ISS end-of-life options

NASA expects to end support of the International Space Station (ISS) in about six years. Several people have already made a number of suggestions regarding the future of the ISS. One obvious choice is to simply de-orbit the million-pound vehicle into an ocean. This can be done by attaching a propulsive de-orbit module that slows the vehicle such that it follows a prescribed atmospheric entry trajectory.

A good part of the station will burn up during reentry, but many large modules with survive to impact the ocean. Since the ISS orbits the Earth at an inclination of about 52 degrees, it overflies most of the world's population as it passes from 52 degrees North to 52 degrees South with each orbit.

There are some risks involved in a de-orbiting operation. For example, if the retro-fire burn to slow the vehicle is not properly executed, the station would miss the planned reentry point and would enter the atmosphere at a point beyond that associated with a safe event. In fact, it is possible the station might stay in orbit for several additional days or weeks before it would naturally reenter.

The result would be an arbitrary reentry location that could end in some property damage and casualties at any location in the populated world. The thought of such an event brings back memories of the Skylab reentry in July 1979.

Although Skylab has a small mass compared to the ISS, its reentry was not controlled by a propulsive module, but was partially steered into a safe orbit through the use of drag modulation made possible by timely reorientations of the vehicle over a period of several months. The result was a safe reentry, but some large pieces did land in Australia.

Another option under consideration for the ISS is to boost the large station out of its low orbit and into a much higher orbit where its natural life would be extended by at least 100 years. Once in such an orbit it could be later used as a commercial resource for space exploitation.

In order to accomplish this extension of life, the station must ascend through the high-density debris and satellite field between the altitudes of 600 km and 1200 km. Six years from now, that field will be much denser due to the added 10,000 satellites that are planned for launch before 2024.

There are some interesting risks associates with this option. A very large boost module must be built and launched well before the actual boost mission. A large Atlas V has been suggested as the boost provider. Assuming all goes well, the ISS would ascend from 400 km through 1200 km in a few hours.

If the boost burn is cut short the station did not achieve its objective, it could get stuck in an elliptical orbit that passes through the debris field every couple of hours. The ISS is sufficiently large that it could disrupt a good part of the active satellite constellations that operate in the 600 to 1200 km altitude range. This risk factor may not be acceptable based on risk-reward factors.

Still others have suggested that the ISS be subdivided into several modules. Most of these would be de-orbited while a few might be kept in orbit and adapted for commercial space station use. Work on these options should start soon, since six years for planning and implementation is not a great deal of time.

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