Earlier this year, an aging, defunct Russian Space Forces satellite, Kosmos 2251, collided over Siberia with the US-based Iridium Satellite, LLC's Iridium 33. The collision happened around noon Eastern time on February 11th. It destroyed the two satellites and left a new cloud of space debris in an already overcrowded orbital neighborhood. This area is already thoroughly littered, thanks, in part, to China's decision to target one of its own weather satellites with an anti-satellite weapon. Within a day of the collision, the US space tracking systems had identified hundreds of individual pieces of debris.
When the two satellites collided, they were each traveling at a speed of around 17,000mph relative to the Earth, and nearly 22,000mph relative to one another. The collision occurred over northern Siberia (72.52 oN, 97.39 oE) at an altitude of 490 miles. Given the limitations of the US space tracking system—it can only reliably track debris particles greater than 5 to 10cm (2 to 4 inches)—it is hard to know exactly how much debris this collision has generated. To date, there have been 352 items identified as coming from this collision.
Had the two hit head-on, instead of at an obtuse (102.2o) angle, models developed at NASA calculate that there would have been more than 62,000 pieces of debris greater than 1cm in diameter. Despite this small size, the extreme orbital velocities mean that 1cm debris can significantly damage an orbiting satellite. While the collision poses no immediate concern to astronauts in the International Space Station—it is far below where the collision occurred—Chinese scientists have expressed concern that the debris field could damage some of their country's satellites, specifically the ones in Sun-synchronous orbits.
This event creates a unique opportunity to look at the real dangers satellites face on a daily basis. In a blog post over at ArmsControlWonk,
Dr. Jeffrey Lewis highlights some of those. Right
now, according to the Union
of Concerned Scientists' Satellite
Database, there are 441 satellites, both operational and
non-operational, in Low Earth Orbit (LEO means altitudes less than 1200km). And satellites represent only a small fraction of the debris greater than a
few centimeters that is orbiting the planet. In order to calculate
the danger a given satellite faces, one needs to know how much junk
could potentially be in the orbit of the craft.
The simplest way to address this problem is to assume a constant or average flux around the entire planet, e.g. that there are N objects per volume of space per unit time. The problem is that reality isn't that simple, as the amount of debris is highly dependent on the altitude. At the equator, the debris flux has two strong peaks around 800 and 1500km high, with little material in between.
The other complication arises from the manner in which we, humanity, put stuff into space. A not-so-insignificant number of the satellites are put into near-polar orbits—that is, they travel around the planet going from the North pole region to the South, or vice-versa. This creates a sort of traffic jam of satellites at the Earth's poles; since a large number of craft are passing over this small region of space, the chances for a collision go up dramatically. According to NASA calculations, there is 10 times more debris per cubic kilometer in the high latitude regions (?75?) than there is at the equator (0?).
Like many other satellites in the sky, Iridium-33 was in a near-polar orbit, having an inclination of 86.3? (meaning it passed the equator at an angle of 86.3?). To understand the risks associated with this flight path and altitude—it had an apogee of 780km and a perigee of 774km, putting it right in a high-debris band—we need to know how much junk it is flying through. This is a serious challenge, but NASA has an Orbital Debris Engineering Model (ORDEM2000) that is free for people to download and use.
Using the ORDEM2000 model, I calculated the debris flux that the Iridium-33 satellite would have to contend with during its 2009 orbits. As you can see from bar graph, the amount of debris is considerably higher at the poles (90? and 270?) than it is in the equatorial region (0? and 180?). Integrating this flux value over the orbital path and using some published data on the LM 700 satellite—and hoping I did all of my unit/coordinate conversions correctly—produces a result that indicates, on average, the Iridium-33 satellite would be struck by an object large enough to do damage once every 44 years. (Again, assuming that the debris flux calculated by ORDEM2000 for 2009 is representative and I interpreted the results correctly.) Clearly this not a common occurrence, but also not so remote that the possibility can be ignored.
Since 1991, there have been 8 major satellite collisions, not including China's ASAT test. This would lend credence to the number I calculated: given the number of active and inactive satellites, the chance of collisions are small, but non-zero.
This collision happened to occur in a particularly bad area, as mentioned previously. The altitudes where this collision occurred are already so congested with space debris that they are referred to as supercritical. That means debris is being generated through collisions at a faster rate than atmospheric drag can remove the existing debris from orbit.
As more and more satellites are launched into orbit, the potential for debris issues is becoming widely recognized. In a press release from the Union of Concerned Scientists, issued shortly after this collision, David Wright pointed out that a number of countries have developed a set of debris mitigation guidelines that have since been adopted by the UN. One key measure was to have countries remove defunct satellites, such as Kosmos 2251, from highly polluted orbital areas. Wright also suggested that, as "space becomes more and more crowded, the international community must begin to develop and put in place measures for space traffic management, similar to what we now have with air traffic control around busy airports."
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