There have been several times that I was out observing either at an organized star party or just with a small group of friends/neighbors. A good amount of time is spent simply scanning the skies with the naked eye enjoying the splendors of the universe. Eventually, someone catches a glimpse of something moving across the background of stars and calls out for the rest of us. As we all turn our attention toward the new-found object, it suddenly brightens like a flash; brighter than anything else in the sky! And then, just as quickly as if flashed into existence, it fizzles back out to its former dim self and continues across the sky moving through the background of stars. Discussion and conjecture ensue about this celestial interloper, and invariably, someone asks, "Was that a UFO?"
In short... No!
What was just witnessed was a satellite flare; more specifically a Iridium flare. Satellites orbiting around Earth out in space are made of many shiny parts. Most are covered in some sort of highly-reflective metallic sheathing and may also have large solar array panels that are quite reflective. Any of these satellites can and do reflect sunlight back down toward the planet's surface when the angle between the satellite, Sun, and Earth are just right.
However, there is a special breed of satellites that do this better than all others: Iridium Satellites. The Iridium Constellation is a fleet of 66 active communications satellites that each orbit the planet roughly every 100 minutes. The satellites are oddly shaped with three large, highly-polished antennaes each about the size of a door. When sunlight glints off these panels, they can reflect a bright cone of light about six miles wide onto the surface of the planet. Depending on where you are in relation to the center of this reflected cone of light, the Iridium flare can appear as bright as -8 magnitude (occasionally -9.5), which is brighter than any other object in the night sky. Only the full moon is brighter at -12.9 magnitude. The bright flash of an Iridium flare lasts several seconds. Before and after the flash, the Iridium satellite appears as any other dim satellite traversing the night sky.
In the photo below, we see an example of an Iridium flare zipping past the Helix Nebula. Photo taken by JD Maddy, an experienced astro-photographer and member of the "Astronomers of Verde Valley."
To view a short video clip of an Iridium flare, check out the YouTube submission below. A telescope on a tracking mount was used to capture this flare. You can see the satellite in the right of the video just above center. Other stars are flying past as the scope tracks the satellite. The flare begins to brighten at about second 15 and lasts roughly 10 seconds:
There are several websites and apps for your smart device that can provide forecasts for Iridium flares. OneDarkSky regularly uses www.Heavens-Above.com to provide its satellite fly-by updates. Check the Satellites section over at the OneDarkSky forums for upcoming Iridium flares and other interesting satellite fly-bys.