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Comment by TeMPOraL | Hacker News Reader

KSP gave me the exact same impression, but there's one thing I wonder about: with sufficiently smart global optimization algorithm, maybe a collector craft could approach and collect several targets on a low Δv budget? It's not something you could do manually, but space has also this nice property that you can predict trajectories accurately far in advance, so it may be amendable to optimization.

Wonder if there's a paper somewhere that computed possible Δv requirements for a collector if you squeeze your flight plan very tight.

(Also in-orbit refueling will change this equation a lot. You could keep a bunch of such collectors continuously in space, and wait for the trash to align right. There's no hurry.)

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Another thing some KSP mods gave an impression to me was how seemingly magical ideal encounters and transfer windows they could make.

Even though I couldn't make it myself by the usual orbit tools I also wouldn't be too surprised if it was possible to gather bunch of stuff with like < 100m/s delta v or whatnot.

KSP taught me that flying a rocket is "easy". But calculating the stuff is hard (well, computationally intensive. So it can be easy, but hard for humans in their head).

Yeah. I think KSP stock makes building rockets seem way too easy and docking way too hard.

I thought so too at first, but after a couple of (dozen) hours, I came to appreciate it. The rocket building leaves for incredible variety, it means as a beginner you can strap a few engines to a cockpit and get to the Mun, without having an understanding of why, while at the same time needing to actually plan and calculate delta for longer missions. Similarly the docking is fiddly and hard at first, but once you've done it a few times, it's a (mostly) rote exercise.

Agreed. It's good to learn and do it a few times, because it teaches you a lot about how objects in orbit behave in relation to each other.

Docking itself has always been my favorite part, though I "cheat" by using a mod[0] that gives me a display[1] of relative velocities and orientations. My brain isn't designed to eyeball relative velocities from an outside-ship camera. But that's just the final part, applicable when you can literally see the target out of your window.

Approach, now that's another story. Took me a while to learn how to match orbits without carefully planning a series of maneuvers. My new way is slightly less Δv-efficient, but much faster in terms of both in-game and wall clock time. I plot a "good enough" intercept (say within 5-50km separation, relative velocity doesn't matter, as long as it's less than my remaining Δv), and then as my ship is a couple dozen to couple hundreds kilometers from the target, I start doing small adjustment burns to minimize distance at flyby and relative speed. The goal is to eventually reduce flyby separation and relative speed at intercept to something reasonable (say, flyby at 100 m, 10 m/s relative speed). From there, I can just dock easily.

It's not the way real docking happens today (though it's how it might in the future!), but I can execute a full intercept and docking in less than one orbit, which is more fun. Doubly fun to do this with transfers between celestial body - docking straight out of a transfer orbit, without first establishing a proper orbit around a moon/planet.

EDIT: Triply fun with planetary landers, if you don't have enough Δv to reach orbit, much less to transfer to a station/orbiter module. You launch the lander into ballistic trajectory that intersects your target's orbit at its peak, and pray you time it right, so that the orbiter/station can decelerate and do a suborbital docking, and then boost back to safety. One mistake here, and you can lose both ships. God, I love this game.

[0] - Docking Port Alignment Indicator.

[1] - https://i.imgur.com/J69AYEw.png. It looks trivial, but it packs enough information that you can easily dock looking only at that display. And something like this is what real astronauts would use, so it's actually making things more realistic :).

> , maybe a collector craft could approach and collect several targets on a low Δv budget? It's not something you could do manually, but space has also this nice property that you can predict trajectories accurately far in advance,

So, the issue is, in the low orbit environment there's no kinds of slingshot or other maneuvers available. You need to match height of the orbit at a point, and need to at least coarsely match velocity at that point, which means you're basically in the same orbit as the object. Sure, you can play games with slowly matching phase, etc, but the delta-v requirements are simply inherently high.

Assuming you manage to grab a hold of debris without ripping off even tiny parts, how would you dispose of it? You'd need to deorbit it and you need a working engine for that. So a reusable collector doesn't seem like a plausible concept to me.

Could an astronaut grab a smaller satellite and simply chuck it towards the sun? Would it have enough velocity break out of earth orbit or would it just get stuck in some really large, wide, graveyard orbit?

If not the sun then maybe the earth?

That would be a fun spacewalk.

The ∆v to escape Earth orbit is generally quite large, and it's far, far greater still to collide with the Sun (though this wouldn't normally be considered necessary).

