Therefore, for the sake of those who, as it were, will presently be on hand to attempt this voyage, let us establish the astronomy, Galileo, you of Jupiter, and me of the moon.
Kepler imagined that human flight would soon be possible and that people would soon want to fly to the planets he and Galilei where studying. So, for practical purposes, he suggested that they were to prepare the way for those travellers.
The complete paragraph is also interesting:
Meanwhile I cannot refrain from contributing this additional feature to the unorthodox aspects of your findings. It is not improbable, I must point out, that there are inhabitants not only on the moon but on Jupiter too or (as was delightfully remarked at a recent gathering of certain philosophers) that those areas are now being unveiled for the first time. But as soon as somebody demonstrates the art of flying, settlers from our species of man will not be lacking. Who would once have thought that the crossing of the wide ocean was calmer and safer than of the narrow Adriatic Sea, Baltic Sea, or English Channel? Given ships or sails adapted to the breezes of heaven, there will be those who will not shrink from even that vast expanse. Therefore, for the sake of those who, as it were, will presently be on hand to attempt this voyage, let us establish the astronomy, Galileo, you of Jupiter, and me of the moon.
Well, despite Kepler being wrong on a few important points and very optimistic regarding human flight and space travel, it was a good idea.