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

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.

While Suni Williams and Butch Wilmore’s situation is unusual, their return trip will be pretty routine, as they were already slated to fly home on a SpaceX capsule as part of a scheduled crew rotation.