In comparison, a deorbit burn is usually relatively inexpensive.

From low Earth orbit, delta-v to Earth and to escape the solar system are about the same. Delta-v to fall into the sun is two orders of magnitude bigger.

Directly. Not if you take it the long way - near escape to reduce orbital speed, a little push to cancel the rest, and you will fall directly into the Sun (with a high velocity).

It would be difficult for an astronaut to chuck an object with over 32,000m/s of delta-v.

For comparison, the fastest bullets go like ~1,500m/s, and they are very very very small.

I think the goal would be to first accelerate the astronaut to that level of velocity and then relatively adjusting its trajectory isn't so difficult. But, that is just my understanding that speed is relative (especially in space with no drag)

Speed is also extremely expensive due to the rocket equation- changing your velocity, that is.

It would surely be easier to accelerate space trash into the sun than it would be to accelerate both the astronaut and space trash into the sun.

I don't think we should do either of those, but less mass is always easier to speed up.

If I chuck a baseball out of an airplane I don't have to worry about accelerating the baseball to the airplane's speed, as I am also traveling at that speed.

Similarly, if a satellite is light enough in low-G to grab and manipulate from a craft that has straddled up along side it, would a single astronaut have enough strength to use his arms to put a satellite on a path that is effectively out of the way? Or could they just chuck it back at the earth so it can burn up on reentry?

edit: it's 24k, not 32k, I had the wrong number initially

edit2: actually now I'm not so sure, I might have had the right number initially. It's either 24k or 32k. Either way it's impractical.

24,000m/s is the difference in speed that the astronaut is already traveling with the speed necessary to get to the sun

Getting to the sun is really hard.

Chucking it back into the atmosphere is much easier, but GP was asking about throwing things into the sun.

> If I chuck a baseball out of an airplane I don't have to worry about accelerating the baseball to the airplane's speed, as I am also traveling at that speed.

But the speed of the baseball you just thrown out will be (ignoring air friction) that of the airplane +/- 40 m/s, assuming you're some super strong thrower.

> if a satellite is light enough in low-G

Microgravity doesn't mean the satellite is light now. It still has the same mass, it's just that you're both in free fall, so it doesn't pull away from you. For the purpose of throwing it, the satellite is just as hard to throw as it would be on the surface of Earth.

But even assuming it weighs as much as a fastball, when you throw it super hard, you'll change its velocity by... 40 m/s at best. Compare that with the orbital speed, which is many kilometers per second (e.g. about 7.6 km/s if you're on ISS). So e.g. instead of going 7.6 km/s, the thrown satellite now goes... 7.56 km/s. Which translates to one end of the orbit dropping by a couple hundred meters. So instead of 408km, it'll now go as low as... 407.5km, or something in that ballpark.

That's the limit of what you can achieve by just tossing things with muscle power.

It takes a "huge" amount of energy to get something to the sun.

Something in earth orbit is also orbiting the Sun. The Earth is traveling ~30km/second around the Sun. If you want to fall into the sun, you have to cancel out all that sideways speed.

Another fun thing I didn't really understand all that clearly until I spent time playing KSP: Why it's theoretically easier to get to the Sun from Pluto than it is to do so from here.

Related: my mind was blown when I realize how easy it is to fall down to Earth (well, Kerbin) if you're in high enough orbit. I've had cases where my orbital velocity was on the order of 200 m/s, which means that at apoapsis you can just fart, and have the satellite crash into the ground a day later.

(I've also had my fair share of high-orbit missions where in the middle of the launch I realized I'm aiming for a prograde orbit, where the contract specified retrograde. Instead of reloading game from launchpad, I just carried on, put my apoapsis at target altitude, and once reaching it, burned off couple hundred m/s of Δv to reverse the orbit's direction.)

The point isn't really for it to fall into the sun, just to push it to some kind of orbit that is irrelevant to the space junk problem.

Orbits are periodical. Anything you throw at the sun will come back up to the same height soon enough.

(Ignoring smaller effects)

To fall into the sun you would have to negate most of Earth's orbital speed which is about 29.8 kilometers per second. To escape the solar system you need about 42.1 km/s of speed. It actually takes less energy to yeet yourself out of the solar system from Earth than to fall down into the sun. If you threw an object from Earth's orbit along Earth's orbital path an extra 12.3km/s that object would be going 42.1 km/s and eventually but barely leave our solar system.

