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| A Ranger 4 launch cover from "SpaceCraft", postmarked at Patrick Air Force Base with a machine cancel on April 23, 1962. |
Ranger 4 was a particularly significant—and unfortunate—mission in the early U.S. lunar exploration program. A failure in the spacecraft's central computer and sequencer prevented Ranger 4 from carrying out its planned operations. Its solar panels and other systems did not function properly, leaving the spacecraft unable to perform course corrections, transmit television images, or deploy its scientific package as intended. Nevertheless, the launch and trajectory were sufficiently accurate for Ranger 4 to reach the Moon, impacting the lunar far side at approximately 12:49:53 UT on April 26, 1962, at roughly 6,000 mph (9,600 km/h). Although the mission failed to achieve its scientific objectives, it was historically important because Ranger 4 became the first American spacecraft to reach another celestial body.
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| A Ranger 4 launch cover from "Artopages", postmarked at Patrick Air Force Base with a machine cancel on April 23, 1962. |
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| A Ranger 4 crash-landing cover, postmarked at Patrick Air Force Base with a machine cancel on April 26, 1962, commemorating the spacecraft's impact on the far side of the Moon. Although a computer failure prevented Ranger 4 from carrying out its scientific mission, it became the first American spacecraft to reach another celestial body. |
