If something is said to be pie in the sky, it means it's pleasant but not realistic. Visiting every planet in the solar system by your thirtieth birthday is a great dream, but it could be described as pie in the sky.
The phrase comes from a 1911 song by Joe Hill, an important labor rights organizer in the United States: "Work and pray, live on hay, you’ll get pie in the sky when you die." The song ridicules the notion that people should accept their current poor work conditions for a later reward — after death. It's come to mean anything desirable but unrealistic.