red or green lights, it’s almost certainly an airplane, not a satellite.  Satellites may cross the sky in many directions.  However, they almost never cross from east to west, because launching a satellite to the west requires substantial additional velocity to overcome Earth’s easterly rotation.  The International Space Station (or ISS) is the largest satellite currently in orbit, and it’s the easiest one to spot.  The ISS appears as bright as the brightest stars, and at times, it may even rival Jupiter, or even Venus in brightness.

Iridium communications satellites are another type of satellite that can become intensely bright, if only momentarily.  There are more than 66 first-generation Iridium satellites in orbit.  But, these are relatively small and usually too faint to see.  However, these satellites have flat, aluminum antennae that strongly reflect sunlight, just like a mirror.  If you happen to be at the right location on Earth, relative to an Iridium satellite and the Sun, you will see a 5 to 15-second-long “Iridium flare,” which occasionally can be many times brighter than even Venus.  But, if you want to see an Iridium flare, you must act quickly.  Most of the first-generation Iridium satellites will be de-orbited over the next 15 months, making way for the Iridium Next generation of satellites, which lack those highly reflective antennae.

You can get satellite fly-over and Iridium flare predictions for your home or any other location on Earth at websites linked below.  Western Slope Skies is produced by members of the Black Canyon Astronomical Society.  This episode was written and recorded by Art Trevena.

Web links for Earth Satellites:   


General Information: