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Have there been any interplanetary secondary payloads? If there were none, are any such payloads planned?

I consider two options:

  1. launcher rocket's destination is Earth's orbit (LEO etc.), then secondary payload propulses (pushes) itself to interplanetary trajectory

  2. launcher rocket's destination is interplanetary, secondary payload also interplanetary (this is logical)

Edit: The definition of "secondary payload" vs. "multipart spacecraft" can be debatable. Though to set it sharp - payloads should have different investors/operators. An imaginary example: SpaceX launches big satellite for company #1 (primary payload) and company #2 rideshares an interplanetary spacecraft (secondary payload) on same rocket.

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    $\begingroup$ I suspect the answer is that there were no discrete secondary payloads. But clearly, things like Cassini-Huygens existed, and are a slightly more sensible and economical way to do interplanetary transits. $\endgroup$
    – Starfish Prime
    Mar 26, 2021 at 10:31
  • $\begingroup$ I suspect the Artemis 1 mission will carry some cubesats to the Moon which are considered to be secondary payload by NASA (primary being the Orion?). $\endgroup$
    – Kozuch
    Mar 26, 2021 at 10:41
  • $\begingroup$ You are allowed to answer your own question, of course (and I didn't know about those cubesats before you mentioned it, so it is a useful answer). What, if anything, distinguishes the Artemis 1 stuff from Cassini-Huygens? Is stuff like C-H an acceptable answer? $\endgroup$
    – Starfish Prime
    Mar 26, 2021 at 10:55
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    $\begingroup$ I can't understand what "Either payload going interplanetary from launch to Earth's orbit or interplanetary secondary payload complementing (interplanetary) main payload?" means at all! How does a payload go interplanetary from launch to Earth's orbit? I think the best answer will be the MarCO cubesats but I don't have time to write an answer. There are plenty of auxiliary payloads, orbiters that drop landers or atmospheric probes, there is Hiten and Hagoromo, and $\endgroup$
    – uhoh
    Mar 26, 2021 at 12:54
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    $\begingroup$ @uhoh Thanks for comments. Yes your mission examples are interesting and partially valid, but the question is actually where is the borderline between "secondary payload" and "multipart spacecraft". Commercially secondary payload usually has different investor/operator than main payload has so that would be two different "missions" but that is for LEO usually. For interplanetary things may not be so clear but I would still consider different spacecraft investors/operators a better case for an answer. Do we have a question on those "multipart spacecrafts"? $\endgroup$
    – Kozuch
    Mar 26, 2021 at 14:01

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Based on clarifying comments by the author of the question, the answer to the question is "no". There have not yet been any secondary interplanetary payloads that are not associated at all with the primary interplanetary payload. All of the secondary payloads mentioned in comments were instead very closely associated with the primary interplanetary payload.

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    $\begingroup$ And it makes sense because the upper stage typically performs the trans-planetary or trans-lunar injection burn. Operations will want the upper stage to perform most of that maneuver so that the payload doesn't have to do it, saving fuel for TCMs and planetary-insertion. $\endgroup$
    – ChrisR
    Mar 26, 2021 at 16:45
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    $\begingroup$ And this isn't surprising - it takes such a large rocket to go interplanetary it's in your best interest to just make the most of that big rocket than to share it with someone else! $\endgroup$
    – corsiKa
    Mar 26, 2021 at 19:57

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