Let's assume we use the ideal positions of those planets relative to Earth for launch. And let's assume the spacecraft is launched from the same place on Earth. Also let's assume the goal is to get the same mass of payload to those planets, meaning the launch vehicle could be different, depending on the energy requirements.

"Getting to" could possibly be a bit imprecise, so let's define it as meaning directly impacting the surface with the spacecraft.

Alternatively, does anything change if "getting to" means getting into orbit around those planets?

  • $\begingroup$ Getting there... slowing down not included... god speed ye brave souls $\endgroup$ Mar 29 '19 at 10:20

To flyby or impact Venus varies from 3.45 to 3.6 km/s from LEO for the optimal time every 19 months. Mars varies from 3.55 to 3.9 km/s for the optimal time every 26 months. So on average, getting to Venus is a little less energy than getting Mars. But not by much. It could even be a tiny bit more in some years.

If you also want to get barely into orbit propulsively, the ranges are 4 to 4.7 km/s for Venus and 4.25 to 7 km/s for Mars.

Mars is more variable than Venus due to its much larger solar orbit eccentricity (0.09 vs. 0.007).

At either planet, you can aerobrake down to the desired orbit. Aerobraking has been demonstrated at both. Or you can aerocapture directly, with just the flyby costs above. Aerocapture has never been demonstrated, but there are no hurdles that would prevent its use in a mission, other than developing an adequate heatshield for Venus (much higher entry velocity). However you incur the substantial mass penalty of the aeroshell, a cruise stage that is discarded before entry, and the structure and mechanisms to discard the aeroshell and deploy the enclosed spacecraft. Aerocapture at neither body appears to be a win if you can afford the time to aerobrake, measured in months. (Aerocapture is mission enabling at Uranus, Neptune, and Titan.)

  • 1
    $\begingroup$ "Aerocapture at neither body appears to be a win if you can afford the time to aerobrake, measured in months." Did you mean either? This sentence seems contradictory to me as written. $\endgroup$ Mar 28 '19 at 18:32
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    $\begingroup$ The sentence is perfectly grammatical. Neither body is worth aerocapture if you can afford the time to aerobrake instead. @TemporalWolf $\endgroup$
    – Nij
    Mar 28 '19 at 18:40
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    $\begingroup$ @Nij It seems to preclude doing both, which seems strange as I would expect a mission that aerocaptures to aerobrake as well. $\endgroup$ Mar 28 '19 at 19:15
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    $\begingroup$ It's inconsistent with NASA publications concerning the feasibility of single pass aerocapture around Venus, which (on page 8 table 4) shows almost double the delivered mass for an aerocapture system to Venus versus a propulsive capture + aerobraking. Venus capture dV is around 1/10th the dV necessary for a LVO aerocapture, which would require even less of a aeroshield. While it's likely to be worth just going straight for the orbit you want, you could save substantial additional mass by combining the two. $\endgroup$ Mar 28 '19 at 21:39
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    $\begingroup$ @TemporalWolf (& Mark Adler, too) Unless you're supremely confident of your aerocapture system's ability to have the exit velocity right where you want it, you probably wouldn't try to aerocapture directly into LVO, since there'd be little margin between the required ∆V and the ∆V that would result in complete entry. 24-hr, 12-hr, even 6-hr orbits would be fine. But for LVO I think you'd aerocapture into a looser orbit, then aerobrake down to LVO. $\endgroup$ Mar 29 '19 at 2:51

The second table here essentially answers your question. Venus transfer from Low Earth Orbit is 3.5 km/s, Mars transfer is 3.6. This will allow you to impact either body (on Venus you will need to make sure your vehicle is tough enough to actually impact, rather than dissolving in the atmosphere, but that's not really the point).

In either case, you can enter orbit for negligible extra energy, but some risk, by aerocapture. Basically you graze the upper atmosphere, losing just enough velocity relative to the planet to enter a long elliptical orbit. At the highest point of that orbit you make a very small boost to raise the lowest point of the orbit to graze the atmosphere even more gently, and then repeated encounters will lower the high point of the orbit. When it's where you want it, you make a further small correction to miss the atmosphere entirely and you are there.

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    $\begingroup$ Do you perhaps know if any actual spacecraft sent to Venus used aerocapture to get into orbit? Edit: I see Wikipedia saying that no. $\endgroup$
    – stackzebra
    Mar 28 '19 at 12:47
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    $\begingroup$ @stackzebra you have a point. On the other hand not that many probes have been sent to orbit Venus at all. Magellan used aerobraking to adjust its orbit. $\endgroup$ Mar 28 '19 at 12:53
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    $\begingroup$ @stackzebra: Not to get into orbit. Dual-stage atmospheric braking (dive-emerge-dive-land) has been used by "Venus" program landers. And, contrary to the above statement by Steve Linton, there were quite a few orbiters that orbited Venus as part of that program. These orbiters did not use aerocapture though. $\endgroup$
    – AnT
    Mar 28 '19 at 16:17
  • $\begingroup$ @AnT: Depends on what you understand by "not that many". I count 8: en.wikipedia.org/wiki/List_of_missions_to_Venus vs 14 for Mars: en.wikipedia.org/wiki/List_of_Mars_orbiters $\endgroup$
    – jamesqf
    Mar 28 '19 at 16:40

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