A space probe leaves the solar system to explore interstellar space. Once it is far from any

Question:

A space probe leaves the solar system to explore interstellar space. Once it is far from any stars, when must it fire its rocket engines?
(a) All the time, in order to keep moving.
(b) Only when it wants to speed up.
(c) When it wants to speed up or slow down.
(d) Only when it wants to turn.

(e) When it wants to speed up, slow down, or turn.

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