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For example, a mission to Mars for which some bit of hardware ended up entering Venus's atmosphere.

Has this ever happened, or at least are there projections that it may happen?

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    $\begingroup$ I adjusted the wording a bit for clarity, feel free to roll back or edit further. I think there are some theoretical simulations for Starman/Roadster that go way way into the future and cite some nonzero probabilities for various planetary "interceptions" that are mentioned in older posts here. $\endgroup$
    – uhoh
    Mar 19, 2023 at 13:51
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    $\begingroup$ Nonearth @SE-stopfiringthegoodguys $\endgroup$ Mar 19, 2023 at 15:54

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Yes

https://scitechdaily.com/space-junk-just-crashed-into-the-far-side-of-the-moon-at-5800-mph/

A piece of space junk (China or spaceX) hit the far side of the moon

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I do not think it has happened. (Note - when I wrote my answer, "Or large moon" was not part of the question. Space junk hitting the moon is not so surprising, although it is interesting - see the other answers.) The planets are so small relative to how far apart they are that the chances of hitting one without intentionally getting close to it are negligible. So a mission to Mars is just not going to randomly collide with Venus.

However, gravity assist maneuvers (see the linked article for more information) do involve intentionally flying close to a planet other than the targeted one, so in principle a collision could happen in one of those cases. But even then it would be unlikely and seems not to have happened. As far as I can tell, three planets have been used for gravity assist purposes in a way that is relevant: Venus (at least four times), Mars (once), and Jupiter (at least twice). But all of those missions were successful, and even in those cases it would have required great precision to actually hit the planet being used. Even if some part were jettisoned right before closest approach, it would be moving at a similar speed to the probe and would have had to slow down greatly to enter an orbit around the planet, or be aimed carefully to collide with the planet.

As an aside, Venus and Jupiter have been used several more times for gravity assist, but those other times they were also one of the main mission targets. And, as Eugene Styer pointed out in the comments, the earth has been used for a gravitational assist, but a collision with the earth would not count.

Finally, a number of different upper stages and probes have entered solar orbits. Space is very big, but the remaining lifetime of the solar system is pretty long. So one of them could eventually get perturbed into an orbit that meets a planet. And apparently that's actually quite likely over the long term, based on other answers, and completely contrary to what I initially claimed!

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  • $\begingroup$ Any sources @Mark Foskey $\endgroup$ Mar 19, 2023 at 17:00
  • $\begingroup$ Earth has also been used for gravity assists $\endgroup$ Mar 19, 2023 at 18:15
  • $\begingroup$ Upper stages in solar orbit usually are injected into their orbits near Earth. A Keplerian orbit repeatedly returns to the injection point. The orbits in question are either nearly Keplerian or have strong gravitational interactions with Earth, even if not bound. The result is that upper stages tend to repeatedly return to the neighborhood of Earth, and may collide with it (or the Moon) if the perturbations are right. $\endgroup$
    – John Doty
    Mar 19, 2023 at 22:17
  • $\begingroup$ You appear to be wrong, see Catprog's answer @Mark Foskey $\endgroup$ Mar 19, 2023 at 23:23
  • $\begingroup$ I guess for the slingshotting cases, hitting the planet with great speed would need no additional slowdown, just a slight deviation some months earlier. $\endgroup$ Mar 19, 2023 at 23:49
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The SpaceX Falcon 9 upper stage that launched NOAA's Deep Space Climate Observatory to a Sun-Earth LaGrange point in February 2015 did not have enough propellant remaining to return to burn up in the atmosphere or to escape the Earth Moon system. It followed a somewhat chaotic orbit before hitting the Moon in March 2022 at around 5,700mph

https://arstechnica.com/science/2022/01/an-old-falcon-9-rocket-may-strike-the-moon-within-weeks/

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  • $\begingroup$ Is this the same thing in Catprog's answer @Slarty $\endgroup$ Mar 20, 2023 at 12:00
  • $\begingroup$ Yes sorry I missed that $\endgroup$
    – Slarty
    Mar 20, 2023 at 12:19
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or at least are there projections that it may happen?

Yes there is!

SpaceX's Elon Musk's goal of getting people to Mars is no secret, and the demonstration of Falcon Heavy put his old car on a trajectory aimed very roughly at the orbit of Mars. Not really targeting an intercept of the planet itself, but to show that it could get to Mars' neighborhood.

While there's no way to calculate exactly so far in the future, at least until it passes Earth again so a much better trajectory can be determined

...it has been predicted that sooner or later the mission's payload Starman/Roadster will intercept a planet.

Quoting @MarkAdler's answer to Starman trajectory in future:

No, it's not a risk. Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is.

As for the future trajectory, there's already a great preprint paper where they propagated it out several million years with uncertainties to see what can happen: The random walk of cars and their collision probabilities with planets.

By running a large ensemble of simulations with slightly perturbed initial conditions, we estimate the probability of a collision with Earth and Venus over the next one million years to be 6% and 2.5%, respectively. We estimate the dynamical lifetime of the Tesla to be a few tens of millions of years. (emphasis added)

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  • $\begingroup$ I meant that it has a pretty high probability of doing so, not 6% @uhoh $\endgroup$ Mar 19, 2023 at 22:45
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    $\begingroup$ @Starshipisgoforlaunch I wish I could find something closer; I'll keep looking for / thinking about it. Right now this is the only candidate I know of, and mentioning it here might jog someone else's memory of something better. I think the underlying principle is that sooner or later on an astronomical timescale, it's going to happen to a lot of objects in heliocentric orbit that cross planetary orbits. Also, there's the uhoh's lemma #3 aspect; we write answers not only for the benefit of the question author, but for future readers as well. $\endgroup$
    – uhoh
    Mar 19, 2023 at 23:54
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    $\begingroup$ @Starshipisgoforlaunch high is relative. $\endgroup$
    – RonJohn
    Mar 20, 2023 at 6:47

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