Space station astronauts have to be "high-risk-taking" individuals. They volunteer to fly from the Earth to the International Space Station (ISS) onboard a Russian launch vehicle that has a less-than-stellar safety record. Once on board the ISS they are exposed to high radiation levels, weightlessness and the hard vacuum of space. In fact, they are at the mercy of the elements for the duration of the flight without the possibility of a rescue in case of a severe emergency.

In some extreme situations, if there is time, they can use their reentry and return capsule to escape. There are some scenarios in which this works, but there are some events that happen so fast there isn't time to react.

Of major concern is a collision with a large debris object whose orbital path cannot be accurately predicted with current satellite tracking capabilities. There are several examples of large, expired rocket bodies and satellites whizzing by the station that are undetected until after the event or detected too late to maneuver the ISS.

For example, on July 16, 2015, an old Russian weather satellite made a close pass of the ISS. Warnings came too late for the station to execute an avoidance maneuver. NASA gave the crew just 90 minutes to shut down the station and get to safety, in the Soyuz spacecraft, sealing themselves off from the rest of the vehicle. This was the fourth such incident, to date, in the 17-year history of the ISS.

Four such incidents in 17 years does not sound alarming, but there have been many more near-miss incidents with small debris and large objects that were not tracked in time to warn the station. Here is the reality of the situation.

Several hundred undetected and dangerous objects pass through the station's orbital path each year. Every object in a decaying orbit crosses this path as they descent into the atmosphere and burn up. In fact, close examination of the ISS will reveal collisions with small debris articles.

While it is true that no large objects have collided with the station, such an event could happen at any time. One might say that the station and the crew have been lucky, because the statistical probability of this happening has been small. As more and more satellites are launched, this probability increases.

Even today, there are no less than 144 expired rocket bodies that, on average, cross the station's altitude every 5.3 hours. Relative closing speeds can be at least 6,000 MPH. These rocket bodies, the size of small school buses, are the remains of geostationary-transfer-orbit (GTO) stages that were used to boost large satellites toward their final orbits, roughly 22,400 miles above Earth.

The exact paths of these expired stages are very difficult to predict because their orbits continually change in unpredictable ways. As they pass through perigee, about every 10 hours, upper atmospheric passage tends to slow them down. This decay of energy results in significant and unpredictable trajectory changes.

In conclusion, dealing with space debris is a big problem for the ISS. Yes, it can withstand small debris impacts. Given several hours to maneuver, the station can avoid larger objects. However, a vehicle roughly the size of a football field cannot respond rapidly or gracefully.

Although the ISS is scheduled to retire within 10 years, there will be other space stations that will be exposed to the same situation. The good news is that this threat can be reduced, but it will require improved and expanded satellite tracking capabilities.

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