He joined an x-ray astronomy group at MIT and conducted all-sky balloon surveys with George W. Clark. Through the late seventies, there were about twenty successful balloon flights. These balloon surveys led to the discovery of five new x-ray sources, which doubled the number known at the time.
Furthermore, some of these x-ray sources were found to be varying, and some were x-ray flares. The rockets used by other researchers could not have discovered that the x-ray sources varied because they were only in the air for a few minutes, whereas the balloons could be in the air for many hours. The surveys also resulted in the discovery of GX 1+4, which was the first slowly rotating X-ray pulsar found. Walter Lewin received the "2003 Everett Moore Baker Memorial Award for Excellence in Undergraduate Teaching" award. Some of his lectures are available online in video format.
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