The Horrible Thing That Happened to Enos the Chimp When He Orbited Earth 50 Years Ago


Few remember the second chimp launched into space by the United States. Even fewer remember the terrible equipment malfunction that subjected the animal to 76 electric shocks in orbit.


The chimps of space -- Ham, the first primate in space, and Enos, the second primate (after Yuri Gagarin) to orbit Earth -- have a special place in our memories of NASA. These animals paved the way for the United States space program by convincing biologists that animals' bodies *and* minds could function in orbit.

But there was a dark side to the missions. The chimps were the first to be trained by "avoidance conditioning" during which electric shocks were administered to the soles of their feet when the animals responded incorrectly in carrying out simple tasks. So, for example, the animals would be presented with three shapes and were trained to pick out the one that was not like the two others. They made their selections by pressing one of three levers that corresponded to the three symbols. On problem one below, the chimp should press the middle lever. On problem two, the chimp should press the right lever, and so on. Scientists call these oddity problems.


After Enos was in orbit, his first battery of oddity problems went as well as could be expected. After 18 problems, Enos had received 10 shocks. But on his next battery of tests, the center lever malfunctioned as did the switch controlling which question was presented. Enos kept being presented the same problem -- number one above -- in which the correct answer required pressing the center lever, but his center lever was broken. Enos, strapped into a space module orbiting the earth, was subjected to 33 shocks in a row, no matter what he did. The chimp kept trying to press different levers, NASA researchers record, but he kept getting shocked. Mercifully, the test ended after 35 shocks, and Enos performed normally on the other tasks he was given.

But then, as per the preexisting schedule, he was presented with the oddity problem again. Just like the time before, the apparatus malfunctioned and Enos was shocked 41 times. Even NASA scientists were amazed that the chimp soldiered on, despite the horrible malfunction.

Note that the malfunctioning of the center lever, which resulted in the subject receiving 35 shocks on the second session of the oddity problem, did not disrupt his subsequent performance. ... And likewise, the 41 shocks received during the third oddity session did not affect performance during the subsequent fourth session of the CA-DA tasks. Certainly, following a malfunction of this nature, it might be expected that behavior would be disrupted, but this was not in evidence.

Eventually, Enos' flight ended and he came back to Earth. His capsule did not land where NASA anticipated, so he was stuck in the capsule for 3 hours and 20 minutes. By the time the USS Stormes crew extracted him, "The subject had broken through the protective belly panel and had removed or damaged most of the physiological sensors," a NASA report records. "He had also forcibly removed the urinary catheter while the balloon was still inflated."

A little less than a year later, Enos died of dysentery. We know his body was inspected, but the location of his remains is unknown.

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Alexis C. Madrigal

Alexis Madrigal is the deputy editor of He's the author of Powering the Dream: The History and Promise of Green Technology. More

The New York Observer has called Madrigal "for all intents and purposes, the perfect modern reporter." He co-founded Longshot magazine, a high-speed media experiment that garnered attention from The New York Times, The Wall Street Journal, and the BBC. While at, he built Wired Science into one of the most popular blogs in the world. The site was nominated for best magazine blog by the MPA and best science website in the 2009 Webby Awards. He also co-founded Haiti ReWired, a groundbreaking community dedicated to the discussion of technology, infrastructure, and the future of Haiti.

He's spoken at Stanford, CalTech, Berkeley, SXSW, E3, and the National Renewable Energy Laboratory, and his writing was anthologized in Best Technology Writing 2010 (Yale University Press).

Madrigal is a visiting scholar at the University of California at Berkeley's Office for the History of Science and Technology. Born in Mexico City, he grew up in the exurbs north of Portland, Oregon, and now lives in Oakland.

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