All inactive satellites spin about some axis of rotation (some would call it "tumbling"). Since inactive geosynchronous (GEO) telecommunications satellites orbit at an average of 37,000 km from the Earth's surface, some would think that they cannot be seen with the unaided eye. This is normally the case, except for those rare exceptions when the observer sees the solar panels nearly face on and sunlight is being reflected off of them directly to you.

Echostar 2 is a typical "box-wing" type GEO satellite, which consists of a central cube portion (the "box") and two large solar panels (the "wings"), as shown in the image below. These solar panels are enormous. In the case of Echostar 2, they are about 8.5 x 3.1 metres (26.5 square metres) in area ---- each. Together, the two solar panels were designed to generate 7000 Watts of power: all so that you could comfortably watch satellite television in the comfort of your home using a small satellite dish.

An Artist's Depiction of the Echostar 2 Satellite - Lockheed-Martin

The Echostar 2 GEO satellite has been inactive (AKA dead) since July 14, 2008 when a solar storm knocked out its main power system. As a result, its attitude (orientation) is no longer being controlled from the ground. This means that the satellite is at the mercy of all natural forces acting upon it, including solar radiation pressure, causing a "solar sail" effect that causes these satellites to spin. As the satellite spins, its apparent reflectivity is seen to change periodically as the different pieces of the satellite are reflecting the sun's rays. The CASTOR movie of Echostar 2 (below) shows this periodic behaviour. A labelled image depicting a portion of Its measured light curve is also shown below.

The Changing Brightness of Echostar 2 as seen from the Earth - May 11, 2012 - CASTOR

A Portion of the Measured Light Curve of the Echostar 2 GEO Satellite - September 8, 2012 - CASTOR
 

As the satellite orbits the Earth, we see constantly differing lighting perspectives of the spacecraft. However, when the angle at the spacecraft (subtended by the observer and the sun) becomes small, the observer is basically seeing what the Sun sees, thus we can somewhat see the spacecraft's orientation with respect to the sun. If we see very bright reflections (often called "flares"), we know that one or both of the solar panels are face-on to the sun. If we see no flares or very dim peaks between the "box" peaks on the light curve during this time, that means that the solar panels are edge-on to the sun and any flares would be very difficult to see without telescopic aid.

The concept in astronomy where a larger telescope aperture will allow the detection of dimmer objects is certainly applicable to the apparent brightness of a satellite flare. If we are seeing the solar panel nearly face-on, we are seeing the largest amount of area and therefore we would see the largest amount of reflected sunlight (if the angle at the satellite between us and the sun is low). Just imagine how much light you could collect with a telescope using a 26.5 square metre mirror! The sun's light would be lethal not only to your eyes but to your whole head in this case!

It is currently unknown whether we are seeing one or both solar panels reflecting light at the same time during a flare. Echostar 2 spins too quickly for CASTOR to currently collect a lot of data points during a flare. In fact, the solar panel peaks in the light curve above appear to be so different in height because sometimes the detector is seeing just the beginning and ending of a flare and not the brightest part of the flare.

Solar panels are not 100% reflective because they ideally need to absorb most of the light to be the most efficient. This means that they are not technically mirrors in space. They reflect only about 10% of the incident sunlight, therefore we only see about 10% of the sunlight reflecting off of them. However, a 10% albedo is at least 5 times the albedo of the Moon's surface (but the Moon is much larger). The best examples of "mirrors in space" are the Iridium satellites which use highly polished metal high gain antennas. These satellites can reach a brightness of magnitude -9, but are much closer to us than the GEOs.

Below is my first colour image of an Echostar 2 flare. The exposure time was long enough to cause the stars to streak. The satellite flare was brief enough (only a few tenths of a second) to make it appear as a stationary dot. I also saw this flare with my naked eye. You could just see the flare ramping up and dimming down very quickly. The brightest portion looked like a very distant camera flash. I could just pick out a very light yellow colour. Since our sun is a G-type star, it is actually yellow in colour. Some might think it is white because of its very high apparent brightness.
 

A Naked Eye Echostar 2 Flare in Colour - CASTOR
 

A CCD camera would see this flare much differently. Although the flare lasted for only a few tenths of a second, the CASTOR CCD camera is sensitive enough to record much of the light in such a small span of time. This is especially true when it is coupled with an 11-inch aperture telescope. A typical image of a massive Echostar 2 flare is shown in the movie below. The flare was so bright that it saturated the CCD camera, showing a bleeding streak just below the flare. For a light curve analysis, this saturation would render this data point useless for photometric purposes, however its time of flare would still make this data point useful in determining the satellite's apparent spin period for that night.
 

A Massive Flare of Echostar 2 Captured on CCD - CASTOR

Since the design of Echostar 2 is very similar to the general box-wing design of the modern GEO telecommunications satellites, all of these satellites have the potential to produce brief flares much like Echostar 2. The only deciding factors are the solar panel area, the spin axis orientation relative to the observer, the phase angle and the apparent elevation of the satellites in the observer's sky. It is very likely that by seeing one, you will have seen one of the furthest man-made objects visible with the naked eye, if only for a brief time.