Space Travel (sometimes referred to as "starwalking"), is the art of journeying to other worlds directly through the vacuum of space, rather than utilizing trans-planar portals such as moongates.
The high level of technology required to do this is primarily found on the planet Earth in the late nineteenth through early twenty second centuries, although due to either encounters with extraterrestrial beings or magical alterations made to common historical continuity, the vessels and instructional information necessary for achieving planetary orbit were briefly available to the natives of early Sosaria.
 Ultima I
During the First Age of Darkness, the wizard Mondain made contact with a savage alien race that lived beyond the heavens of old Sosaria - beings possessed of technology to travel among the stars. The natives of the realm, while unable to devise how to perform such a feat themselves, eventually came to discover a document among the foothills of Mount Drash which appeared to give instructions for the operation of star-faring vehicle. Having heard it prophesied that the eventual champion who would defeat the wicked arch-mage would have need to battle the star-walkers, the people of the land preserved the document and spread knowledge of it as far as was possible, in the hopes that the promised hero would find their way to these crucial writings.
During this time, it was possible to purchase a space-faring shuttle from a number of vendors, albeit for a steep price. Given the general populace's ignorance of how to operate such crafts, however, it was probable that the merchants trading in such vehicles sold them primarily as curiosities or as scrap, although it is not known for certain that the Stranger who eventually took on the mission of Mondain's defeat was the only person capable of mastering interstellar travel.
In order to operate the shuttle, specially prepared armour was necessary to protect the pilot from the rigors of space flight. Once in orbit over Sosaria, it was discovered that a docking station had been constructed within the gravitational circle of the planet, although its architects have never been revealed. From this waypoint, the Stranger was able to commandeer star ships better suited to battle the Mondain's sky-riding vassals, and hyper warped throughout the various sectors of the Sosarian system, slaying twenty of the starwalkers in a series of dogfights that the champion might attain the title of "Space Ace" and with it the favor of the imprisoned princesses who guarded the secret of Sosaria's time travel device.
 Ultima II
In the Second Age of Darkness, three CCCP rockets could be found in the Soviet settlement of Pirates Harbour in the post-apocalyptic Earth's twenty-second century as it occurred within the warped alternate history of Minax's devising. Once more, the Stranger found need to voyage into the stars, and stole one of these guarded vessels.
Unlike the anachronistic Sosarian crafts of the past age, the communist-developed rockets of the future had a simplified navigational system and were capable of interplanetary flight within the Terran Solar System. They still required specialized gear for the pilot and additionally required stocks of the compound tri-lithium as fuel and an ankh as some manner of operational key. Once launched, the rocket required only that the hero enter Xeno, Yako and Zabo coordinates into its computer that it might auto-pilot a hyperjump to the correct coordinates.
Using this technology, the Stranger was able to discover the previously theoretical Planet X, upon which dwelt the sage Father Antos, a figure crucial to the hero's quest to slay Minax. The mystic imparted upon the Stranger his blessings, which allowed the champion to at long last convince a nameless old man of prophecy to part with his magical ring.
 Martian Dreams
During the later 1890s, astronomer Percival Lowell discovered the miraculous substance phlogistonite, a material whose explosive properties allowed for the design of a capsule which could be propelled as far as Mars. Having long had a passion for Giovanni Schiaparelli's theory that the red planet bore canals, Lowell was convinced that the Martian surface was home to civilization and organized an expedition to this distant world. After exhibiting his vaunted plans for space travel at the 1893 Columbian Worlds Exposition, however, the space cannon which Lowell had assembled was sabotaged while a number of Victorian luminaries were on board the capsule. This act forced the vessel to launch a day earlier than anticipated, with a significant number of unintended passengers.Nikola Tesla, who had been in communication with those stranded on Mars during the accidental launch, was able to construct the replica cannon and organize a rescue mission. Hours before the launch of this second space capsule, the Avatar and Dr. Johann Schleimann Spector appeared in Tesla's secret lab within the mountains of Colorado, having been transposed from the same location in the 1990s by a silver moongate. After providing Tesla with a paradoxical commendation letter in his own handwriting, the hero and the scientist were able to accompany the space-farers on their journey to the red planet, upon which they embarked on a series of adventures that would culminate in the mass immigration of the Martian people to Earth. These aliens, disguised in humanoid robotic bodies have lived in secrecy among the inhabitants of Earth ever since.
Upon their return, a celebration was held for the triumphant adventurers, implying that within even the unaltered time stream of Ultima's Earth, knowledge of space travel existed in a far earlier age than is reflected in the technological history of reality.
 Visitation by the Kilrathi
At multiple points in the Ultima series, it is implied that the world of Britannia exists in a complex multiverse which includes the continuity of the Wing Commander series, another set of games produced by Origin Systems. Specifically, the denizens of Ultima's worldscapes have had occasional brushes with the Kilrathi, a sapient race of alien bipedal cat-like creatures which would eventually come to engage in a bloody war with humanity in the twenty-seventh century.
In Britannia, a lone Kilrathi vessel somehow found its way to the Britannian Solar System. Given that time moves inconsistently between various worlds in Britannia's multiverse, it is not know from which point in the Empire of Kilrah's history this lone emissary came, although the creature landed in the Britannia of the early Age of Armageddon, some time prior to the events of Ultima VII.
After crashing in the fields of a simple British chicken farmer known as Mack, the alien spoke the name of his people once, before being indecorously run through with Mack's magical Hoe of Destruction. In the days that followed, the shocked farmer would go on to tell his story of the "tiger-lion" from the stars, which apparently wished the death of someone named "Wrathy," only to be met with dismissal and accusations of mental illness by those who heard him. The ship, however, remained within Mack's fields, testament to an extraterrestrial visitor.
 The Trilkhai
Later, during the trial that beset Castle Britannia during Ultima Underworld II, the Avatar sojourned to the dark world which housed Killorn Keep, and likely met therein the Trilkhai, a race of large telepathic cats who had been enslaved as work beasts by the human populace. Having heard rumors of her race having once lived in much nobler circumstances, one of the Trilkhai, Blackie charged the hero with discovering the history of her people.
The answer could eventually be found by locating an emissary of the Xorinian dimension, which could tell the hero of the ultimate fate of star-faring beasts who preceded the Trilkhai. If asked, a wisp could inform the Avatar vaguely of the Kilrathi's eventual defeat in the Terran-Kilrathi wars in 2669, and would elaborate that over the centuries that followed the devastation marking the end of this conflict, the Kilrathi re-adapted to a more primitive lifestyle, losing the ability to use tools and eventually becoming the quadrupedal beasts of Blackie's clan.
Should a champion emerge from the mists of legend, the means by which to combat this menace from the sky will appear -- so say the prophets. The legends which foretell of this hero include a number of writings and several ballads sung by the bards of our realm. Among the more recent discoveries pertaining to the coming of the starwalkers is an arcane manuscript, found on the foothills of Mt. Drash. Since it appears to hold instructions for the use of some form of transport, it has been broadcast throughout the land in hopes that it might prove useful to one engaged in the quest to rid Sosaria of Mondain.
Your journey will inevitably lead you into outer space. Special attire is needed to successfully travel through the solar system. While this map shows the limits of known space, don't be afraid to search for new worlds.