# Collisions in space

#### Ol'Weedy

##### Banded Mongoose
I've noticed in several threads, people worried about collisions in space, especially when exiting jump space. After a little amateurish math, the following should add some context:
If for simplification, you consider the earths atmosphere to go all the way out to 100KM, you are looking at a volume of roughly 51 Billion cubic KM.
Commercial aircraft here on old Earth, rarely go higher that 15KM, but we are considering craft flying to orbital stations, so 100KM is probably fair.
Since we are talking a way bigger volume of space between 15KM and 100KM, even greatly increasing amount of traffic will still result in a very unlikely chance of hitting something, even with a blind pilot.
With that in mind, the volume of space out to 100 earth diameters, is a whopping 1.08683×10^18 cubic KM ((1,086,830,000,000,000,000). One thousand and 86 Quintilian)
That's a little over 21,310,392 Times the volume in the 100KM thick atmosphere band.
Other than for plot reasons, you are not going to hit another ship coming out of jump. Ever.

Never tell me the odds.

I haven't seen any of these threads worrying about collisions, so I can't address their concerns directly. I do recall some comments that coming out of jump space with a substantial velocity is risky because you don't know what might be ahead of you. That seems reasonable. But it depends on assumptions you make about how jump travel works.

If you think a ship can appear anywhere around the 100d line, that makes it much less likely. But there is a lot of material suggesting that ships jumping from a particular origin generally arrive in the same general area. Accurate jumps are accurate to within 3000km according the JTAS article. So then you are talking about a much smaller area.

Also, it depends on whether you think jump transition is something that is disorienting. It is implied to be, but only debilitating on "bad jumps". If your pilot might be less then 100% for a couple seconds, you don't want to come out at speed.

The ships in my characters' settings make an unorthodox approach. They Jump into a system well ahead of the homeworld, hang in there in space in one of the Trojan points, deploy collectors to soak up exotic particles, and send out aux craft to the planet, usually with cargo bays stuffed with trade goods.
They jump into and out of empty space, and avoid all the pirates and other miscreants hanging around the 100D limit looking for incoming Jump ships to harass. Most of them never notice modular cutters and small craft, and assume the craft come from other worlds in the system rather than outsiders.

If you think a ship can appear anywhere around the 100d line, that makes it much less likely. But there is a lot of material suggesting that ships jumping from a particular origin generally arrive in the same general area. Accurate jumps are accurate to within 3000km according the JTAS article. So then you are talking about a much smaller area.

Jump shadow in particular can be interpreted to result in clustered exit points. It doesn't have to be, but can.

Separately I've been thinking about in-setting reasons that would pull crew numbers back up from "rules say I only need 1 guy, if that after automation." One of those reasons could be safety regs around high pop, high traffic worlds requiring a crewman on the bridge at all times. A pop 7 backwater probably doesn't need to worry about collisions. A pop A+, high tech world on a trade route... still might not have to worry, but then again with enough traffic a collision could become a once in a century risk. Which sounds acceptable to player characters but is just the sort of thing some Administrator or Noble would decide to do something about.

Jump shadow in particular can be interpreted to result in clustered exit points. It doesn't have to be, but can.

Separately I've been thinking about in-setting reasons that would pull crew numbers back up from "rules say I only need 1 guy, if that after automation." One of those reasons could be safety regs around high pop, high traffic worlds requiring a crewman on the bridge at all times. A pop 7 backwater probably doesn't need to worry about collisions. A pop A+, high tech world on a trade route... still might not have to worry, but then again with enough traffic a collision could become a once in a century risk. Which sounds acceptable to player characters but is just the sort of thing some Administrator or Noble would decide to do something about.
You definitely need at least 3 watch standers for any commercial or military ship. Unless you are giving the ship's brain a lot of leeway, "lean manning" still requires humans to make the final decisions, even if the computer is doing most of the actual work.

Assuming you exit exactly where you wanted to, you might not at the time you planned for.

Space is big. Really big. Collisions in deep space aren't going to happen unless you make them happen. Space near objects where ships cluster might get crowded, which is what Pilot roles are for, so there is that. Going through planetary rings might cause a small chance of an impact. There was drama when the Cassini probe passed near Saturn's inner ring. Also, impacts in orbital space might cause a Kessler Syndrome cascade, making collisions around a particular planet likely. It seems like big space battles in near orbital space would be likely to cause this sort of issue to occur.

I think the idea is, if you have 10000+ tons of shipping coming through per day (so that's 10k in and 10k out) and they're all using the standard 'optimal' 3000 square km area to jump, then the possibility is high enough above zero, that astrogators need to take momentum into account.

That doesnt make it likely, any more than it's likely for airplanes to collide mid air, but it's a possibility you simply dont want to ever risk - and even if not you, the owner of the megafreighter you might collide with doesnt want to risk it.

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