'Asho Zoroaster' is a double reference: first to the legendary prophet of that name, and later to the vehicle of Ransom's Crew.
Asho Zarathustra Spitama
This is the name, or title, of a religious leader (native planet unknown) believed to have introduced moral afterlives (i.e.: Heaven v. Hell) and the 'endless struggle of good against evil' (culminating in an escatological battle) to later religions. Little or nothing is known of his life. He is often depicted as a bearded man in white robes, with a turban on his head and a bull-headed golden/brazen staff in one hand. Some legends place him within memory's distance of Antiquity, and identify his 'Ahuras' (gods of virtue) with the Iname and 'Dívs' (gods of oppression) with the Eladamri. Others set him in the Mythical Age.
G.S.S. Asho Zoroaster
This was a courier-class space-ship given Marcus Ransom by the Gudersnipe Foundation, as his vehicle exploring the Known Worlds. Although no more sophisticated than any commonplace vessel, it was the stuff of science-fiction/space-opera on planet Altronis, where it formed the base of Ransom's rebellion against the local kleptocracy. After initial transport, aboard a gigantic cargo-craft, to the system of which Altronis was part, the Zoroaster was launched from the said behemoth into the system proper, where Ransom landed it himself. Because the main controls were damaged in the atmosphere, the operation thereafter was often jury-rigged, and the crew spent many of their free hours repairing the ship's systems.
The Zoroaster was equipped with a UZH Reactor, and some auxiliary systems including a complex solar-sail apparatus, wherein the ultraviolet light, collected by the sun-sails, was concentrated in a set of crystalline capacitors, called 'sunstones', which absorbed sunlight until it achieved a maximum capacity; at which, it radiated energy instead, which powered the engines. For emergency boosts, the engines included a device to manufacture tri-lithium: an unstable, artificial form of that element, whose breakdown supplied immense amounts of power, at some risk to the conduits.