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Back Garden Astronomy - The ISS and Other Satellites
Back Garden Astronomy - The ISS and Other Satellites

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

The Moon isn't the only object orbiting the Earth you can take a look at

International Space Station by Jimmy E.

International Space Station by Jimmy E.

There are two types of satellite visible in the night sky — natural ones like our Moon and artificial ones that we have placed up in orbit. Of all the artificial ones, the International Space Station, or ISS, is probably the best known. Easy to predict, its constant, often bright passage across the heavens is a sight that instills wonder.

Humankind's orbital outpost typically appears as a dot, which gets brighter as it passes across the sky before fading again. Sometimes the ISS appears bright and then fades abruptly from view. The fading occurs when the ISS's trajectory takes it into Earth's shadow, and as most sightings tend to be in the early evening the disappearance occurs when the ISS has reached the eastern half of the sky. If you're an early riser, Earth's shadow will be to the west. This causes the ISS to instantly 'switch on' as it passes out of the shadow back into full sunlight.

Solar power
It's the interaction of sunlight with the surface of a satellite that makes things interesting. Spacecraft that have large reflective areas can flare in brightness, sometimes quite significantly. The best flares are caused by a group of satellites known as the Iridium constellation: 'constellation' being the collective noun for a group of satellites. When you see one of these spacecraft brighten rapidly, this is what's known as an Iridium flare.

The science behind the flare is unremarkable in that each satellite in the constellation has three large flat, reflective antennas. When the Sun's light happens to hit an antenna at the right angle, it will appear bright when seen from a fairly localised region on Earth's surface. What is remarkable, however, is the fact that there are ways to predict, with down-to-the-second accuracy, when a flare can be seen from your location. And we're not talking faint, indistinct flaring here: some Iridium flares can increase the apparent brightness of the satellite's dot from that of a dim star to something brighter than Venus.

The brightest flares tend to be around mag. -8.0, brilliant enough to easily illuminate any thin clouds that may get in the way. In theory, such a bright pass could even cast shadows — not that anyone ever looks behind them when a flare occurs! Not all Iridium flares will reach this brightness, of course; the flare may not be optimal and you may be located away from the position on Earth where the brightness of the flare peaks.

Other satellites can also show flare activity and it soon becomes obvious, especially to meteor imagers, that flaring spacecraft occur all the time. A flaring satellite that reaches peak brightness and is then rudely truncated by the camera shutter closing will look very similar to what you'd expect to see from a bright meteor trail.

It's possible to tell the difference by looking carefully at the brightest end of the trail. If the trail looks perfectly smooth and is cut off squarely at the brightest end, then it's either a rare meteor trail interrupted in its prime or — much more likely — a flaring satellite trail that wasn't allowed to complete its display before the camera shutter closed. Iridium flares also tend to record as white trails, while meteor trails often exhibit a pink start changing to green — an effect caused by the excitation of atoms in our atmosphere.

There are over 1,000 operational satellites orbiting Earth and an estimated 21,000 objects larger than 10cm. If you widen the net and include objects down to 1cm in size, the count moves beyond half a million. In fact, on any clear, moonless night, it would be unusual not to see an artificial satellite passing through the constellations, appearing as a moving dot among the stars.

Predicting a pass
There are many different ways to predict satellite passes — some more reliable than others.

Heavens Above
One of the most popular and respected methods is to use the website Heavens Above (www.heavens-above.com). You can create a free account that logs your location and generates visibility predictions for many different satellites. Sky charts accompany visible passes, and clicking on the date of the pass will typically bring up an all-sky chart showing the passage of the satellite among the stars. So long as you have a basic knowledge of the constellations then the track, adjusted for your location, should be pretty easy to identify. As an added bonus, if you don't know the stars that well, then this is a good way to have some fun while learning the night sky.

Other Prediction Sites and Apps
For the slightly more technically minded, there are many excellent programs available to download such as WXTrack (www.satsignal.eu/software/wxtrack.htm), which is able to predict the passage of many satellites directly from a Windows PC. Apps for other operating systems, including smartphones are also available; many are listed in a 'satellite tracking software index' at celestrak.com/software/satellite/sat-trak.asp. Some of these programs are commercial, requiring you to purchase a licence to use them, but there are plenty of freeware options available too.

Ensuring Accuracy
One problem with computer predictions is reliability. This could be down to problems with the program itself, or that you haven't set your location, date or time properly. And if satellite data isn't updated regularly, this too may affect accuracy. If doubts start to creep in, compare the predictions for an easy to identify satellite, such as the ISS, with Heavens Above. If they don't match, update the software's satellite data, and your time and location details, before trying another program.

Copyright Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Date Taken: 02/06/2017
Author: BBC Sky at Night Magazine
Category: Astronomy

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