Method version 1.0, 2026-09-26. The same rule applies to every provider, vehicle and launch site.
Launch Library 2 (The Space Devs) publishes the world's launch manifest but keeps no history of it: when a date moves, the old date is overwritten. Since 2026-08-29 SpaceNexus has recorded every change of more than a minute to a launch's date the moment our sync sees it, with the date before and the date after. Nobody can rebuild this ledger for earlier dates, including us, so every figure states when recording began.
A launch with no announced date is still stored with one. “NET 2026”, “NET Q4 2026” and “NET December 2026” are all kept as December 31, 2026 at midnight UTC, with a precision field saying how much of that date is real. When the year turns, “2026” becomes “2027” and a naive ledger records a 365-day slip that nobody announced. We read the precision, and a move only counts as a slip when both dates were known to the day or better and the launch moved at least 24 hours later.
Only slips, pull-ins and same-day changes are “real” moves. Firmings, lost dates and placeholder rolls are published in their own tables and never added to slip counts.
The CSV names the basis used for each end (recorded, chained or inferred).
Moves of one launch observed on the same UTC day are netted into one: the earliest “from” and the latest “to”, classified again as a single move. A launch nudged twice in an afternoon is one change, and a move that returns to where it started is dropped.
For every launch that flew (success, partial failure or failure alike), the baseline is the first firm date we have on record for it: the date it held when we started watching, or the first date it was given to the day. The delay is liftoff minus that baseline. A launch is on time within one day, or within seven days, when it flew no more than that long after its baseline; flying early counts as on time. Averages and medians are of the delay in days.
A launch that went from a placeholder period straight to liftoff, with no firm date on record before it flew, is listed but not scored. Rates are shown only for a provider, vehicle or site with 3 or more scored launches.
Known limit. We cannot yet tell how far ahead a never-moved launch's date was set, so a launch added to the manifest a few days before it flew counts as on time. This affects every provider the same way, and weighs most on providers whose launches appear late in the public manifest. From September 26, 2026 we record when each launch first appears, and a later version of this method will add a minimum-notice rule once that history exists.
A frozen edition is stored once and never recomputed, so a figure you cite reads the same next year. If an edition turns out to be wrong, the correction is logged on /corrections with what it said and what it should have said. The live Slip Explorer and on-time record keep moving; the editions do not.
It is not a judgment of any company's engineering. Weather, range conflicts, customer readiness and regulators move dates too; the ledger says how often and how far dates moved, not why. It covers the public manifest only. No sponsor or advertiser sees, shapes or pays to change any figure (advertising policy).
Data is licensed CC BY 4.0. Cite as: “SpaceNexus Launch Ledger (spacenexus.us/launch-slips), method v1.0” and link the edition you used. The full move-by-move ledger with each move's kind and precision basis is a CSV on the datasets page.
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