A dwarf planet coming within 11 AU of the sun over the next 10 years

Like, imagine a collapsible rod about a kilometer long sticking off the end of a space probe, lined up so it hits the surface as close to perpendicular as possible, each segment made of appropriate material for its impact speed. (I think once you go past the speed of sound in a material, you can't transfer any more force)

With the far end of the rod, which impacts first and with the most force probably vaporizing/creating a crater on the surface (useful to align the rest of the rod), and later sections crumpling in on themselves predictably, like a highway crash barrier or car hood. With a certain max amount of Acceleration, Jerk, Snap, etc... that the probe can survive.

I would very much like someone to explain why decelerating a spacecraft like this is infeasible/inefficient so I can stop thinking about it. Failing that, I wish to devote the next few years of my life to jamming a massive spear into the moon.

First off, mass. Mass is everything in spaceflight. A rod like that would weigh thousands of kg at the very minimum, likely much more than the rest of the spacecraft combined. Spending the same mass budget for propellant and a big rocket engine would be much more efficient, never mind being useful for arbitrary velocity changes rather than just deceleration.

Second, shape and volume. How would you even launch a km-long rod to space? Not going to fit onto any launch vehicle ever devised. Besides, even at 1g it would collapse under its own weight. Making it telescoping would just increase total volume besides adding complexity – and mass, did I mention mass? Never mind that a collapsible rod is going to have a vastly lower compressive strength than a solid one, making it nigh useless for the intended purpose.

Third, moment of inertia. A long, massive rod stuck to your spacecraft is going to make orientation changes really difficult. And orientation changes are pretty important in spaceflight due to heat management, course corrections, and, well, being in the exact right orientation for your braking maneuver.

Fourth, the concept of a hypervelocity rod falling from space reminds me of something… yeah, kinetic bombardment, aka "rods from God" [1]. The rod and whatever it's going to hit are not going to behave like solid objects crumpling like a crashing car. Stuff at the point of impact is just going to instantly vaporize and result in an explosion likely in the kiloton range, a fried spacecraft, and a big crater on the surface.

Fifth, even if you first decelerate to more reasonable speeds by other means (which is going to take a lot of fuel because of the extra mass (see, again the m word)), a rod much longer than its diameter is not going to nicely crumple into itself under compression. It will buckle, and then snap, like a piece of spaghetti, failing to decelerate much at all but sending your spacecraft tumbling out of control.

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All that said, there are instances where crumple zones have had a small role in spaceflight, including the the Apollo Lunar Module which included crushable honeycomb shock absorbers in the landing gear struts.

[1] https://en.wikipedia.org/wiki/Kinetic_bombardment