Complex sentence: Many people believed that intelligent beings might live on the moon, Mars, and Venus; but since travel to other planets was not yet possible, but some people suggested ways to signal the extraterrestrials even before radio was discovered.
Split into simpler sentences: Many people believed that intelligent beings might live on the moon, Mars, and Venus; but since. Travel to other planets was not yet possible, but some people suggested ways to signal the extraterrestrials even before radio was discovered.