Humanity has ranged an area approximately five hundred light years in diameter by two hundred. It is estimated that there are between four- to five hundred inhabitable planets in that area alone, though the more pessimistic always think that the next planet found is the last one. To date, only sixty-four have been found and logged.
Despite the A-S drive, space exploration is time consuming, since it's safer to appear outside a star system and approach on sublight. Once inside a system, a beacon is set in orbit around the planet the explorer finds most interesting. The beacon helps second-stage explorers find the planet and acts as a way for the person who discovered it to lay claim. A system can only be claimed if it is surveyed and logged, and even basic surveys of new systems take time. An explorer can expect to spend years on even a dead system.
What most people think of when they think of an explorer is actually a founder - someone who discovered and settled an inhabitable planet. Most explorers go their whole lives without finding an inhabitable planet.
An exploration team can be anything from one person to a small crew to a colony ship of four-hundred. A colony ship is less likely to stop and do research of uninhabitable systems, instead jumping from system to system hunting for a place to land.
A second-stage explorer is a scientist concerned with tracking down already-discovered uninhabitable star systems and giving them a more thorough study. Sometimes they find young colonies and report their whereabouts.
A road crew is a ship with a large cargo of hub beacons, to lay out shipping lanes to connect colonies to the rest of the inhabited planets.
Colony ships are a very specific design, mostly cargo space and manufacturing equipment. The chief feature of their design is that they're modular and easily deconstructed, so a starting colony can quickly make a small yet fully-functional settlement out of the ship itself.
Provided they abide by the basic laws of humanity, a new planet belongs to the group that founded it to do with and live on as they wish. It is illegal for a government to attempt to expand its territory through colonisation of other star systems, if only to keep the richer worlds from turning themselves inside-out in an attempt to snatch up every inhabitable planet they can because it's the kind of stupid thing humans would do if you let them.
Unless a colony has planned ahead and brought a scoutship to send back to inhabited space to immediately establish a shipping lane, it can take decades for a new colony to be found and integrated into the network. Most don't, preferring to use the time to more firmly establish the colony before announcing themselves. If there is no contact for more than fifty years, the colony is considered lost.
Jethin Basaday: ( b. AU1813 Earth, d. AU2095 Tethys ) Founder of Shub-Niggarath and Tethys. Ended up settling on and exploring Tethys rather than heading out into space again. He is one of the most famous modern explorers, having founded two planets, and because he had the already-famous bioengineer/doctor Thalassa Zefonith as his sidekick.
Leo Bressica: ( b. AU1923 Miranda ) Gained fame not only through his work but because he recorded all of it, making New Worlds With Leo one of the longest-running reality science shows. He teamed up for a mission with Faadiya Fahraj as a publicity stunt and the two wound up married by the end of it. ( The show was since renamed New Worlds With Leo And Faadiya. ) He founded Rhea by himself, co-founded Tiga with Faadiya, and is still actively adventuring.
Faadiya Fahraj: ( b. AU1943 Capricorn ) Faadiya was such a fan of reality science shows that she became an explorer herself. She agreed to appear on New Worlds With Leo as a publicity stunt to secure more funding for her own work, but ended up marrying him instead. She founded Hebat on her own, co-founded Tiga with Leo, and is still actively adventuring.
Kiace Mahi: ( b. AU2019 Durga, d. AU2068 Dead Zone Planet ) Primarily a second-stage explorer, but these days infamous as the leader of the expedition that accidentally brought the Yogzarthu to this universe.
Habynah Mahi: ( b. AU2063 Durga ) Kiace's daughter. Notable for the Second Mahi Expedition which took a small crew to the Dead Zone Planet under sublight rocket power so they wouldn't be detected by the Yogzarthu. This was a sixty year round-trip from Monitor, so the crew was in stasis most of the trip. She returned in AU2147 with all the records of Kiace's doomed expedition, studies of the Dead Zone Planet, and copies and translations of the monument that tells the Qohathoth's side of the Qohathoth-Yogzarthu War. She was last seen on the lecture circuit.
Ehra Scriber: ( b. AU1968 Terra Nova II ) A very prolific second-stage explorer. She records her work as the reality science serial Worlds Well Lost.
Larnce Stelton: ( b. AU2035 Thari ) AKA King Larry I, founder of the Kingdom of Larry. As the story goes, as a minor soldier, Larnce had boasted that if he survived the war, he would either retire to a farm or found a planet and rule it as a king. After the war, he founded a planet, declared himself king, and set up a dairy farm for himself and his family.
Miranda Thill: ( b. AU1195 Earth, d. AU1490 Miranda ) Founder of Miranda.
Robert Veras: ( b. AU383 Earth, d. AU469 Veras ) Founder of Veras. The Veras family is still high-ranked and active in the politics of the planet, but doesn't rule it.