Most spacecraft are still at little risk of collision with space debris during their operational lifetimes, but given the numbers of new satellites launched each year, the orbital environment in the future is likely to be less (i) __________
We are told that at present there is little risk of collision of spacecraft with space debris. We then have the thought contrastor "but", so we must expect the opposite scenario in the second part of the sentence. Thus we can predict that the orbital environment in the future is likely to be dangerous or less safe.
We are looking for a synonym of safe. The word
benign goes well as a descriptor of the environment and a synonym of safe.
The word
protected looks good too but we are moving from a less risky scenario to a more risky scenario. It is not that the environment was well-protected in the past and now its not so, rather it was safe in the past and not so safe now.
It is also a question of usage, and not just grammar; The environment is better described as benign rather than as protected, in this particular context.
_________________