Interplanetary means happening between planets — a ship that leaves Earth and goes toward Mars, dust floating between worlds, a signal sent from one planet to another. Scientists and news writers use it; it is not a kitchen-table word. Though actually, people often say ‘space travel’ and stop there, even when they mean this narrower idea: not a trip to the stars, just from one planet to another in our own solar system. That smaller trip is already huge. Do not mix it with international either; the inter- is the same kind of ‘between’, but the second half is planets, not nations.

行星際的。口語常說「星際」，但「星際」比較像星星與星星之間；地球到火星這種，精確是行星際。