Back in 2009 I posted on the risk of GPS satellite positioning system degrading over the next few years, both in terms of coverage and accuracy, due a decrease in the number of operational satellites. This risk was the main finding of an audit performed by the Government Accountability Office (GAO), where Monte Carlo simulations predicted that the number of operational satellites would fall below the threshold required to provide positioning at agreed service levels. In short, too many satellites that were approaching, or had passed, their expected operational life were being relied on to continue functioning in the absence of replacements. Engineers know that satellites have very finite operational lifetimes, and at some point will simply stop working and start drifting.
And one significant satellite did just that last month, as reported by the Economist for example. The satellite in question was Envisat, one without GPS responsibilities thankfully, launched in 2002 to provide a wide range of environmental data which it has delivered handsomely in the terabyte range. It is a critical primary source of data for scientists, providing continuous observations until contact was lost last month. The European Space Agency has formally announced that the mission of Envisat has been completed, and successfully so, after celebrating it’s tenth year of operation when only five were expected – both from an engineering and funding perspective.
So Envisat was living on borrowed time, five years of it or 100% additional mission time, as the GAO report on the GPS satellite constellation was asserting. The Economist article goes on to name some culprits in the case of Envisat, with governments being allocated the lion’s share due to lack of funding. Both NASA and ESA are unwilling to sure up their Earth-observation programs without additional government guarantees.
There is another risk beyond the loss of service provided by Envisat or GPS satellites, and that is additional space debris created by these satellites once they stop functioning. It is estimated that Envisat will orbit the Earth for the next 150 years before being drawn down into its atmosphere. During this time it will be at risk from colliding with other existing space debris, breaking into smaller pieces upon impact, producing even finer debris. This is known as the Kessler Syndrome, proposed by NASA scientist Donald J. Kessler in 1978, who commented on the Envisat demise as follows
It seems ironic that a satellite intended to monitor the Earth’s environment is at risk from the space environment and is likely to become a major contributor to the debris environment.
Orbital debris and the collisions that may result from its presence, are a significant risk for NASA. There is a 180-page report on this topic which, apart from the specific subject matter, contains many useful risk principles and guidelines.