Launched from a Juno I rocket, the mission remained secret from the public for six months.
The satellite telemetry was analyzed for three Operation Argus nuclear weapons tests at high altitude.
An unexpected tumble motion of the satellite made the interpretation of the detector data very difficult. The low-power transmitter and the plastic scintillator detector failed September 3, 1958. The two Geiger-Müller tubes and the caesium iodide crystal detectors continued to operate normally until September 19, 1958. The high-power transmitter ceased sending signals on October 5, 1958. It is believed that exhaustion of the power batteries caused these failures. The spacecraft decayed from orbit after 454 days on October 23, 1959.
^Herlihy, Ed (Narrator). Project Argus — “Greatest Experiment”: 3 A-Blasts In Space (video). Universal International News. Event occurs at 29s. Retrieved September 9, 2012. "“To monitor the radiation shell in outer space, the satellite Explorer 4 was launched. And all of this in a secrecy not broken for six months.”"
Payloads are separated by bullets ( · ), launches by pipes ( | ). Manned flights are indicated in bold text. Uncatalogued launch failures are listed in italics. Payloads deployed from other spacecraft are denoted in brackets.
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