Unfortunately throwing things at kilometers per second by hand is not possible, 1 km/s is just under 2237 mph.

Of course with atmospheric drag for most low orbit satellites things will come down eventually, in which case throwing something the right way could add or subtract from the time it will stay in orbit, but that would be highly dependent on where you started. And LEO is already like 7.2 km/s or about 16,000 mph. Standing on a massive enough satellite using a large stick or like a sling staff would certainly help you get more speed in your throw though. A 100 mph baseball throw really wouldn't be that significant though.

Shoot it with a dart that has a really long wire attached to it, then release the wire to drag in atmosphere, so the satellite is slowly pulled down?

You can attach a wire to an object in Earth orbit and use the magnetic field of the Earth to change that object's orbit over time, as well.

That's an interesting idea, I wonder if you would need a loop or a winding method to get enough counter-emf?

There are at least some proposals that use ionised particles in the earth's upper atmosphere as a conductive elements, meaning you don't need a loop as such- just a wire pointing towards the earth with a big metal ball on the end. Lorentz force means you can accelerate prograde or retrograde.

I find it fascinating to use the natural things you have available as a motive force, and it's really cool to think all you might need is a few bits of conductor and the earth's own relatively static magnetic field to move a satellite around.

I wish humans were less interested in the noisy, explosive things like engines and rockets, and more into subtlety.

There are some interesting ways of adding drag to objects.

Increasing surface area will (eventually) de-orbit due to drag.

The wire methods mentioned earlier could possibly offer the option for magnetic / induction drag.

Ion engines are an option, though they're spendy.

Geosync orbit is the tough one. It's much larger, but the sweet spots (equatorial orbits) are relatively scarce.

Mandating methods of clearing critical orbits (for GEO) or deorbiting (for LEO) implemented prior to deployment is probably the more viable option.

Thankfully GEO is still a bit self cleaning as Sun and Moon influence makes it unstable - you can clearly see it in orbital debris models, there is a blurry plane shifted ring around the primary GEO stripe.

Still that will only prevent a dead sat in sitting in a valuable spot for ever, it migh still drift the wrong way and require active sats to do avoidance maneuvers, not to mention the chance of two inactive sats colliding.

Self-disposing satellites are a good idea, but it's not perfect. If the control unit or the engines of the satelite fail too early during the satellite's life span, the thing will simply stick around.

Right. And this actually is rocket surgery.

There's also the problem of retaining propellants over long periods without experiencing some negative consequence (detonation, unintended firing, volatilisation, etc).

But in general, minimising the special-handling cases would be an improvement.

Long-term propellant issue could probably be solved by using a cold-gas thruster with a non-reactive propellant - basically a pressurized gas tank welded to a valve and a nozzle. It should have enough Δv to push you into a graveyard orbit, or even deorbit straight from GEO (though that's probably dangerous, and I imagine you could see spectacular fireworks from the ground, as the satellite would be hitting the atmosphere with a couple km/s more velocity than it's typical).

Solid thrusters being another possibility.

> Assuming you manage to grab a hold of debris without ripping off even tiny parts

There are some ideas that could be repurposed from asteroid mining concepts. Many asteroids are more like pile of gravel loosely bound by gravity than solid rocks, and either way, trying to extract anything risk launching lots of tiny pebbles in a random directions. One idea I saw floating around NASA website was to wrap such an asteroid in a net or sheet of material, to keep it from shaking itself apart. I could imagine similar lightweight net being used to wrap a satellite.

Once wrapped, you can do two things with it. You can try to deorbit captured trash - for instance, with a light net and a solar sail attached (which would be used to increase atmospheric drag). Such trash would not only deorbit faster, but a sail would make it easier to track. Nets and sails would have to be light and disposable, they could be shipped to orbit in bulk, or (later) manufactured in space.

Alternatively, you can try to recycle the trash. If you have in-orbit refueling using propellant sourced from space, you probably already have - or are about to have - some facility in orbit that could reprocess parts of dead satellites. So you can move the trash to some designated recycling orbit.

