Satellites – No Impact – Weather

Satellites—Impact Turn—Debris

Space debris experts attending the 38th Congress of the Committee on Space Research (Cospar) here July 18-25 said an event last January brought home just how much of a threat to the low-Earth orbit environment Envisat will be. That was when the U.S. Space Surveillance Network warned ESA that a 3,306-pound (1,500-kg) upper stage from a Chinese rocket was bearing down on Envisat and that the "conjunction assessment" pointed to a likely impact. With Envisat still operational, ESA's European Space Operations Centre (ESOC) control facility in Darmstadt, Germany, fired Envisat's on-board thrusters to perform a collision avoidance maneuver. Heiner Klinkrad, head of ESA's space debris office at ESOC, said here July 21 that a post-event analysis showed that the Chinese stage probably would have collided with Envisat if the avoidance maneuver had not been done. Such maneuvers will not be possible once Envisat is retired. ESA officials, more accustomed to speaking of Chinese, Russian or American debris issues, are uncomfortable discussing the danger that Envisat represents, especially since the agency has showcased the satellite as a major success. Klinkrad is no exception. But he did say that if the collision with the spent Chinese upper stage had occurred, it likely would have polluted a highly used portion of low Earth orbit with 10 times as much junk as what was caused by the 2009 collision of an operational Iridium communications satellite with a retired Russian Cosmos spacecraft. That event occurred at about the same altitude where Envisat flies. Huge satellite debris risks Envisat's 17,636-pound (8,000-kg) mass alone would be enough to put it onto the top tier of space debris threats, even though there are nearly a dozen spent Russian rocket upper stages that weigh as much as or more than Envisat. But Envisat's configuration in orbit makes it a unique concern, even beyond its weight. The satellite's in-orbit size is 26 meters by 10 meters by five meters. Its suite of observing instruments uses a small farm of antennas that likely have become more fragile after a decade in orbit. That means that even a small piece of debris pieces too small to be cataloged by the Space Surveillance Network could cause what debris specialists refer to as a "fragmentation event" that would produce its own population of space garbage. An analysis of the space debris environment at Envisat's orbit suggests that there is a 15 percent to 30 percent chance of the satellite colliding with another piece of junk during the 150 years it remains in orbit. But that likelihood is based on the current population of space debris in low Earth orbit remaining constant during the period a scenario no one believes is remotely possible.

In the past, imaging satellite operators have generally not needed to worry much about the risk their satellites might face from collisions with other satellites or with orbital debris. More recently, however, they have begun to take notice of environmental conditions in Earth orbit. One immediate reason for increased interest in orbital safety is the February 10, 2009, collision of the Iridium-33 satellite with the defunct Russian Cosmos 2251 satellite. That collision created more than 800 pieces of debris 10 cm in length or above, in two clouds. Over time, these two clouds will slowly expand outward, threatening other working satellites that move in nearby orbits. Because the destroyed Iridium satellite flew in a near polar orbit, debris from this collision now poses an additional hazard to some remote sensing satellites. The Canadian RADARSATS 1 and 2 have been cited as threatened by this debris.(1) Other Earth observing satellites are likely to be affected in the future. Normal space operations unavoidably add debris to the space environment. In addition, fragmentations of spent rocket bodies in orbit and occasional explosions of old satellites add to the threat. Finally, anti-satellite tests by the Soviet Union and the United States in the 1970-80s and by China in January 2007 resulted in thousands of pieces of space debris. U.S. Air Force officials estimate that some 18,000 pieces of debris greater than 10 cm now circle Earth.