The Mars-Earth return, by way of Venus

Discontented outer-planet visitors stranded up some other outer planet, wanting off the rock to go home, can count on a solar fly-by not too long after arrival. They can use Venus, which has an atmosphere fit for braking; someday its L2 might get a tether for acceleration, as well. Until then, trajectories tend from planet D to C by way of Вене́ра. Mars-to-Earth on "opposition", some thirty days after Earth-to-Mars, is the best-studied by far.

Robert Zubrin dismissed the ex-Martian 290ish-day slog as the "Fry-By". He figured you'd really have to hate Mars and/or be homesick for Earth, if you wanted to trade that (admittedly long) Martian time plus Hohmann, for a 290 day sun tan. The Mars Society disapprove too. But it still has defenders (pdf).

I said "about" and "ish" because it happens that Mars' own eccentricity makes some Mars / Sun / Earth opportunities faster and/or less energy-intensive than others. What you do from Mars is target Venus when that planet is in line for a Hohmann up to Earth. Robert L Sohn in "Venus Swingby Mode for Manned Mars Missions" (1964) looked at fourteen opportunities in the 29 years 1971-99 inclusive. The best windows recur every fifteen Earth years, roughly. Sohn flagged 1971; 1986 (in the chart) was even better than 1971.

Venus comes in handy for the bad years - like 1980 (and 1993). Venus shaves off Delta V - slows the vessel down - on said vessel's way back to Earth, so they don't have to slow down near Earth. Sometimes (1978, 1997) even Venus doesn't make the Fry By worth the bother, and the Earthbound on Mars might as well stick it out for Hohmann. Usually Venus makes all the difference, though.

For an idea on the timetable Zubrin had excavated another chart, with one such triple-planet alignment starting 15 January 2014. That craft would have got to Mars 27 August, and departed Mars 26 September. It then would have passed Venus 23 February 2015, to get to Earth 14 July.

For this blog's purpose, Mars is particularly rich in materiel to hand over to Venus. It does have the disadvantage - for locals - that speeds and angles will differ dramatically from one visit to the next.