[jessriedel]

Yes, this is an aspect of orbital mechanics that people find unintuitive before they study it. You can't be captured by a planet's gravity alone. If you come in from infinity (i.e., not already captured) you will escape to infinity (remain not captured). The basic idea can be seen from the fact that gravitational dynamics are time-reversible, so if gravity could capture you like this you could also start in orbit around a planet and spontaneously be ejected.

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.

[azernik]

It can reduce the delta-V requirements, though - by the same principles as a gravity assist, a capture burn (especially into a loosely-bound planet-centric orbit) often takes less work than burning into the equivalent heliocentric orbit on your own.

[JumpCrisscross]

> by the same principles as a gravity assist, a capture burn

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.

[azernik]

The usual way I explain it is as a transfer of kinetic energy and momentum from the large body to the small one. The interaction is through gravity, rather than the mix of electrostatic, degeneracy, and strong/weak forces involved in collisions; but the equations are more or less the same.

(Usually textbooks use a baseball bouncing off a semi truck to illustrate.)

[beckingz]

Right! The Oberth effect for a smaller body is not as large as it would be for earth, but would still be non-zero.

[lmilcin]

"Non-zero" is the key word here.

Oberth effect from fast flyby of a body with low gravity would be negligible.

[a1369209993]

> 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.

[jessriedel]

Ok, good point. Thanks.

[azernik]

Still definitely not worth it for something so small, though!

[tmhrtly]

What if we crash landed into it? That would probably tick the irreversible box. Clearly engineering difficulties quite high though!

[Sharlin]

This is often jocularly called lithobraking and is typically not advisable.

[mianas]

I came here to reply to the parent comment's remark about "irreversible interaction like aerobreaking, but this dwarf planet has negligible atmosphere." by mentioning lithobraking because it's been consuming my thoughts for the past couple of months.

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.

[Sharlin]

So, uh, let's see…

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.

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

[fouronnes3]

> not advisable

Unless you're NASA landing a probe on Mars in 1997.

[Sharlin]

Seriously, the tandem failures of the Mars Polar Lander and Mars Climate Observer missions were probably something NASA as an organization needed at the time. A reminder that Space Is Hard, and you can only pick two of "faster", "better", and "cheaper". Since then, NASA's Mars program has grown in both scope and ambition, yet remarkably has had zero loss-of-mission failures during that whole time!

[fouronnes3]

I was referring to the successful landing of Pathfinder but yeah... Space is Hard and NASA is good at it are both very true.

[phkahler]

After Pathfinder someone from NASA wrote a book about their new "faster, better, cheaper" approach to missions. Usually you can't have all 3 but they managed to get lucky. That made it particularly amusing when the book and concept were getting popular as the next 2 missions were failing.

[garmaine]

Most of the velocity of Pathfinder was shed using aerobraking and parachutes. The crash-balloon landing system just shed the last tiny sliver of velocity after cutting the chutes.

[Sharlin]

Ah, yeah, you could call airbags lithobraking. I thought you referred ironically to the Mars Polar Lander, but it of course flew in the 1999 launch window rather than 1997.

[Cthulhu_]

Didn't they launch a projectile into an asteroid not long ago as well?

[ehsanu1]

It's a cool idea, but seems super sci-fi. Might need some wonder materials to make it feasible, even then that would be a really big crumple zone. Or flubber. Another crazy idea: latch on to the planet from the side, like a skateboarder hitching a ride by hanging on to a truck. Again, wonder materials required.

[azernik]

NASA and the Soviets didn't need any wonder materials, just airbags. (Though they did a lot of braking first - either with the atmosphere for NASA at Mars, or with thrusters for the Soviets at the Moon.)

[gifnamething]

That crash still needs to absorb the same amount of speed that was just defined as too much to expel in orbit

[shkkmo]

> You can't be captured by a planet's gravity alone

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.

[Armisael16]

In a two-body system it doesn’t matter what you do; gravity is a conservative force so conservation of energy demand that you leave the body’s SoI at the same speed you entered it (in the body’s frame of reference).

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).

[shkkmo]

> In a two-body system it doesn’t matter what you do;

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.

[lxmorj]

Saturn's captured satellites might also be the result of incidental aerobraking or whatever you want to call smashing into a bunch of very tiny satellites during a close periapsis, no?

[shkkmo]

You need some sort of subsequent acceleration to raise the perigree out of the atmosphere so the orbit doesn't continue to decay. This could happen due to a slingshot effect, but atmospheric braking alone is not enough to allow you to establish a stable orbit.

[prince781]

> gravitational dynamics are time-reversible, so if gravity could capture you like this you could also start in orbit around a planet and spontaneously be ejected.

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.

[lmilcin]

It would be like saying all the school children exercises and train timetables are invalid because they don't take into account relativistic effects that obviously are still present at 60mph.

[a1369209993]

Technically? Yes! Incoming gravitational radiation of precisely the correct shape will in fact un-decay a orbit under exactly the same (modulo appropiate symmetries) circumstances as a orbit would decay by emitting (the reverse of) that radiation. (The same applies to thermal radiation cooling things off - see Liouville's Theorem.)

For practical purposes, that'll never happen, but for practical purposes gravitation radiation doesn't matter anyway.

[himinlomax]

Over a very long time, we are all very dead. From what I understand, the loss in gravitational energy would be so tiny, the length of time required for it to eventually matter in any way would be way beyond the lifespan of the sun. So it's only a finite duration if you have infinite time, which you don't.

[CamperBob2]

That's pretty cool. It's wildly counterintuitive, but if it weren't the case, the planets would be orbited by lots of captured asteroids and debris, instead of/in addition to being covered in craters. The only explanation for why that doesn't happen is that it can't happen.

[shkkmo]

The presence of natural satellites indicates this can indeed happen. It just requires a pretty unlikely orbital configuration.

[stickfigure]

Or an impact:

https://en.wikipedia.org/wiki/Giant-impact_hypothesis

[dragonwriter]

> The presence of natural satellites indicates this can indeed happen

No, it doesn't, because natural satellites are generally not captured, and for those that are captured, the process involves interactions with other bodies.

[perl4ever]

The first hit when I search "planets capturing moons" says natural satellites generally are captured.

"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.

[CamperBob2]

One of the criteria for planethood is an assumption that the body clears its own orbit. Moons don't just come hurtling out of the cosmos; they either result from a collision of some other body with the planet, as with our Moon, or they're already close to the planet's orbit at the time they are captured.

[perl4ever]

>One of the criteria for planethood is an assumption that the body clears its own orbit.

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...

[hinkley]

Jupiter’s moons are I’m harmonic orbits because they trade momentum until they balance out.

From what I understand any eccentric orbits would either flatten out or crash into Jupiter.

[CamperBob2]

No, it doesn't. No (rocky) planet has more satellites than craters, but every planet's gravitational field is larger than its surface.

[8note]

Do we know how many of those craters used to be satellites?