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I just came across this video about manned Mars and Venus missions that were planned in the 1960s. They considered a Venus flyby the best way to get humans to Mars. Indeed, there are also modern papers arguing for a Venus gravity assist to get to Mars, citing shorter total mission times (~700 instead of 850 days), more frequent travel windows (every 19 months vs. 26 months), less total radiation and less required propellant.

Why then are the planned NASA and SpaceX Mars missions always presented as a direct flight to Mars, while Venus gravity assists are almost never mentioned?

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The Venus flyby does indeed make the mission shorter, but it has some pretty serious negative consequences as well.

  1. The mission spends much more time in deep space.
  2. Approaching the sun will increase the amount of radiation exposure by a large amount.
  3. The thermal design would need to be rethought to survive that close to the Sun.
  4. The time on Mars for a Venus flyby mission is about 30 days. Compare that to 1.5 years for the standard trajectory.
  5. The radiation exposure as a whole is higher for the Venus flyby mission.
  6. On the surface of Mars, there are things that could be done to fix a spacecraft. There are fewer options for one in deep space.
  7. Returning via Venus actually requires more delta-v.

I'm sure there are others. The bottom line is, while the mission is longer by a year, it is generally considered safer, and provides much more valuable time on the surface of Mars.

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    $\begingroup$ Thanks for this nice answer. This paper states: "[...] even the longer EVME [Earth-Venus-Mars-Earth] opposition Mars missions of ~700 days would result in less total exposure than the shortest EME [Earth-Mars-Earth] conjunction missions of 850 days". So unless I'm reading that wrong the Venus gravity assist has less radiation exposure, because it is shorter. But of course if we consider radiation exposure per day spent on Mars, then the direct mission to Mars is better. $\endgroup$
    – KarlKastor
    Commented Apr 28, 2021 at 20:55
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    $\begingroup$ I don't know how they can seriously make that claim. I can only guess they're ignoring the effect of shielding by the ground and atmosphere at Mars...they mix flyby and landing missions and aren't very clear about what figures are for what missions. Some of their other "advantages" are also remarkably silly...landing ellipse error reduction for Venus landers by manual remote piloting by humans? $\endgroup$
    – Christopher James Huff
    Commented Apr 28, 2021 at 22:35
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    $\begingroup$ For a quiet Sun with no CME events, does being near the Sun actually expose astronauts to more radiation through a spacecraft's skin? It takes a 50 MeV proton to go through 3 mm of aluminum for example 1, 2 Or is the radiation concern only related to unexpected CMEs? $\endgroup$
    – uhoh
    Commented Apr 28, 2021 at 23:23
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    $\begingroup$ My understanding is the main "background" solar radiation hazard is the x-rays produced when those protons (and the electrons, alpha, etc) hit the hull, not the protons themselves. $\endgroup$
    – Christopher James Huff
    Commented Apr 29, 2021 at 0:55
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    $\begingroup$ Maybe if we ever set up a long term Martian occupation, the Venus flybys can be used to send unmanned supply drops, since these could be one-way and don't need to worry about life-support and radiation exposure. $\endgroup$
    – Darrel Hoffman
    Commented Apr 29, 2021 at 14:42

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