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Why does the InSight Mars lander mission intend to launch from Vandenberg, which does not allow for prograde launches? Will it launch into polar or retrograde orbit? Why? It will do an interplanetary transfer, so would the best orbit not be around the ecliptic, so about 23.5º and prograde, because it saves fuel?

I googled around for a bit and the only thing I found was, that it is the first interplanetary launch from Vandenberg, which totally makes sense to me, because why would you launch those from there.

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  • $\begingroup$ This is a good question. I note that it's a very lightweight spacecraft, so even from Vandenberg it can launch on an Atlas V 401 (the minimum, no-booster Atlas configuration), but that doesn't explain why Vandenberg is preferable in this case. $\endgroup$ – Russell Borogove Mar 4 '18 at 16:22
  • $\begingroup$ They could have put it on something even lighter (Antares maybe?) or put some additional payload like earth observation satellites or a boatload of cubesats on it to deploy in LEO, if they had launched east. $\endgroup$ – Hans Mar 4 '18 at 16:40
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    $\begingroup$ Antares doesn't have a restartable upper stage, let alone a hydrogen-fueled one. The only other rocket we've used to send anything to Mars in this century is Delta II, and ULA has stopped production of those. $\endgroup$ – Russell Borogove Mar 4 '18 at 16:42
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According to the article "Seven Ways Mars InSight is Different", the driver was launch site availability:

InSight will ride on top of a powerful Atlas V 401 rocket, which allows for a planetary trajectory to Mars from either coast. Vandenberg was ultimately chosen because it had more availability during InSight’s launch period.

InSight is a very lightweight spacecraft (at 360kg, it's about one-tenth the mass of Mars Science Laboratory), so the Atlas has enough performance margin for the mission, even in its minimal 401 (no-boosters) configuration, launching from Vandenberg without the benefit of Earth's eastward rotation.

The "Vandenberg penalty" isn't really that severe, though. Launching into the 158º azimuth (south-south-east-ish) limit from the West Coast loses about 200 m/s of eastward rotation speed compared to Cape Canaveral. That's about 2% of the total delta-v budget for ascent to LEO -- not negligible, certainly, but not insurmountable for small payloads.

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The eastward launch is somewhat of a misnomer. As Russell mentioned in his answer, the amount saved is only a few percent. The real difference, however, comes for missions going to GTO to launch closer to the equator. The cost of an inclination maneuver is very significant. You can see more about this in the following video:

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  • $\begingroup$ As @MagicOctopusUrn points out, this affects both launch sites pretty strongly; between Vandenberg's 34 N and KSC's 28 N there's not that much improvement. $\endgroup$ – Nathan Tuggy Apr 17 '18 at 6:23

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