A series of adventure modules for Classic Traveller, published in pairs.
From an Overview of Traveller Material appendix in Book 0: An Introduction to Traveller:
While adventures are intended as large, multi-session events, there is also a need for shorter situations: what we produce as double adventures. Perhaps you remember the old Ace double novels – two paperback science-fiction novels printed back to back. That's the format we have adopted for double adventures. Each is about 20 pages, with a complete situation, pre-generated characters (if necessary), background, maps or deck plans, additional data, and everything else needed for an evening of Traveller fun.