An ETF that can trade all day is investing in an asset that rarely changes hands. Figuring out what it’s worth is a guessing game.

Elon Musk's space exploration business, SpaceX, got a lot of headlines late last year when it was valued at $350 billion -- a gain of more than $100 billion from its previous valuation, according

SpaceX, the private space technology company owned by Elon Musk, is asking federal authorities to approve a new round of tests for a prototype reusable rocket, but some of them may never make it back to the surface intact and will crash in the ocean.