At this time there is no public domain English text of Kepler's Somnium, the first science fiction story. Johannes Kepler wrote the Somnium in Latin between the 1590s and the 1630s. The Latin text is in now in the public domain but there is no public domain English text, because the English translations are still under copyright.
In England, where Kepler's reputation was eclipsed by Newton, the Somnium was almost ignored, or read only in Latin. In Germany, interest in Kepler revived in the 19th Century when his collected works were edited by Fristch. Two English translations were published in the US, Lear/Kirkwood in 1965 and Rosen in 1967, are still under copyright and won't be in the public domain for many years.
Extracts from the best-known Rosen translation and Lear translation, both first published in the 1960s, can be found on Google Books. But the online samples do not include the storyline of Kepler's Somnium itself, only the introductory essays by the translators.
The English text of the Somnium on the Frosty Dew website are copied from the sleeve notes for the Mannheim Steamroller 1983 album Fresh Aire V, which was inspired by Kepler's Somnium. Translated by Normand Raymond Faladeau, S.S.S, who died in 2004, this English translation also is still under copyright and will not be in the public domain for many years. It looks like Frosty Dew have no copyright at all to the Faladeau translation.
You can read a new English translation of Kepler's Somnium at http://somniumproject.wordpress.com and follow @SomniumProject on twitter for line-by-line tweets from the full text of the story. More on the origins of Kepler's Somnium and other early science fiction: http://somniumproject.wordpress.com/faq/