Now that I think of it, you could probably make a recycling collector work without all the infrastructure in space. In orbit, you have time, you can take things slow. I can imagine a collector chasing light trash using ion thrusters, or whatever is most promising in high-Isp department these days (how small can a VASIMR get anyway?), grabbing it and parking at the recycling orbit. Might take weeks per piece of trash, but if you launch a bunch of such collectors and feed them trajectories from the ground, they'll eventually clean the orbit out of light trash. It might require some actual developments in the beamed power department, and perhaps launching a power satellite to microwave some electricity to the collectors, but think it would work in (sufficiently modded) KSP :). Anyway, the biggest problem isn't mission design, but figuring out who will actually pay for this.

Not sure what to do about the most dangerous space trash - the tiny bits that can't be reliably tracked. Track them better and vaporize with lasers?

'Laser brooms' are one idea. I think you still have to be able to resolve the debris, but the idea is to end up causing assymetric light-induced thrust on the debris, deorbiting it faster (by heating the prograde side).

Another very 'interesting' idea is deploying a large amount of fine dust on an appropriate polar orbit. Everything intersecting that orbit will be sandblasted, increasing drag; this will have the greatest effect on the smallest debris, and hopefully anything operational in that orbit will be well enough armored or able to dodge the cloud. The dust deorbits quickly for the same reason as the tiny debris (greater surface area/volume). One of the more metal ideas for space debros handling I've heard.

> Assuming you manage to grab a hold of debris without ripping off even tiny parts, how would you dispose of it? You'd need to deorbit it and you need a working engine for that.

meet up with the debris, reorient yourself and the debris such that the debris is closer to the earth (reaction wheels, magnetic torquer bar and patience is all that's required), and then just mechanically kick it down (store up energy from the sun and then trigger a solenoid). the debris itself becomes your propulsion. you get a tiny boost up, perhaps towards your next target, and it gets a (admittedly!) tiny boost down.

obviously it doesn't instantly dispose of everything, but progressively pushing material further down the gravity well is the next best thing.

not claiming this would be cost effective.

Clever! A 'hopper' bot. Capacitor + Discardable Mass == Thrust.

> then just mechanically kick it down (store up energy from the sun and then trigger a solenoid). the debris itself becomes your propulsion. you get a tiny boost up, perhaps towards your next target, and it gets a (admittedly!) tiny boost down

This doesn't actually work. If you push something down in orbit, you just make its orbit elliptical. After half an orbit it will be at the same altitude as you again, and after a full orbit it will hit you from the top. If you want to lower its orbit, you need to push it back.

Orbital mechanics is weird.

> This doesn't actually work. If you push something down in orbit, you just make its orbit elliptical.

Came here to say this as well! The only caveat to that I can think of is if you're already low enough in orbit, the push making the satellite's orbit elliptical could put it in a drag zone, and then air would do the slowing down for you.

Also, thank you Neal Stephenson for introducing me to the wonderful would of orbital mechanics via Seveneves!

Also plays a role in another of his books, Anathem.

For me, accidentally losing contact with the space ship during a Kerbal space walk taught me exactly how tricky manual orbital piloting is and inspired me to learn more.

The Kerbal never got back to his ship before running out of propellant.

> If you want to lower its orbit, you need to push it back.

if you want to lower its orbit, you probably need to take a lot of complicated stuff in to account which doesn't fit into a comment. i won't charge the orbital dynamacists implementing my idea more if they replace "down" with "the optimal direction for this target, at the optimal moment".

mass of the target, cross sectional area of the target, any out gassing it might be doing, where we're trying to maneuver to next, etc, etc.

if the optimal direction ends up being exactly "down" or "back", i'll buy you a beer.

As a general rule of thumb, the only way to decrease the energy of an orbit (what most people would consider to be "lowering the orbit"), is to apply acceleration in the opposite direction that the object is traveling.

You may be to do this with atospheric drag if you add enough ellipticity but they would only be lowering the orbit indirectly.

> would only be lowering the orbit indirectly.

you know, in the wildly hypothetical world where this was implemented, i don't think the folks cutting the check would mind if you deorbited things directly, or indirectly.

But you could just kick it in the retrograde direction and to de-orbit it. In fact you could use centrifugal force and a cable to generate a significant amount of acceleration using reaction wheels powered by solar panels which would not consume any propellant.

You could attach a simple lightweight drag sail or a Echo style very low pressure balloon. That can increase drag of the thing by orders of magnitude and get it down fast while you tug goes for the next target or returns for refueling or restocking.

Why does it need to be deorbited? Why is collected and held not effective?

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