Understanding Iridium Flares

Iridium flares
A pair of Iridium satellites flaring. Jupiter is to the right and the bright star Arcturus is to the lower left. Jud McCranie, Creative Commons Attribution-Share Alike 4.0.

Our night skies are packed full of stars and planets to observe on a dark night. However, there are more objects closer to home that you can plan on seeing every so often. These include the International Space Station (ISS) and numerous satellites. The ISS appears as a slow-moving high-altitude craft during its crossings, while most satellites look like dimmer points of light moving against the backdrop of stars.

Some satellites appear to move east to west, while others are in polar orbits (moving nearly north-south).

There are thousands of artificial satellites around Earth, in addition to thousands of other objects such as rockets, reactor cores, and pieces of space debris (sometimes referred to as "space junk"). Not all of them can be seen with the naked eye. There's a whole collection of objects called Iridium satellites that can look very bright during certain times of day and night. Glints of sunlight bouncing from them are referred to as "iridium flares" and they can be observed fairly easily if you know when and where to look during the satellite orbits. Many people probably have seen an iridium flare and simply not known what they were looking at. It also turns out that other satellites can show these glints, although most are not as bright as the iridium flares.

What's Iridium?

If you use a satellite phone or pager, chances are the signals you receive or send come through the Iridium satellite constellation, a set of 66 orbiting stations that provide global telecommunications coverage.

They follow highly inclined orbits, which means that their paths around the planet are close to (but not quite) from pole to pole. Their orbits are roughly 100 minutes long and each satellite can link to three others in the constellation. The first Iridium satellites were planned to be launched as a set of 77.

The name "Iridium" comes from the element iridium, which is number 77 in the periodic table of the elements. It turns out that 77 were not needed. Today, the constellation is used largely by the military, as well as other clients in the airline and air traffic control communities. Each Iridium satellite has a spacecraft bus, solar panels, and a set of antennae. They go around Earth in roughly 100-minute orbits at a speed of 27,000 kilometers per hour.

The History of Iridium Satellites

Satellites have been orbiting Earth since the late 1950s when Sputnik 1 was launched. It soon became obvious that having telecommunication stations in low-Earth orbit would make long-distance communications much easier and so countries began launching their own satellites in the 1960s. Eventually, companies got involved, including the Iridium Communications corporation. Its founders came up with the idea of a constellation of stations in orbit in the 1990s. After the company struggled to find customers and eventually went bankrupt, the constellation is still in operation today and its current owners are planning a new "generation" of satellites to replacing the aging fleet. Some of the new satellites, called "Iridium NEXT", have already been launched aboard SpaceX rockets.

This new generation of Iridium sats will no doubt enable more flare-watching among Earth-based observers.

What Is an Iridium Flare? 

As each Iridium satellite orbits the planet, it has a chance to reflect sunlight toward Earth from its triad of antennae. That flash of light as seen from Earth is called an "Iridium flare". It looks very much like a meteor flashing through the air very rapidly. These brilliant events can happen up to four times a night and can get as bright as -8 magnitude. At that brightness, they can be spotted in the daytime, although it's much easier to see them at night or in twilight. Observers can often spot the satellites themselves crossing the sky, just as they would any other satellite.

Looking for an Iridium Flare

It turns out that Iridium flares can be predicted. This is because the satellite orbits are well known.

The best way to find out when to see one to use a site called Heavens Above, which keeps track of many known bright satellites, including the Iridium constellation. Simply enter your location and get a feel for when you might see a flare and where to look for it in the sky. The website will give the time, brightness, location in the sky, and length of the flare.