The astronauts who desire to work on the Space Station have to be high risk-taking individuals. They intend to fly to the International Space Station from Earth, riding on a Russian vehicle that has less-than-stellar safety records. Once these astronauts reach the Space Station, they must keep in mind that they would be exposed to high radiation levels, the giant vacuum of the space and also the weightlessness. In fact, the astronauts would have nothing to do in case of an emergency but to wait for the rescue missions to come and help them.
In certain situations, if the astronauts are lucky enough, then they can plan for their re-entry and return their capsule for escaping. Otherwise, specific scenarios are so intense that it would not give much of a time for the astronauts to even think of what to be done. There can be situations where large debris comes within the path of the ISS and hit it poorly. Often it becomes tough to identify the orbits of such debris. There is the presence of several such expired rockets bodies or even damaged satellites that are moving t a tremendous speed around the ISS. By the time they are detected, it might become too late for the ISS to maneuver.
For instance, ISS escaped a near accident on July 16, 2015. An Old Russian satellite passed closely of the ISS. The Space Station received the warning signal too late to react. In fact, NASA gave the crew only 90 minutes to shut down the station and get to a safety zone, aboard the Soyuz aircraft, get them detached from the rest of the station. This was the fourth occasion till the date of a 17-year-old ISS.
You will feel that four incidents in 17 years do not sound very alarming. But there had been such grave incidents whereby the ISS got saved from near-miss from several small debris as well as astronomical objects whose track could not be detected until the last moment. There are numerous deadly objects which move within the orbital path of the ISS. Every such object moving along the decaying path burns up while entering the atmosphere. In fact, if we analyze deeply, we will see that collision of ISS with small debris have taken place in reality.
The correct path of these expired stages is challenging to predict their orbital paths changes on a continuous basis.