The U.S. Department of Defense (DOD) 1960 World Geodetic System was produced by Irene Fischer and published in 1960. Today they are commonly known as 1960 Fisher Ellipsoids, as they are called today, is a mapping of the shape of the Earth, published by Irene Fischer in 1960. This was the best unclassified ellipsoid available at the time. As a result, NASA used them for the Mercury project, which had it's first manned launch in 1961. Thus they gained the nickname "Mercury Dadum". 

Sources:

* https://books.google.com/books?id=B97bWOGPHIUC&pg=PT225&lpg=PT225&dq=Mercury+Datum+Ellipsoid&source=bl&ots=SzXFVvB8Zs&sig=2h30pt-mkIEWZr11UopREFvx8ro&hl=en&sa=X&ved=0ahUKEwjZ-4Wi0-bXAhXFKCYKHR8cAWoQ6AEIWzAJ#v=onepage&q=Mercury%20Datum%20Ellipsoid&f=false

* https://www.nap.edu/read/12884/chapter/16#79

* https://books.google.com/books?id=QeGL4Rw9nxgC&pg=PA153&lpg=PA153&dq=Mercury+Datum+Ellipsoid+Fischer&source=bl&ots=cGX_w5QZeo&sig=1NQHBYXcJf4OSxHdFik-dLmIRUA&hl=en&sa=X&ved=0ahUKEwjqyafR1ObXAhWMSSYKHb1sCIEQ6AEIQzAD#v=onepage&q=Mercury%20Datum%20Ellipsoid%20Fischer&f=false