Practically all of us have already played reflecting the sun rays with a small hand mirror. During those experiences we saw how is possible for rays to reach long distances, to reflect on windows and to illuminate very distant places.
Rescue manuals also contain vast literature showing how to use the mirrors to call the attention of the rescue teams, which at many kilometers away can see the reflex made by those asking for help.
With the objective of popularizing telephony via satellite on the decade of 1990, a group of American investors created the company Iridium and placed in the space dozens of communication satellites in low orbit, around 780 km, each one composed of 3 plane and metallic antennas of 188cm x 86cm, each one separated by an angle of 120 degrees. Now, the system Iridium is rarely used, but the constellation satellites are still very active in the space and counts with 81 objects in orbit.
What the company Iridium didn't imagine was that the three antennas, for being highly polished and tilted in 40 degrees, could reflect the sun rays exactly like the mirrors game and could be a true physics and optics class in schools and universities. The term Iridium Flare appeared then, that intense light flash provoked by the reflex of the Sun on the metallic antennas of the Iridium satellites.
When we say intense, we are not exaggerating. In lots of occasions the flash is so strong that it can reach magnitude -9, which represents 30 times the bright of the Venus planet. The flashes can be very easily seen and the only requirement is that the observer knows where to look and at which moment. The duration time is not fixed, but we already witnessed flashes of up to 15 seconds.
Why does it happen?
As explained, one of the flash responsible mechanisms is the Sun's reflex on one of the three main antennas, called MMA (Main Mission Antennas) but at some occasions the reflex can also be originated in the solar panels. The antennas are built of aluminum with a silver bathing and mounted with a 40 degrees inclination in relation to the satellite's body.
Besides the antennas and the solar panel, the other element responsible for the flashes is the satellite's position, which has its vertical axis rigidly pointed in the direction of the Earth. This configuration maintained, one of the antennas will always point forward.
It is the precision of the position of the elements above that allows the flashes to be foreseen and calculated with accuracy, since the position of all the elements is perfectly known.
The other necessary data for its calculation are the satellite's position, the observer's location and the position of the Sun in relation to these two. Once these parameters are known, the traditional formulas of spherical trigonometry are used and specifically the mirror's Law, necessary to calculate the reflection angle, also called the specular angle.
Here on Earth
The reflex of the Sun on one of the three MMAs produces a small spot on the surface of the Earth, or a luminous circle, of approximately 10 km of diameter, which moves as the satellite and the Earth move.
The flash or Iridium Flare occurs when the observer is inside the spot and it will be the more intense, the closer it is of its center. That means that the calculations for its observation will be more accurate, the more exact the coordinates are supplied by the observer, since small variations can move the spot focus.
The lateral illustration helps understand the process of the flash generation. Observe the spot projected on the surface.
Watching the Flashes
Our application allows you to know the exact moment the flashes will take place, besides giving information about the azimuth and the elevation of its position in the sky. The informed values are very accurate, therefore keep your clock perfectly adjusted.
The most intense and interesting flashes are those with the larger intensity, reminding you that the smaller the magnitude informed on the table, the larger the flash bright will be. Therefore, a flash of magnitude -8 is more intense than one of magnitude -5.
Once you know where to look, you only have to wait for the settled hour. An intense light point will mark the sky. Just like the mirror game!
Have good Flashes!