Sen—Just the other day I was in rural western Colorado, standing under magnificently dark skies at the Waunita Hot Springs Ranch. I was with a few dozen other science enthusiasts for Science Getaways, a vacation trip planned by my wife and me. After a long and fun day we were enjoying a moonless night of stargazing, and had just finished viewing Saturn and Mars through my telescope.
The Milky Way was rising in the southeast, so I took a few minutes to point out some of its features. While we were all looking up I noticed a faint star, almost lost among the thousands of others. This one was different, though. It was moving.
I drew their attention to it, a dot of light gliding silently through the constellations. “Oh look,” I said. “It’s a satellite.”
The response I got was one I expected—surprise. Many people, even ones interested and engaged by astronomy, don’t know that human-made satellites can be easily spotted on any given night. Some satellites are quite large, many meters across, and can be in full sunlight in their orbits high above the Earth even as we observers are shrouded in night.
Everything from rocket boosters used to launch satellites to the International Space Station itself can be seen from the ground, and because their orbits are well-known, the times and brightnesses can be predicted. Some objects tumble as they orbit, presenting their long side and then their short side to us, so their brightness changes slowly as they move across the sky. Others can be more dramatic.
The best example of this comes from the fleet of Iridium satellites. These are communication satellites that circle the Earth in polar (north/south) orbits about 800 kilometers up. Nearly 100 such Iridium satellites are flying now, and each is covered with highly reflective antennae. Most of the time the satellites are faint, nearly invisible. But if the angles are just right one of the mirror-like antennae can catch the Sun, reflecting sunlight to the ground. If you happen to be at the right place at the right time, an Iridium satellite can grow in brightness rapidly, getting extremely bright. They can rival and even outshine Venus for a few seconds before slowly fading away as they continue to move through the night.
Photographer Mike Taylor was at the right place at the right time to catch one such Iridium flare (as astronomers call this phenomenon). He was out in western Maine, taking photographs to put together in a time-lapse animation. During the 30-second exposure he was able to capture thousands of stars together with the glow of the Milky Way, as well as low clouds illuminated by the light pollution from nearby the nearby city of Farmington.
But he also caught an Iridium flare, so bright it made Saturn (seen through the trees on the right) pale in comparison. During the exposure the satellite moved a few degrees, starting faint, intensifying rapidly, then dimming into obscurity once again. The whole event probably took 10 – 15 seconds to play out.
The location of a visible flare seen from the ground is very tightly constrained. The geometry of the angles of the Sun, the satellite, and you is so narrow that if one of them changes even slightly you’ll miss the flare. If you’re seeing a flare, someone even 50 kilometers away may see nothing at all.
This makes each flare something special, even personal. Not long after we saw the faint satellite at Waunita Ranch, I happened to catch the peak of an Iridium flare out of the corner of my eye. I quickly turned toward it and was able to point it out to everyone around me before it faded out; several people gasped in delight as the bright white light dimmed in seconds as they watched.
Over the course of the night (and the rest of the week) we saw dozens of satellites, a handful of meteors, and of course celestial objects both near – in cosmic terms — and far. Getting the note from Mike about his photo of the Iridium flare around the same time we happened to see one was a fun and happy coincidence.
And it was an excellent reminder that there are wonders going on over your head you may not even know about. Spending time under a dark sky is never wasted.