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I know that in real life, asteroids tend to be hundreds of thousands of kilometres apart.

In fiction, asteroids are often very close together, maybe a few hundred metres. Obviously this is much more exciting.

In the short story I'm writing, I want to include a battle scene set in a dense asteroid field like the one in "The Empire Strikes Back".

This leads me to my question: is there any scientifically plausible reason for the existence of an asteroid field or belt containing millions of asteroids tightly packed together?

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    $\begingroup$ Hello Anonymous! Welcome to Worldbuilding. When you have a moment, please take our tour and read through our help center to better understand and make full use of the site. $\endgroup$ – JBH Jun 22 '20 at 4:50
  • $\begingroup$ Do they have to be asteroids? Ringo's absolutely amazing Troy Rising series has some scenes (albeit not battles) set in the wreckage left after major fleet battles. Maybe that would work for you? $\endgroup$ – Matthew Jun 22 '20 at 14:53
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    $\begingroup$ There is only one plausible reason: Star Wars Reenactment. In the far future people will be rich (compared to us) beyond belief and have so much free time that even the most bizarre hobbies will be mainstream. In such a universe, there will be people who reenact Star Wars (but probably not the prequels, they're shit). Obviously asteroids will have to be towed into place so that the Millennium Falcon replica can try to escape the TIE fighters within the field. The alien creatures will be animatronic, since genetically engineering womp rats would be considered unethical... $\endgroup$ – John O Jun 22 '20 at 19:00
  • $\begingroup$ Wasn't that dense asteroid field simply the rubble from the recently-destroyed Alderaan, or am I misremembering? $\endgroup$ – Hearth Jun 23 '20 at 14:41
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We need look no further than Saturn's rings.

Saturn's rings are made up of billions of particles ranging from grains of sand to mountain-size chunks.

Pioneer 11 passed through the outer rings and although it came close to hitting one of the two new moons it discovered, it survived, suggesting that the density of the outer rings is low enough to not give you the effect you're looking for. However, I believe the density of the inner rings are. The Cassini probe discovered evidence that there may be millions of moonlets in the rings. That suggests mountain-sized to me.

So, can we have a large field like in the Empire Strikes Back?

Of course we can. But there are some caveats.

  1. Gravity is not your friend. When you get mountain-sized asteroids in close proximity, gravity wants to pull them together to form planets. This process takes a honking long time — but it's nevertheless going on. So why doesn't Saturn's rings coalesce? Because the gravity of Saturn and the orbit of the rings keep them churned up like a food mixer.

  2. All that cool flying around in The Empire Strikes Back, without the use of Star-Trek-grade "inertial dampers" would turn most passengers on a space ship into a thin pink marmalade on the side of a bulkhead. This cool bit of space math points out that the orbital speeds of Saturn's rings are up to 24 Km/s. That doesn't sound like much until you convert it into Km/h (86,400 Km/h or Mach 70). This means you need a lot more space between those asteroids to give you maneuvering room than the folks at Lucasfilm led us to believe. So if you want "real," you're not actually looking for what the film did. You need the asteroids spread out much more (there'd still be plenty of high-stress flying around!).

I don't believe we know yet of any such fields outside planetary rings, but we're only just beginning to view our galaxy with detail. That being said, I can think of no reason why they can't. In fact, I think for the sake of a story it would be very plausible to find them in or around nebulae or orbiting newly-formed stars.

