The Iridium satellite constellation is a large group of satellites used to provide voice and data coverage to satellite phones, pagers and integrated transceivers over Earth's entire surface. Iridium Satellite LLC owns and operates the constellation and sells equipment and access to its services.
The constellation requires 66 active satellites in orbit to complete its constellation and additional spare satellites are kept in-orbit to serve in case of failure. Satellites are in low Earth orbit at a height of approximately 485 mi (781 km) and inclination of 86.4°. Orbital velocity of the satellites is approximately 17,000 mph (27,000 km/h). Satellites communicate with neighboring satellites via Ka band intersatellite links. Each satellite can have four intersatellite links: two to neighbors fore and aft in the same orbital plane, and two to satellites in neighboring planes to either side. The satellites orbit from pole to pole with an orbit of roughly 100 minutes. This design means that there is excellent satellite visibility and service coverage at the North and South poles, where there are few customers. The over-the-pole orbital design produces 'seams' where satellites in counter-rotating planes next to one another are travelling in opposite directions. Cross-seam intersatellite link handoffs would have to happen very rapidly and cope with large Doppler shifts; therefore, Iridium supports intersatellite links only between satellites orbiting in the same direction.
At 16:56 UTC on February 10, 2009 Iridium 33 collided with the defunct Russian satellite, Kosmos 2251. This was the first time two intact satellites have collided. Iridium 33 was in active service when the accident took place but was one of the oldest satellites in the constellation, having been launched in 1997.
Iridium moved one of its in-orbit spares to replace the destroyed satellite, completing the move on March 4, 2009.