The ISS, Space Shuttle and Iridium Flares, Oh My!

Though the summer season unofficially begins with the Memorial Day weekend, astronomically speaking the summer solstice doesn’t occur until June 20 at 7:59 pm. On that date the Earth’s northern hemisphere is tilted at its maximum towards the Sun and at local noon the Sun reaches its highest point in our sky here in Southern New England.

With warm weather and longer days, many folks spend more time outdoors. And with gasoline prices skyrocketing I will bet families will opt for more local outings, perhaps camping at nearby campgrounds. You don’t have to know very much about astronomy to appreciate the beauty of the night sky. But while you are out there casually stargazing from a dark sky location, you might see some other interesting objects. You’ll see some familiar constellations, and of course there are a few meteors on any clear night. And you might see the Moon and a naked-eye planet or two. But every now and then some unknown object may traverse the sky. You might even see a brief, but bright flash.

The uninformed might think UFO, but more than likely there is an earthly explanation. I am often amazed that folks forget that we earthlings have launched thousands of satellites into Earth orbit. In dark skies a keen eyed-observer can see several dozen satellites on a clear and moonless night. And the brightest of them all right now is the International Space Station (ISS).

This orbiting laboratory is so large and reflective that it can be easily seen when it makes a favorable pass over Rhode Island. Hey, if it can be seen from Ladd Observatory on Providence’s East Side then you shouldn’t have any difficulty from elsewhere in Rhode Island.

And when the Space Shuttle is in orbit you can see it as well. In addition, if the shuttle docks with the ISS, the then combined spacecraft is even brighter still. But how does one know when these vehicles will pass through our skies? There’s a great website called Heavens Above (http://www.heavens-above.com/)That provides this information. After inputting your geographic location you can obtain a schedule of ISS/Space Shuttle flyovers. You can also find the passages of other satellites as well.

The Heavens Above site also provides other information. And one of my favorites, and that of many of our guests at Seagrave and Ladd observatories, is predictions for events known as Iridium flares. Iridium satellites are low Earth orbit communications satellites designed to be the “cell phone” technology of their day (1997). The service was too expensive for the general public, so the business venture went bankrupt. Lucky for us, in 2001 the satellites were bought by private investors and spared a fiery re-entry into Earth’s atmosphere.

Here’s what we regularly observe. Each Iridium has three main mission antennas which are highly reflective. The orbit of each satellite is precisely known, as is its orientation to the earth. Therefore, for any location on the ground, it is possible to calculate the angle between the satellite, the sun and an observer. When you get a perfect reflection of the sun off one of the antennae, the observer gets flashed by the satellite.

The brightness of the flash or flare depends on how perfect that angle is. From one location the flash may appear 30 times brighter than Venus, while at a location just 27 miles away the same satellite may flash no brighter than Sirius, the brightest star seen in our nighttime sky!

It’s incredible when you get flashed! You can usually spot the satellite just a few seconds before a flare occurs. The satellite will first appear dim, then all of a sudden it dramatically increases in brightness. It then fades just as quickly. It’s exciting to watch and makes the experience a little more interactive. An Iridium flare’s visibility ranges from five to as long as twenty seconds. On average I’d say most of the flares are of short to medium duration.

How can you know when an Iridium flare will be visible from your (or any) location? As with the predictions for ISS and Space Shuttle flyovers, you must input your geographic location to facilitate the calculation of satellite viewing opportunities over your airspace. You can ask for Iridium predictions for the next 24 hours, as well as for the next 7 days. The predictions are usually quite accurate.

So keep informed about what is happening in the sky, and you will certainly see evidence of humankind’s presence in low Earth orbit. Hopefully you will no longer be surprised when you get flashed by an Iridium satellite!

Don’t forget to visit Seagrave Memorial Observatory on any clear Saturday night for a tour of the heavens. Visit our website for additional information: www.theskyscrapers.org