FTSE Russell will, after Friday’s close, add SpaceX to its Russell U.S. index—part of its semiannual index rebalancing. This means that passive, index-tracking exchange-traded funds (ETFs) that track Russell indexes, such as the iShares Russell 1000 ETF, will also have to add SpaceX stock to their portfolios. Because fund managers will try to minimize the “tracking error” between fund performance and the index (i.e., the discrepancy that can arise from differences between the purchase price and the closing price), this adjustment is likely to be carried out during a brief window at the end of trading on Friday.