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Debris Alert: A Crack in the Window
by Staff Writers
Bethesda MD (SPX) May 18, 2016

In terms of ISS safety vis-a-vis the debris field, an important question is, "Should we be concerned about a large debris object hitting the ISS?" The answer, unfortunately is "Yes."

In recent days media coverage of space debris activity has been intense. Some commentaries are reminiscent of the hysteria in Chicken Little's report of the sky falling. Others recommend that the issue be ignore. They say, "Space is big, so don't worry." However, just last week a report revealed the existence of very small cracks in an International Space Station Window.

Apparently, a tiny piece of space junk collided with a window in the Cupola, a European-built viewing compartment installed on ISS in 2010. While it provides great views of Earth and has facilitated the taking of many stunning images and videos, it also provides a window into small debris hits with the station.

If all parts of the station were inspected with a magnifying glass we would undoubtedly discover literally hundreds and possibly thousands of collision sites. This would certainly be unnerving to the astronauts, but not necessarily dangerous. Almost all of these sites would surely confirm that most of the orbiting debris consists of very small particles.

While there may be a noticeable number of small holes in the solar arrays, the station can withstand a considerable number of such impact sites before losing significant electrical power. The important thing to remember is that most of the debris resides in orbits above the station. Therefore, the ISS is exposed to only a very small portion of the total mass of orbiting junk. On the other hand, as we already know, the station is not completely immune to debris effects.

A cautionary note is appropriate here. Each debris piece whose orbit is decaying may have started life as an object flying above the station. As its orbit degrades the piece will drift through the altitude used by the ISS. As it turns out, the only guarantee against a collision between a debris object and an operational satellite is when the two do not occupy the same altitude at the same time.

Thus, when a debris piece crosses the station's altitude, it is possible a collision could occur, even though the probability of such an event may be infinitely small at that moment. Given that there are probably thousands of such altitude crossings each day it is not surprising that one small object hit the Copula window.

In terms of ISS safety vis-a-vis the debris field, an important question is, "Should we be concerned about a large debris object hitting the ISS?" The answer, unfortunately is "Yes."

Although the level of concern may not be high, the possibility is real. First, there are a few debris objects that come from expired satellites and pieces of exploded spacecraft that pass through the station's altitude as their orbits decay.

Second, almost every time a geostationary satellite is launched the launch vehicle's upper stage is left in a highly elliptical transfer orbit whose perigee occurs at an altitude below that of the station. Thus, each of these discarded rocket bodies, while its orbit is slowly decaying, passes the station's altitude twice during each of its orbits.

As an orbit decays the associated orbital period ranges from about 10 hours initially to less than about two hours as atmospheric reentry is approached. At any given moment there may be as many as 15 to 20 or more of these derelict rocket bodies in orbits that cross the station's altitude. Although the probability of a collision is very small, such an event would be catastrophic.

Of course, NASA and the international station partners are aware of this situation. The station's movements are precisely and continuously tracked. Discarded rocket bodies are also tracked, but not as precisely. In fact, when it can be determined, in a timely manner, that one of these large objects may pass close to the ISS, avoidance maneuvers are executed.

There still, however, is some angst at the space agency. These old rocket bodies are in orbits that are rapidly changing in unpredictable ways due to atmospheric drag near each perigee passage.

This phenomenon, coupled with an inability to continuously track and predict exact orbital debris paths leads to increased uncertainty about orbital conjunctions with ISS. As a result, the station crew may sometimes characterize the ISS experience like some bomber pilots in World War II, "Hours and hours of pure boredom, followed by moments of stark terror!" Over the past several years there have been a number of such moments when an unexpected rocket body or large object passed close by the station.

One reported near miss with a large debris object occurred on March 24, 2012. The crew was forced to take emergency shelter in the two docked Soyuz capsules. NASA reported that the debris was spotted too late to maneuver the station out of the way. This object passed within 11 km of the station. In this case the debris was a product of a previous crash between Iridium 33 and Cosmos 2251 in 2009, an event that produced over 2,000 pieces of debris.

So, should we be concerned about the growing population of debris objects in orbit?

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