What is "zero-x"

Zero-X (spelling variants include "Zero X" or "ZeroX") is a fictional Earth spacecraft that appeared in two of Gerry and Sylvia Anderson's Supermarionation productions, the 1966 film Thunderbirds Are Go and the 1967 television series Captain Scarlet and the Mysterons. Although publicity material for the various Supermarionation series, and the TV Century 21 comic, made references to connections between the Thunderbirds and Captain Scarlet canons, Zero-X is the only official link between the two series.