By Bailey McCann Thematic ETFs, exchange-traded funds that focus on specific investment themes, are booming And one of the most-talked-about themes lately is space. There is no shortage of ways to invest in the final frontier. The classics Historically, the way to invest in space has been in aerospace and defense ETFs. Before Elon Musk was shooting cars into orbit, space exploration was a purely governmental affair. Aerospace and defense companies were building the rockets, and many of them still are. Funds with large positions in companies like Boeing, Lockheed Martin or Northrop Grumman tend to have lower volatility and turnover than funds focused on the newer, high-growth companies that compete in space. Aerospace and defense ETFs come in both actively managed and passive, or index, varieties. On the passive side, iShares U.S. Aerospace & Defense ETF (ITA) and SPDR S&P Aerospace & Defense ETF (XAR) offer broad exposure to the old guard of compani