Arellano was captain of the San Lucas, piloted by Lope Martín, one of four ships in a fleet commanded by Miguel López de Legazpi.
The fleet sailed from Mexico on 21 November 1564, but the San Lucas became separated from the rest on 1 December.
He discovered a number of islands in January 1565, including Lib, Truk, Pulap, and Ngulu, before sailing back to Mexico,[1] becoming the first navigator to establish the "Urdaneta's Route" in the Pacific Ocean.
Andrés de Urdaneta was part of Legazpi's fleet, who had also separated from the group and discovered the same path afterwards.
Arellano was detained after his return on suspicion of desertion from Legazpi's fleet; however, an enquiry was inconclusive.