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LOL I added it at the end as a quick fact, sorry. It's the duration of propulsion from fuel. How long it lasts. If it lasts longer, you can get further away (and get back). At least that's how I understood it, but I could be mistaken.

I C. It sounded quite odd in the context of the deep science article without inclusion of what the time pertained to. Thx for the clarification.