ELi ON EARTH: Iridium Flares Over East Lansing

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Monday, November 17, 2014, 12:33 am
Aron Sousa

Image: International Space Station, courtesy of NASA.

Early on the mornings of November 20 and November 22, East Lansing residents will have the opportunity to see two bright communication satellites overhead. Every day, many satellites fly over East Lansing, and those satellites become visible in East Lansing when the sun reflects off the satellite in an otherwise dark sky. Next Thursday (November 20) and Saturday (November 22), East Lansing observers should be able to see reflections that will make the two Iridium communication satellites the brightest objects in the sky, other than the moon.

The most common time to see satellites and other spacecraft, like the International Space Station (ISS), is just before sunrise and just after sunset. At these times, the sky is dark but the sun still shines on satellites 200 to 400 miles above the earth. The sunlight reflects off the satellites and down to earth, where ELi readers have a chance to see the steady, fast movement of the satellite across the sky. (In the middle of the night, the sun is on the opposite side of the earth and the satellites over East Lansing are in the shadow of the earth and therefore not visible from earth.)

There are a total of 66 Iridium communication satellites rotating around the earth, each with door-sized metal antenna that reflect especially brightly in the sky. These satellites are part of the Iridium Communication System, a commerical communicaiton system, and orbit the earth from pole to pole (north and south) rather than west to east as the ISS and other human space flights move. There were originally going to be 77 of Iridium satellites, and so the system was named after the element Iridium, which has the atomic number 77.

The solar panels and bodies of all satellites reflect light and make them potentially visible from earth, but the large door-sized metal antenna on the Iridium satellites act like a mirror and can reflect the sun brightly to a spot on the earth about 10 kilometers (6.1 miles) in diameter. East Lansing will be in the path of these extra bright spots on November 20 and 22.

Most satellites are as bright as a common star as they cross the sky, and most of the time that is what an Iridium satellite looks like. On the occasion when the viewer is in the path of the reflection from the Iridium antenna, the satellite will get substantially brighter as the 10 km reflection passes the viewer before returning to a more normal brightness. This flare (getting brighter and then dimmer) can occur with other satellites, but it is most common with the Iridium satellites. The brightest Iridium flares are visible in the daytime, but the Iridium flares later this week will be visible as the brightest objects in the sky except for the moon.

On November 20, 2014 the Iridium satellites will be traveling from nearly due north to the southeast about 20 degrees over the eastern horizon. (If you hold your fist out at arm’s length it covers about 10 degrees of sky, so two fists above the horizon would be 20 degrees above the horizon.) There will be a minor flare in the morning at 6:40:55 (hours:minutes:seconds), followed by a brighter flare (another satellite) 10 seconds later, at 6:41:05. Slightly earlier at 6:09:41, the ISS will be traveling west to east from the northwest to northeast about 15 degrees above the horizon.

On November 22, 2014, the ISS will again precede an Iridium flare. The ISS will be visible in the sky from 6:07:29 am to 6:11:16 am, moving from the north-northwest to the east-northeast about 15 degrees above the horizon. There will be an Iridium flare at 6:19:11 am in the east-northeast about 15 degrees above the horizon. As ELi’s astronomy reporter, and as someone who loves to see the products of science among the stars, I am hoping for clear skies.

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