Is that really true? If we manage to get a spacecraft get captured by the dwarf planet's gravity and orbit it, would that not be a lot less delta-V compared to if we made the spacecraft achieve the dwarf planet's orbit around the sun just by itself?
Now, something like this can work if you use an irreversible interaction like aerobreaking, but this dwarf planet has negligible atmosphere. You could also use the dwarf planet for a gravitational assist (basically bouncing off it like a billiard ball), but I think gravitational assists from the other planets are almost always more convenient and effective.
Unless you're NASA landing a probe on Mars in 1997.
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.
~ ~ ~
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.
No, it doesn't, because natural satellites are generally not captured, and for those that are captured, the process involves interactions with other bodies.
"Most satellites of the outer solar system didn’t form with their host planets"
https://astronomy.com/news/2016/12/captured-moons-of-the-gia...
Even Triton, which is the size of a planet and in an almost circular orbit, is thought to be captured, the last I heard.
From what I understand any eccentric orbits would either flatten out or crash into Jupiter.
Is that so?
"The generic definition of a centaur is a small body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets"
https://en.wikipedia.org/wiki/Centaur_(small_Solar_System_bo...
There are tens of thousands, so perhaps the definition of a planet is even more abstruse than people let on.
And apparently at least dozens have been identified as probably of interstellar origin, while it is thought that a centaur can become a moon, (e.g. Phoebe) so I wonder if we can really rule out that moons "come hurtling out of the cosmos":
"Being able to tell apart interstellar asteroids from native asteroids born in the Solar System has long eluded astronomers, but the team’s results identified 19 asteroids of interstellar origin. These are currently orbiting as part of the group of asteroids known as Centaurs, which roam the space in between the giant planets of the Solar System."
https://ras.ac.uk/news-and-press/research-highlights/interst...
Yes, it is. The definition of "clearing an orbit" isn't precisely defined, but it doesn't have to be since there appears to be a large natural gap in how much orbit clearing an planet does vs. a dward planet.
> A large body that meets the other criteria for a planet but has not cleared its neighbourhood is classified as a dwarf planet. That includes Pluto, whose orbit intersects with Neptune's orbit and shares its orbital neighbourhood with many Kuiper belt objects. The IAU's definition does not attach specific numbers or equations to this term, but all IAU-recognised planets have cleared their neighbourhoods to a much greater extent (by orders of magnitude) than any dwarf planet or candidate for dwarf planet.[0]
[0] https://en.m.wikipedia.org/wiki/Clearing_the_neighbourhood
Oberth effect from fast flyby of a body with low gravity would be negligible.
Pretty sure the problem would be, rather, that a flyby of a body with low gravity would be negligibly fast (relative to your speed when not flying by). Oberth effect is because of high speed (a given increase in momentum gives more kinetic energy at higher speed than at lower speed) - it's just that dipping deep into a gravity well is the obvious way to get that speed.
Note to the audience: these mechanisms don’t violate the conservation of energy because you aren’t tapping the object’s gravitational energy per se but instead its orbital energy around the sun. Put another way, you can’t do a gravity assist or capture burn in any direction.
(Usually textbooks use a baseball bouncing off a semi truck to illustrate.)
Technically, this isn't completely true. There are gravity assist techniques that will allow you to dump speed by essentially adding your momentum to the object you are trying to orbit. The is basically an anti-slingshot manuever.
In practice, I believe the range of scenarios when this is possible with a dwarf planet is so small as to be practically useless.
You can lose speed or alter course relative to another body in a single encounter, and those changes can reduce speed in future encounters, but if you’re on an escape trajectory heading in you stay in one (without forces beside two-body gravity, which is a pretty safe assumption 11 AU out of Saturn doesn’t come close).
Two-body systems do not exist in reality.
Energy is also conserved in 3 body problems. When you utilize the slingshot effect, some of the energy of the orbit of the body you are swinging around orbiting is transfered to you. The transfer of this energy does not depend on the closeness of the sun, but rather on how deeply you descend into the gravity well of the object you are slingshotting around.
> which is a pretty safe assumption 11 AU out of Saturn doesn’t come close
No, it really isn't. The "safeness" of the assumption entirely depends on your margin for error. The existence of the naturally captured saturnian satellites clearly indicates that you are simply wrong about the relevant margins for error.
I don't have a strong background in physics, and perhaps this is splitting hairs, but is this true if we consider gravitational radiation? Over a very long time a body's orbital energy will be lost to gravitational waves.
For practical purposes, that'll never happen, but for practical purposes gravitation radiation doesn't matter anyway.