Vacuum is primarily composed of next to nothing and is synonymous with the Interstellar Medium.
- The space within a star system is called the interstellar medium.
- Most sophonts require the use of a vacc suit to survive within a vacuum.
- It is also known as a Type 0 Atmosphere or Hard Vacuum.
Worlds of Size 1 or less are asteroids, orbital complexes, space stations, and other small satellites, and are much too small to retain a breathable atmosphere. They are uninhabitable by most forms of life without technological support. 
- Although a trace atmosphere denotes a thin veneer of gasses around a world (such as Mercury), it is of such barely measurable pressures that it may as well be a vacuum. Animal and plant life is non-existent on an airless world, although exotic and incredibly rare examples of vacuum life might exist (…and certainly be worthy of an IISS Scout mission).
- Mainstream sophonts must resort to living in bubble cities, underground settlements and other artificial, sealed environments. It can be difficult for a scout to differentiate these vacuum habitats. Try to take note of distinctive features at these settlements such as interesting geographical locations: at the entrance to a huge mine, straddling a small meteorite crater, along the walls of a canyon, inside old lava tubes, in the caldera of an extinct volcano, inside a network of vast, natural caves ... and so on. 
References & contributors (Sources)
- Marc Miller. Worlds and Adventures (Game Designers Workshop, 1977), 5.
- J. Andrew Keith. Exotic Atmospheres (Game Designers Workshop, 1983), 1-16. (Special Supplement 2 from the periodical Journal of the Travellers' Aid Society 17)
- Marc Miller. Referee's Manual (Game Designers Workshop, 1987), 22.
- Joe Fugate, J. Andrew Keith, Gary L. Thomas. World Builder's Handbook (Digest Group Publications, 1989), 64-68.
- Terrance McInnes, Dave Nilsen. World Tamer's Handbook (Game Designers Workshop, 1994), 11-17.
- Jon F. Zeigler. First In (Steve Jackson Games, 1999), 70,72.
- Paul Drye, Loren Wiseman, Jon F. Zeigler. Interstellar Wars (Steve Jackson Games, 2006), 125-6.
- Gareth Hanrahan. Mongoose Traveller Main Rulebook (Mongoose Publishing, 2008), TBD.
- Marc Miller. T5 Core Rules (Far Future Enterprises, 2013), 409.
- Paul Elliott. The Universal World Profile (Zozer Games, 2016), TBD.
- Traveller Wiki Editorial Team
- Author & Contributor: Lord (Marquis) and Master of Sophontology Maksim-Smelchak of the Ministry of Science