Knowledge Zone · Card 11 of 13
The mid-course correction
Nobody aims a 259-day coast perfectly, and nobody has to. A mid-course correction is a small burn made in the middle of the cruise that moves the far end of the arc by a great deal, because there is a lot of time left for a small change of velocity to act over. The run grants two of them.
Their price is entirely a matter of when. On a Scout departing one 5 m/s slider step above the best burn, correcting on day 30 costs 7 m/s, on day 60 it costs 9, on day 130 it costs 26, and on day 200 it costs 80. By day 255 the same button asks for 889 m/s — every metre per second that ship has left at Mars — to undo an error one slider step wide, because there is no time left for a small burn to act over.
Waiting does buy something, which is why this is a judgement and not a rule: the very late corrections land closest, since by then the error they are cancelling has fully developed. And one burn along one line cannot cancel a miss that has two dimensions, which is the whole reason there are two corrections rather than one.