15 December 2008—Russian space engineers think they are closing in on the cause of two dangerous off-course Soyuz landings in the past year, but recent activities suggest there is still a dispute about the leading theory. Top space officials have asserted that the problem—the failure of one or more explosive bolts during reentry—is already solved. But in fact, in a space walk from the International Space Station (ISS), scheduled for 23 December, astronauts will install an instrument that is supposed to confirm that theory, for which there is as yet no physical evidence.
The instrument, a small Langmuir probe, measures the electric potential of plasma flowing across the space station’s outer surface near where the Soyuz spacecraft are docked. The voltage difference between that plasma and the station’s electrical systems was an early headache for engineers, requiring the addition of two ion jets that ground the station’s electrical system to the surrounding plasma.
In October 2007, and again in April 2008, one of the six bolts that ring the interface between the Soyuz crew cabin and propulsion module failed to fire when commanded. As a result, the two modules remained attached far longer than they should have been, exposing lightly shielded portions of the craft to scorching reentry heat and setting the craft down hundreds of kilometers from its designated landing point. The bolt in that same position on the next Soyuz was removed by two cosmonauts on a space walk in July 2008, clearing the attachment point of any potential hangup for its October 2008 landing. That bolt, along with the two cosmonauts and space tourist Richard Garriott, landed without incident on 24 October, after a replacement Soyuz and crew had been launched two weeks earlier.
In an exclusive telephone interview last month, Bill Gerstenmaier, NASA’s associate administrator for the Space Operations Mission Directorate, told IEEE Spectrum that after exhaustive analysis the Russian team had converged on the so-called electromagnetic interference (EMI) hypothesis.
The EMI was supposedly caused by the flow of plasma around the station. There are two possible EMI failure modes that could lead to a dud pyrobolt, explained Gerstenmaier. One is that EMI can ”harden” the igniter wire so that it requires a higher current level to spark ignition. Or the EMI may cause ”the slurry of combustible material around the wire [to] migrate away from the wire, so there’s a gap between the slurry and the wire,” he said.
The hypothesis is supported by what was hitherto thought of as an inconsequential fabrication feature of the Soyuz intermodule interface. For one and only one of the six pyrobolt locations, Gerstenmaier explained, ”there was actually a cutout in the [metallic insulation] blanket that allowed a strut to attach to the Soyuz, and that cutout then allowed an arcing path around that area.” On both missions that experienced a dud pyrobolt, it was this position that failed.