What's the legal/licensing situation with Traveller? Why so many versions? Who owns the IP?
Marc W. Miller owns the Traveller IP, lock, stock and Barrel, with some caveats. Mongoose is a licensee. QLI was a licensee. 13Mann is a licensee. DGP was a licensee.
FFE is working on a conversion doc for HT to HSR 6 (well, actually, Don McKinney is).
The GT license ended 31 Dec 2014, and was handshake-extended to 31 Dec 2015. A deal to make a CD of it for continued but unsupported availability is in the works.
Mongoose holds a license from Marc that is slated to expire in 2018 last I heard. At that point, all rights revert to Marc, as I understand things. It's possible it may be renewed.
Mongoose released its core for 1E as closed content, but has an SRD - but the SRD's sample careers are NOT open content, and the SRD includes no damage ratings!
There is a Traveller Logo License that Marc authorized, it expires with the MGT license, it's free. There is the Foreven Free License that allows OTU elements. It also expires with Mongoose's license.
Commercial licensing for T5 supplements is "Call Marc" territory.
The "FFE Fair Use Policy" (FFE FUP) is essentially an open non-commercial supplement license to non-mongoose non-GURPS material. You can write whatever you want in the way of expansions for CT, MT, TNE, T4, T5, T20, T2300, or 2300AD, provided you acknowledge Marc's ownership of the IP, don't charge for it, and don't claim officialness - further, Marc has a longstanding "no sexploitation in Art" policy - he considers indecent or demeaning artwork to violate the FUP.
As for why so many editions? Largely, edition churn...
CT 1E was 1977
CT 2E was 1981
MT was 1987
TNE was 1992
T4 was 1996
GT was 1998 - due to much dislike of T4.
CT 2E reprints began around 2000 - first with QLI's GRIP: Traveller edition including a reprint of the 3 books of CT core in a single digest volume, included in the "General Roleplaying Internet Protocol" VTT.
T20 was released in 2002 - but spent just shy of 2 years in development. Done by QLI under license. Sold to Marc by Hunter Gordon, along with the COTI BBS (
http://www.travellerrpg.com) the Electronic version was 2006. Hunter's license was supposed to run to 2010...
MGT was 2008. Mongoose snuck in an exclusivity clause...
HT was licensed in about 2004 from FFE and from Hero Games, didn't release until 2007, and the Hero 6E came out shortly after...
T5 started around 1998, went beta about 2006, was released in 2013, and updated in 2015.
MGT 2E was started about january... hit beta this month.
Traveller 2300 was 1987 as well.
2300AD was 1988 or 1989. Just a new logo, new print run, and some minor upgrades and a rewrite.
2320 was 2005.
QLI had secured licenses to the Honor Harrington and Aldenata series, but apparently they didn't meet David Webber's approval, or weren't done in time. QLI purchased the Judges Guild Traveller offerings. Marc since bought those IP's.
Marc has secured rights to several 3rd party CT publishers' Traveller materials: Gamelords, FASA, Paranoia Press, Games Workshop, and some fanzines.
Mongoose has a license for Judge Dredd, Slaine, and Strontium Dog, from the Rebellion Group. When MGT sunsets, those go the way of the dodo.
Mongoose has a license for 2300 from FFE/Marc.
What Marc doesn't own: the mechanics for TNE, but he has a royalty free license to them as part of the breakup of GDW. He likewise lacks rights to GURPS rules and Hero System rules, but is licensing both from their owners. Marc has an agreement in place to sell CDRoms of Challenge Magazine (which was jointly owned by the GDW partners).
FFE also is electronic publisher for Dark Conspiracy 1E/2E, and Twilight 2000 (1E, 2E, and 2013). DC is under license from Loren Wiseman, and T2K from Frank Chadwick. T2013 I'm not certain about, either Marc licensed it from them, or it reverted to Frank and Frank has Marc keeping it available on CD.