Editing Explorer 4 (section)

== Mission ==
Explorer 4 was launched on 26 July 1958 at 15:00:07 GMT from the Cape Canaveral Missile Test Center of the [[Eastern Range|Atlantic Missile Range]]. The spacecraft was injected into an initial {{cvt|263|xx|2213|km}} orbit with an inclination of 50.30° and a period of 110.20 minutes at 15:07 GMT.<ref name="Trajectory">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/displayTrajectory.action?id=1958-005A|title=Trajectory: Explorer 4 1958-005A|publisher=NASA|date=28 October 2021|access-date=12 November 2021}} {{PD-notice}}</ref> This was a much higher inclination and apogee than previous Explorer to allow it to sample more area at higher altitudes. Soon after orbit insertion, the spacecraft developed an end-over-end tumbling motion with a period of about 6 seconds, which affected the measurements and signal level throughout the mission.

The mission remained secret from the public for six months.<ref>{{cite video|people=Herlihy, Ed (Narrator)|title=Project Argus — "Greatest Experiment": 3 A-Blasts In Space|url=https://www.youtube.com/watch?v=J-A1wQ_qo2c |archive-url=https://ghostarchive.org/varchive/youtube/20211211/J-A1wQ_qo2c| archive-date=2021-12-11 |url-status=live|medium=video|publisher=Universal International News|access-date=September 9, 2012|quote="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".|time=29 seconds}}{{cbignore}}</ref> The satellite telemetry was analyzed for three [[Operation Argus]] nuclear weapons tests at high altitude. Explorer 4 was in orbit and operational during the three Project Argus launches 27 August 1958 to 6 September 1958, part of the mission objective was to observe the effects of these high-altitude [[Nuclear weapon|A-bomb]] detonations on the space environment.<ref name="Display">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1958-005A|title=Display: Explorer 4 1958-005A|publisher=NASA|date=28 October 2021|access-date=1 November 2021}} {{PD-notice}}</ref>

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 on 3 September 1958. The two [[Geiger-Müller tube]]s and the caesium iodide crystal detectors continued to operate normally until 19 September 1958. The high-power transmitter ceased sending signals on 5 October 1958. It is believed that exhaustion of the power batteries caused these failures. The spacecraft decayed from orbit after 454 days on 23 October 1959.<ref name="Display"/>