Economic growth estimates for many a country are often easy to get but are seldom sensible to trust. Thus you report that Vernon Henderson, Adam Storeygard, and David Weil of Brown University are proposing to track the changes in the intensity of artificial light over a country at night as a proxy (“Light Relief,” August 8, 2009). American military weather satellites collect these data every night for the entire world. The main advantage of this proposal is that errors in measurement of brightness are unlikely to be correlated with errors in the calculation of official growth figures. Splendid. However, that means that American military weather satellites can be used to obtain unbiased estimates of a whole slew of other useful statistics, such as industrial pollution, violent crime rates, or consumption of illegal drugs. Outer space is the right place to gauge all sorts of things down on earth besides economic growth. Which is what Martians must have known for quite some time.