Thursday, February 2, 2012

How to see the ISS

It is easy to view and see the International Space Station (ISS), you only need to know where to look and what to look for. The near space around us is constantly active. Dozens of satellites circle the earth in different orbits. Many satellites can be viewed from Earth shortly after sunset or sunrise. Satellites do not shine their own light, but can be seen as they reflect the bright sun light.
Despite the difference between a satellite and an airplane, it is possible to confuse between the two. Eye equipped, satellite seen as a single point of light which is not flashing, in a single color and moving quickly in the sky. Aircrafts, on the contrary, are usually seen as several colored lights, flashing and moving much slower. In addition, the apparent brightness of a satellite varies relatively in fast pace during the transition, and the airplane brightness varies much slowly. The satellite disappears when it enters the Earth's shadow and does not receive more sunlight (or appears when it comes out from Earth’s shadow and starts to receive light from the sun). When the sun is low below the horizon satellites can not be seen (During the summer, satellites can be seen almost the entire night). Several reasons to observe satellites can be found at this link. With little experience, you will be able to point a moving dot in the sky as a satellite.

Large satellites
The satellites which are the most comfortable for watching are the largest:  The International Space Station (ISS), space shuttles (STS - No more!) and the Hubble Space Telescope (HST). The ISS is orbiting at an altitude of 350 km, with many visible transitions. At its peak, the station can reach brightness magnitude up to -4 (brighter than Jupiter and almost as bright as Venus). Space shuttle travels to the ISS occasionally. When the shuttle and the station had not yet joined, or soon after they parted, the pair can be seen as two successive points of light chasing one after the other on the same path within a few seconds to a minute difference. Repeated observations over two or three days will show how the two dots receding or approaching each other.

During one of the launches of a space shuttle, I noticed that it is already dark here at Israel. Although there were no data for the pass, I went up to the roof immediately after viewing the launch at NASA site and looked toward the approximate place where the shuttle had to appear (by my own calculations). In about 20 minutes after launch, a bright dot of light appeared exactly as expected. Much to my surprise soon after, another bright spot appeared on a similar route but dimmer. After thinking I concluded that the second point was none other than the external fuel tank of the shuttle. After a shuttle launch, its main fuel tank separate after about ten minutes, losing speed and altitude and finally falls into the Indian ocean waters. Since the phenomenon which I watched is quite rare (Because It can be seen only for about 20 minutes and at most of them from unpopulated areas), I found no other amateurs who viewed it. But I found confirmation that it exists in the following picture. Due to the low number of remaining shuttle missions the chance to see this behavior again is little (at least until the next generation of shuttles will be operational).

Even the Hubble Space Telescope can be seen with the naked eye. The HST cruise in higher orbit than the ISS (600 km) and is much smaller. Therefore, its visible magnitude is only about 1.5 at most. Space shuttle mission STS-125 in October 2008 was to upgrade and repair the telescope, giving him at least 5 more years to serve and sends more amazing pictures and data.

Iridium Flares
Another type of observation is on Iridium satellites. The Seattleites are in polar orbits (moving around the earth at an angle of 90 degrees to the equator), passing over the poles.  The Iridium satellites are used for communication purpose from anywhere on Earth. Not all of them are operational but all still orbits the earth (except one which crashed in space with Russian satellite). The satellite brightness normally is barley seen with a naked eye but they have relatively large radio antennas which return the sunlight like a mirror to the area along an imaginary strip over the earth. Whoever found within the stripe or a short distance away, will see a flash in the skies for a few seconds. The closer to the center of the strip, the flash intensity is higher and take longer. Iridium flares can be seen almost every day. Particularly bright flashes can be seen even in daylight if the location of the flare in the sky is far enough from the sun (angular distance). This is the time to note that caution is required in observations that occur while the sun is in the sky, as it might cause irreversible damage to eye of the observer without protective measures.

Communications Satellites
Communications satellites are Geostationary. They circle the earth around the equator and remain at the same point in the sky all the time (Their speed is identical to the earth own rotation speed and their height is about 36,000 km). Such satellites are difficult to see and they will appear as a dim star. Their movement speed is very slow, in fact, they will move in the direction opposite the movement of the sky. If photographed without star tracking they will be sees as a single point (compared to stars which will produce arcs). If tracking is used they will appear as an arc while the starts are fixed as dots.

How to photo satellites
Photographing satellites is relatively easy. It requires aiming the camera at the corresponding area in the sky (using a wide field lens). It is best to find a nice constellation that the satellite will pass through, or integrate an Iridium flare with a lovely landscape (buildings, landscape, etc.). Use a long exposure of several seconds, or preferably in manual mode. The result is a strip of light passing through the constellation or in the landscape. Iridium satellite flares start as a narrow point, become wider and narrow again as can be seen in the photo.
Iridium flare in the constellation Lyra
Iridium flare in the constellation Lyra

When aiming the camera at the area of the satellite leaves or enters Earth's shadow, see how the brightness changed from white to red;

The Space station in the Virgo constellations. Entrance to the earth shadow is changing the brightness.

For telescopes owners
It is hard to observe satellites with a telescope due to the high speed of the object. Small satellites will look only as a bright dot, but the International Space Station structure can be seen at 60x magnifications and higher. Another interesting option is to watch satellite on as they move across the sun. This course requires use of special solar filter. Do not look at the sun without appropriate equipment or you will damage your eyes. Satellite's passage over the sun take around a second, but at least you know where to aim your well protected scope; similarly, one can see passes on the surface of the moon.

How to find satellites
You can of course watch the sky looking for satellites. When observing from a dark area you will usually see several satellites during the hour’s right after sunset or before sunrise. However, it is better to come prepared and download location information from the Internet before starting the observation.

Here's an example of two recommended sites.
HeavensAbove: Registration is not mandatory, but it helps to keep your data for future use. The use is required to choose is location (by country and city, or exact coordinates). The site is very friendly and easy to use, and provides detailed maps showings where every satellite should pass the sky (including the direction and height of the starting points, peak and end). The site includes information on the movement of planets and comets as well.

CalSky:This site is rich in information. You can perform queries and receive a detailed report that includes watching a lot of events. The reports are little harder to read but it provides much more information such as transitions over the sun or moon.


  1. Note that the ISS can be seen in binoculars when it is in the Earth's shadow. This is due to the onboard lights. It is then very dim, about magnitude 9 when overhead.

    The easiest way to observe this is when the ISS is at first visible to the naked eye and then moves into the Earth's shadow. You can then follow the ISS using binos and continue to follow it into the shadow. You'll see that it becomes dimmer quite rapidly when it enters the shadow, but it remains visible.

    As it approaches the horizon it does become too dim to see it. Usually I lose it about 20 degrees above the horizon, the brightness is then magnitude 12 or dimmer, and there is a lot more light pollution close to the horizon.

    To follow it all the way to 20 degrees above the horizon, I do need to use my peripheral vision.