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    $\begingroup$ While the orbital speed of Saturn's ring are quite high, the relative speed between objects is far lower - and it is this speed that determines how difficult it is to maneuver. $\endgroup$ – Klaus Æ. Mogensen Jun 22 '20 at 8:26
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    $\begingroup$ @JBH, no, but the size of turn needed is based on relative speed. A maneuver that looks like a 90-degree left turn in the 1 km/s ring-relative reference frame is just a 2.3-degree radial-inward turn in the 25 km/s Saturn-relative reference frame. $\endgroup$ – Mark Jun 22 '20 at 21:24
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    $\begingroup$ @JBH There's no such thing as "actual velocity". All velocity is relative. And, yes, if you're moving slowly compared to the asteroids, you can turn as tight as you want without excessive G forces, regardless of your velocity relative to Saturn. $\endgroup$ – HiddenWindshield Jun 22 '20 at 21:55
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    $\begingroup$ @JBH the Earth is also orbiting the sun at 107,000km/h, by your logic, if I'm running in a straight line and change direction by 90 degrees at a street corner, I should experience a G-force strong enough to squish my body into a pancacke. The fact this doesn't happen should show why the orbital velocity of Saturn's rings is equally irrelevant $\endgroup$ – Fred Stark Jun 23 '20 at 0:00
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    $\begingroup$ @MatthieuM. you very definitely won't be "standing still". If you're orbiting Saturn, you'll be nearly still relative to the asteroids and have plenty of time to dodge. If you're doing a flyby, you'll be moving at Saturn's escape velocity - not orbital velocity - relative to Saturn. Best case, your relative velocity would be (escape velocity - orbital velocity). Not sure how those relate numerically. $\endgroup$ – John Dvorak Jun 23 '20 at 16:36
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Make the field a very recent creation. Say a planetoid was recently captured by the planet and torn apart because it's orbit was inside the Roche limit; just enough time has elapsed to break it's pieces into a nice debris field, but not enough for collisions and orbital decay to make it too sparse. You would of course be cherry picking a rather brief window of time (astronomically speaking,) but what interesting story isn't?

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    $\begingroup$ Since we are looking at Star Wars universe (or alike), there was a nice little thing that can generate a lot of rocky debris in little time. It just needed one planet as input and no pesky kids dropping bombs down the vents. $\endgroup$ – Yksisarvinen Jun 23 '20 at 15:15
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Trojans might help suspend disbelief

Asteroids are collected at the Lagrange points of planets, in our system notably the Trojan points of Jupiter. While they are still not dense enough, a soft SF story might mention that the battle happens "in a dense patch of the trailing (or leading) Trojan point."

Alternatively, consider going to the rings of a gas giant. Again less dense as you want, probably, and it might not contain sufficiently large rocks.

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The asteroids carry electrical charge.

https://www.youtube.com/watch?v=qHrBhgwq__Q

water droplets and a knitting needles

This great video from the space station shows droplets of water orbiting a charged knitting needle. Rather than the attraction of gravity, they are attracted because of charge differences. You have to have a fair bit of mass to produce serious attractive forces. But electrical charge can pull lightweight things together.

This is the case for your asteroids. Because of their composition and the fact that they are whacking into each other and rubbing against each other, many of the asteroids have acquired electrical charge. They attract each other and so remain in a cloud. Perhaps a solar wind inputs energy and charge as well, keeping the assembly dynamic.

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If these object don't have to be asteroids but could also be comets, there is a simple mechanism by which you could end up with a trail of many comets.

A comet can break up into many pretty large fragments as it passes near the Sun. Those fragments will all have almost identical orbits, so they will be very near one another (see for instance the Levy-Shoemaker comet).

You don't say how big the field has to be, but this might be one option.

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I do not think this is what you are asking for, but I have it like this in my space horror novelette:

Hero's planet was cut off from civilization for 1000 years . In reality, most of the humanity was destroyed by bio-engineered virus - aliens just overlooked this little colony and with little bit of luck, supply ship never arrived and virus never got there. It took the colony 1000 years to built themselves back to interstellar technological level (they had knowledge, just not resources). Hero one day found another shard of human leftovers - mining and refining space station in asteroid field (It is huge station - where miners and metallurgist lived with their families + required infrastructure). Despite being dead, the station operated for next 500 years and while no FTL ship was available they had at least short-range vessels and pulled near asteroids together and used them for whatever they needed. For expanding station, fuel, spare parts etc. Over 500 years the Mining station accumulated thousands of various asteroids in huge cluster around station - the field in my settings is really small - just a bubble in diameter 100,000km. But perhaps you could use this idea and make it as a some huge junkyard or storage field for mining facility - perhaps even whole ring around some industrial planet. They just pre-stocked asteroids. Sorry I love One-Product Planet and Planet of the Hats trope so I use it a lot haha

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Very recently there was an Earth-shattering kaboom, and now there's 6×10²⁴ kilograms of junk out there. It's mostly all moving away from the center of mass of the planet, and it's all superheated and glowing white-hot. Gradually it will spread out and form a ring, but it will be millions of years before the collisions become rare.

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