Chronology of Sputnik/Vanguard/Explorer Events 1957-58
Sputnik (83.6 kg) launched
Sputnik II (508.3 kg), with dog Laika as passenger, launched
Vanguard TV-3 explodes on launch pad
First try to launch Sputnik III fails
A second Vanguard try fails
Explorer 2 fails to orbit
Vanguard 1 (1.47 kg) successfully orbits, establishes the pear-shapedness of Earth
Explorer 3 orbits, collects radiation and micrometeoroid data
Another Vanguard fails to orbit (third failure)
Sputnik III (1,327 kg) orbits, carrying large array of scientific instruments, but tape recorder fails, so it can't map Van Allen belts
Vanguard fails for the fourth time
Vanguard fails for the fifth time
Explorer 4 orbits and maps Van Allen radiation belts for 2 1/2 months
Explorer 5 fails to orbit
Vanguard fails for the sixth time