Ask HN: Looking for a letter(?) by Elon Musk(?)

Some time ago I read an article about Elon Musk where he talks about a woman (in Africa? Doing humanitarian work?) asking him why he is focussing on space instead of helping the people on earth. He wrote a very well-written letter back, detailing on why he thinks that we should look at the bigger picture instead of focussing on short term goals. I want to read this letter again, but I can't find it anywhere. Does anyone know what I mean and/or where to find it?