In the story “Debris-control Report Card Cites Improvement by Geo Sat Owners” [Oct. 28, page 1], the description “if … an object poses a greater than one-in-10,000 risk of surviving atmospheric re-entry” is not completely accurate. Most re-entering spacecraft are estimated to have at least some portion survive atmospheric re-entry heating. The guideline is based on whether the risk of injury to the public from such debris exceeds one in 10,000.
The ability to perform a controlled re-entry depends not only on having sufficient fuel reserves, but also on a design that supports controlled re-entry (sufficiently large thrusters, for example). Many of the spacecraft in low Earth orbit don’t even include a propulsion system, so this is not an option for them.
The story says, “The [Inter-Agency Space Debris Coordination Committee] and many of its member space agencies, including NASA, typically plot debris-buildup scenarios in different orbits assuming that, in the future, 90 percent of all satellites and rocket upper stages will be disposed of correctly at the end of their missions.” This 90 percent assumption also includes proper passivation, to prevent the potential for post-mission breakup through explosions or other mechanisms. There are a number of good reasons that this may not be possible at the end of a mission, but when passivation is considered, we are even further from the 90 percent goal.
Careful consideration of the decommissioning and disposal phase of the mission is needed throughout the mission and hardware design, in order to allow for a responsible disposal later. This is an important part of the design process because all space users suffer when a disposal is less than complete.
Scott Hull
Columbia, Md.
