One issue with various Traveller rules is that operating a merchant vessel while taking only cargo and passengers is generally going to cause you to go broke. This introduces the speculatory cargo rules. While these can cause tremendous profits, they can also cause tremendous debts. All of these roads point to forcing the crew into potentially "unsavory" pursuits, such as possibly joining in with various portentially risky or even criminal enterprises (all kill, loot, and ploot stories were "adventures" way back in the early days).
The speculatory rules and adventure activities "force" crews to go places they wouldn't ordinarily. Pirates and void krakens may lurk thereabout. Some pirates may divert target craft into ideal unpatrolled local space with complicated confindence games.
"Yes, of course the Rialto Grain will sell for 7,678 Cr per dTon on Dinomn at High Fest, it's required in local religious rituals! There has been a run on Zuchai Crystal overproduction on Dinom and now there are no independent ships left to carry the last lots of Rialto Grain that will be required on Dinomn. Too bad there are no Zuchai Crystal shipments left to buy for the current market, that would have been some real money. Come on, it's only J-2 from Regina, one jump! It's not like its far out of the way or anything. You'll be back in time to watch the Regatta for the Purple! I hear Duke Norris is favored to win this year!"
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That is like asking why is there a thriving pirating industry in Somalia...
There is piracy all over the Earth today. There was all over the Earth last century. Piracy on Earth has never really stopped.
Did they give up multi-masted sailing vessels and yelling, "Yo, ho, ho and a bottle of rum," a while ago? Some of them did. Some pirates used sail well into the 20th Century and may yet be using it.
So, in theory piracy should not exist in the OTU. In theory, modern day earth shouldn't have pirates either...
There is precious little information, in my opinion, to do more than try and form hypotheses about the true reality of piracy in the OTU.
The most serious issues regarding piracy in the OTU are technologically related, in essence, what can the technology of ships in Traveller do? These are debated issues across many editions.
Pirates on the high seas (or roads) of Earth have numerous advantages, most notably the coverage of the Earth itself.
In space, there is basically little or no cover. You can hide behind an object, but that's about it. It's a matter of who can spot who, who can run down who first, and who can force boarding (or whether it can be forced at all: grav-pong, anyone?), which goes to issues of planting passengers or crew in order to obtain a hijack (by whatever means), preferably one that looks like a malfunction rather than a theft. The most ideal hijack would be to suborn the navigator who enters a false jump course. The ship does not even know anything has gone wrong until it appears with no remaining jump fuel in an empty hex with no choice but to starve or surrender to a waiting and fully-fueled vessel (which may be a foregone conclusion if the navigator has control of the ship's computer). Said waiting ship would be carrying collapsible fuel bladders in the cargo hold to refuel the target, or might have a cargo hold or boat bay big enough to just take the ship whole, allowing it to be better hidden for however long the journey to the sale site or chop-shop will take.
Does your anti-hijack program check to make sure the destination course entered by the Navigator is the one on ship's flight plan? What does it do if they're different? How does this interact with emergency situations with new orders flying about at the last second? Does the anti-hijack forbid a new course until a tedious security procedure is cleared?
One GM may allow all of this and another GM may not. It leads to some potentially strong differences of opinion regarding these matters.
And all that aside, pirates in an RPG are fun.
In an RPG game, definitely. In real life, I dodge.
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Of course it could be argued that the IN will step in,
I just have to issue a general MTU ruling that the IN does not pursue pirates. If they see a piratical action in progress, yes, they'll step in. They do not deploy forces in response to piratical activities. That is the realm of subsector and planetary navies.
however they don't have the resources to occupy a planet, merely blockade it (which arguably can be worse than the piracy problem).
I'm a little confused here. The IN would never "occupy" a planet, so it's resources availability regarding this is moot. The IM or IA would, potentially, occupy a planet. It's going to be fairly rare for an Imperial member world to be occupied (or even assaulted) in the modern OTU. A lot of political mediation, covert intelligence activity, and PCs or big story-NPCs sent in for the last minute rescue, will have to have all gone wrong in order to get there (IMO).
IMTU high pop worlds do not suffer from piracy (high pop = big budgets available for anti-piracy/smuggling etc).
Yes. Absolutely. The budget's of the four TL-15 High-Pop worlds in the Spinward Marches are immense. Mora and Rhylanor especially so. They can afford squadrons of capital ships and could easily field so many SDBs that every planetary body in the star system would be thoroughly covered. A pirate might not even dare enter the system under legitimate business for fear of being caught peripherally on a background check.
And therein lies the adventure.
Absolutely.
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[...] the notion that every single moon could provide a moderate base for a well-heeled outfit [...]
How about every single asteroid belt, kuiper belt, and Oort cloud object big enough to dig a base into? Of which there could be tens or even hundreds of thousands per star system...