1. Wilson, A: A look at Juno II. Spaceflight, vol. 19, no. 5, May 1977. For the record, Juno III, IV and V never flew, but the Juno V became the basis of the Saturn V (LePage, Andrew: Juno V - the early history of a superbooster. Space Views, September 1998).
2. LePage, Andrew: Vintage Micro - the second generation Explorer series; NASA’s forgotten lunar program, www.drewexmachina.com .
3. The Explorer satellites. NASA Facts.
4. Marcus, Gideon: Earthbound Pioneer (Explorer 6). Quest, vol. 19, §1, 2012.
5. 17th August (Pioneer 0); 11th October (Pioneer 1); 6th November (Pioneer 2); 6th December (Pioneer 3). Pioneer 4 passed the Moon in March 1959 at 60,000km.