These are, in some sense, "spinoffs" from Gemini, created by people active in the Gemini community during the earliest stages of its development, who wanted to explore taking the protocol in a slightly different direction, or to add more capabilities. None of them are an official part of the Gemini project, but some Gemini clients also support some of these additional protocols, and some people are active in both communities.
Titan is an add-on for Gemini clients and servers, created by Alex Schroeder, to support uploading data. It was designed with the goal of making it possible to build wikis in Geminispace (and Alex has done precisely this!). Titan is named after Titan II GLV rockets used the Gemini spacecraft in the original Project Gemini. Learn more at the link below:
Spartan is a protocol strongly inspired by Gemini but is substantially more, well, spartan! It was created by mozz (of Astrobotany fame!). If Gemini "sits between Gopher and the web", then perhaps it's fair to say Spartan sits between Gopher and Gemini. Like Gopher, there's no TLS or UTF-8, but like Gemini there are MIME types, virtual hosting, and some capacity for redirects. Then again, Spartan supports clients uploading to servers, so in some ways it goes beyond both Gopher and Gemini. The overarching goals of Spartan are to be simple, familiar, fun and inspiring!
Learn more at the official Spartan Gemini capsule
Mercury was an entirely hypothetical alternative to Gemini, substantially simpler, with Solderpunk sketched out in a gemlog post purely to illustrate something which could exist in principle, not with the intent that anybody actually implement it, but for it to act as a "philosophical navigation aid", to help make sure the general consensus in the community was that Gemini was not becoming overly capable and overly complicated. Mercury was supposed to be something that Gemini could be mindfully steered either toward or away from. The majority preference at the time seemed to be "away from", and the idea was never really developed further. Despite Mercury not being "an actual thing" (there's no official gopherhole/capsule/website, no leader, etc.), some people have actually implemented in software nevertheless! You're a less likely to bump into references to Mercury in the wild than Titan or Spartan, but it could happen.
The original sketch of the Mercury protocol
Circumlunar Space is another, earlier project by Gemini project leader Solderpunk. It started as a a simple free Gopherspace provider, quickly morphed into a public access unix system, and later expanded into an experimental "pubnix confederation" of multiple servers.
For several years, the official Project Gemini capsule was hosted at gemini.circumlunar.space. But Gemini and Circumlunar Space didn't share a domain name because one was an integral part of the other. This was simply a historical accident. The gemini.circumlunar.space host was setup quickly in the very early days of the project when it was not expected to grow to anything like its current size, and registering a separate domain for it did not seem warranted. This was finally corrected in September 2023 when the official home of the project was moved to geminiprotocol.net.
"The small internet", also affectionately known as the "smolnet", is an online counter-cultural movement which has a lot of currency in the Gemini community. The term has no official, precise definition and it is not always used consistently. Often it is used simply as a shorthand term to refer to Geminispace and Gopherspace together. More properly, it refers to internet technology that embraces at least some of:
Of course, the above is only a rough outline of a fuzzy concept without a precise meaning and not everybody will agree fully with every dot point above, but this should provide the gist of it.
As mentioned, the two common uses of the term are not entirely consistent; it's certainly possible to use web technologies to build spaces which abide very closely by the smolnet philosophy as sketched above, and it's possible to use Gemini to build spaces which fly in the face of it. But it's not a terrible conflation, either; the small internet philosophy is a nearly invisible minority stance on "the big internet", whereas it is fairly mainstream on Gemini and Gopher.
Nevertheless, the term itself and certainly the ideology behind it both predate Gemini. You don't necessarily need to embrace the "smol" philosophy to find Gemini useful or enjoyable, but if you spend a lot of time in Geminispace or talk to a lot of people about Gemini, you are very likely to bump into the term and find yourself talking to a lot of people for whom this philosophy is almost the entire appeal of the project and the community. It's part of the "local culture", if you like. Even if it's not entirely your thing, being aware of the basic idea and having an open-minded, respectful attitude toward it will go a long, long way toward making your interactions with the community smoother and making the protocol itself seem less like a straitjacket.
The nascent Permacomputing movement seeks to apply ideas from permaculture to the subject of computing and society's relationship to them. A core concern is the environmental impact of computing, stemming from both the large and ever-growing energy demands of consumer computing devices and the datacenters and other internet infrastructure used to supply them with content, and the burgeoning piles of hazardous, recycling-resistant e-waste produced by non-repairable computing hardware with rapid, largely artificial, cycles of obsolescence.
The "permacomputing" label was coined about one year after the Gemini project was started, without any immediate connection or mutual awareness between the two. However, many Geminauts and other small internet enthusiasts (see 6.3) were very receptive to permacomputing ideas upon encountering them, and many in the permacomputing community have embraced the small internet.
Just like the small internet philosophy, you certainly don't need to embrace the permacomputing to find Gemini useful or enjoyable, but if you spend a lot of time in Geminispace you'll likely run into the concept, so it might help to have some awareness going in.
Next section: Responses and rebuttals to common criticisms of Gemini