This is a risk mainly for manned spacecraft in low Earth orbit (LEO), where the amount of space junk is sufficient to cause concern that a damaging collision may occur. Manned spacecraft that operate long-term in the LEO environment, such as space stations, can be equipped with debris bumper shields (see discussion of Whipple shields in Chapter 6) to protect them from debris impact. For example, a considerable amount of design effort and mass has been invested in the ISS to provide debris shielding. Most of this shielding is deployed on the forward-facing surfaces of the station, as most damaging impacts are likely to be caused by debris coming from the forward flight direction. On the other hand, the Space Shuttle is an example of a manned vehicle that cannot be equipped with such shielding, due to the fact that it has to fly both in the environment of space and in the Earth's atmosphere. The use of Whipple-type shielding would compromise its ability to fly through atmospheric reentry and landing. Over recent years, there has been a growing appreciation of the threat from debris impact to the Shuttle orbiter and its crew. In an attempt to at least minimize the threat to the crew, the vehicle adopts a particular attitude in orbit—upside-down and with the main engines facing forward. In this way the crew is protected from impact with the potentially most damaging debris coming from the forward direction.
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