The classic edition of Traveller is a great game, and no game has yet touched its fame for science fiction tabletop RPGs. Traveller covered all the "space" things in a detailed setting called the Third Imperium, a unified star community ruled by an Emperor. Traveller spawned an extensive series of supplements, which you can take or leave as needed for the emphasis of your particular game, whether star merchants or space mercenaries or explorer scouts.
Star sector maps are laid down in hexagons, each is a parsec (3.26 light-years), and starships have Jump Drives which depending on the engine rating of the ship can Jump from 1 to 6 of those parsecs but each Jump takes a week. There is no faster way to get news or goods across the stars, so the political feel resembles 18th-century sailing. The Imperium is held together but its size is causing strain. The Imperium also borders on alien empires which range from coldly neutral to hostile.
The Traveller Starter edition (1982) did some reorganization of the original LBBs (Little Black Books) of 1977 and presents all the tables of the game in the second booklet, cross-referenced to the pages of the rulebook. A third booklet presents two short adventures.
Traveller is "old school" and the best way to learn it is to take its systems one at a time. Randomly generating star-maps is particularly fun, a kind of mini-game within the game, with the main world in each system defined as to size, atmosphere, population, and government with rolls of six-sided dice (only). Nowadays the fans offer web-based computer utilities to instantly generate vast tracts of space, or use the huge, defined Imperium map already available online at travellermap.com.
Characters are generated by taking an unformed 18-year-old and choosing one of six military or other careers which are determined in 4-year Terms that you must survive. First you roll 2d6 six times to establish Characteristics by the names of Strength, Dexterity, Endurance, Intelligence, Education and Social Standing. High or low values of these traits may give you a Dice Modifier on Skill Rolls (in the most basic dice-rolling rules, you must roll 8 or more, "8+" by rolling 2d6+ a Skill Level). Characters without a Skill in something are penalized at -3, so even 0-Level Skills are useful to avoid this Unskilled penalty. Gain Skill levels and promotions each Term, but also run a chance of character death or dropping out of the career. Physical Characteristics have a chance to decline each Term starting at age 34, and even mental Characteristics may decline for seniors, an incentive to voluntarily "muster out" instead of collecting yet more skill-levels by being as old as you can be. Once your old career is done, you roll randomly for Mustering Out (cash or final benefits based on number of Terms). This leaves you with an older, more seasoned character ready to adventure. However, you can never directly pick your skills! You can place yourself in the desired career, and choose to roll on one of several skill tables but at the end of a term one or more d6 rolls determines what skill is granted! Again, as a game-within-a-game this activity can be fun for some. Characters who are failed or not-quite-right spacers can populate your universe as NPCs!
The personal combat system, in basic Traveller, is NOT on a grid-map but more abstractly defined with "bands" similar to a football field! There are also rules for trade (buy low, sell high of course), which is easy to do as you note the characteristics of the world you're going to (Trade Codes like INdustrial or Non-Industrial, Agricultural, etc.) which give bonuses or penalties to the negotiated price of cargo when it is bought at the source or sold at destination. Nothing is a sure thing, however, providing yet another system that is a fun economic mini-game within the larger RPG story! Use profits to fuel up (ships have fusion engines that will run on ANY source of water or hydrogen such as from a planet or gas giant, preferably refined to keep out impurities), maintain your ship, for life-support, for crew salaries (we love R&R) and pay off the expensive monthly mortgage on your ship. The starship combat, in the original version, is not gridded either, but takes note of what you have for weapon turrets and rolls the damage to the other ship and what sub-system was damaged.
Definitely a solid science fiction experience.