On this day in space! April 27, 1961: NASA Launches Explorer 11

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On April 27, 1961, NASA launched Explorer 11, a satellite that contained the first gamma ray telescope to go to space.

On April 27, 1961, NASA launched Explorer 11, a satellite that contained the first gamma ray telescope to go to space. This marked the birth of space-based gamma ray astronomy.

Russia's Saylut 1, the world's first space station, is seen during preparations for its April 19, 1971 launch in this undated photo from the Russian space agency Roscosmos. (Image credit: NASA)

Before Explorer 11 launched to look for gamma rays in space, scientists were pretty sure that gamma rays were out there. However, they had to way of detecting these gamma rays, because they get absorbed in Earth's atmosphere.

Explorer 11 detected 22 cosmic gamma rays during its seven-month mission. These rays were coming from all over the place and didn't seem to point to any particular sources out in space. This observation became the first evidence of a uniform gamma-ray background in the universe.

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Hanneke Weitering
Hanneke Weitering
Contributing expert

Hanneke Weitering is a multimedia journalist in the Pacific Northwest reporting on the future of aviation at FutureFlight.aero and Aviation International News and was previously the Editor for Spaceflight and Astronomy news here at Space.com. As an editor with over 10 years of experience in science journalism she has previously written for Scholastic Classroom Magazines, MedPage Today and The Joint Institute for Computational Sciences at Oak Ridge National Laboratory. After studying physics at the University of Tennessee in her hometown of Knoxville, she earned her graduate degree in Science, Health and Environmental Reporting (SHERP) from New York University. Hanneke joined the Space.com team in 2016 as a staff writer and producer, covering topics including spaceflight and astronomy. She currently lives in Seattle, home of the Space Needle, with her cat and two snakes. In her spare time, Hanneke enjoys exploring the Rocky Mountains, basking in nature and looking for dark skies to gaze at the cosmos.