Sixty years ago, on November 29, 1961, Enos became the first chimpanzee to orbit the Earth. He flew on NASA’s Mercury-Atlas 5 (MA-5) mission, which the relatively new space agency deemed necessary before orbiting an astronaut in a Mercury capsule. But Enos is little remembered today. Ham, his compatriot from the U.S. Air Force’s chimp training facility in New Mexico, overshadows him. Launched on Mercury-Redstone 2 on January 31, 1961, Ham made a publicity splash after NASA stipulated his short, suborbital trip, which NASA required before the United States could put its first person in space. Doctors had a lot of (exaggerated) concerns about the impact of spaceflight on the human body and psyche; engineers wanted to prove the safety of the Mercury capsule. Ham, who was visibly agitated by his rough flight and by the flashing press cameras afterward, became a celebrity.

By the time Enos flew, the U.S. once again was embarrassingly behind the Soviet Union in spaceflight spectaculars. Two cosmonauts had circled the Earth that year: Yuri Gagarin, the first human in space, and Gherman Titov, who had orbited for a full day in August 1961. Two American astronauts, Alan Shepard and Gus Grissom, had also gone into space, but they had only made short suborbital journeys like Ham. Project Mercury’s transition to the larger Atlas Intercontinental Ballistic Missile (ICBM), which would put astronauts into orbit, was lagging. During the first launch attempt that year, 13 days after Gagarin’s triumphant mission on April 12, 1961, the range safety officer blew up the uncrewed Mercury-Atlas 3 (MA-3) when the rocket’s guidance system failed. Fortunately, the launch escape system salvaged the Mercury capsule, which was re-flown successfully on Mercury-Atlas 4 (MA-4) in mid-September. The spacecraft carried a “spaceman simulator”—a device to test the cabin environmental control system. The question then became, could an astronaut fly on the next mission, MA-5? At least the United States could orbit someone in the same year as the Soviets.

The chimpanzee "Enos" made two orbits in this capsule on the Mercury-Atlas 5 (MA-5) mission. (Smithsonian Institution)

The MA-5 capsule is currently on loan at the North Carolina Museum of Life and Science in Durham, North Carolina.

Caution once again prevailed, as NASA’s confidence in the capsule and the rocket were not very high. The MA-4 flight had worked, but there were numerous technical problems with the spacecraft and the booster that needed investigating and fixing, delaying the next mission. The doctors were also still worried about the effects of extended weightlessness on an astronaut and the engineers wanted another test of the cabin environment. Despite questions from the press and from the office of NASA Administrator James Webb, Project Mercury officials under Robert Gilruth confirmed that MA-5 would be a chimp flight. In October 1961, personnel from Holloman Air Force Base near Alamogordo, New Mexico, brought three more chimpanzees to Cape Canaveral to join two already there. One of the new arrivals was Enos, a native of Cameroon in west central Africa, who had been purchased by the Air Force from a Florida wildlife center in 1960. The chief veterinarian described him as "quite a cool guy and not the performing type at all."

Enos looks relaxed as he is prepared for launch. (NASA)

Technical problems pushed the launch back several weeks. Three days before the new target date, November 29, 1961, the Air Force team designated Enos as the prime candidate for NASA’s mission, with several of his compatriots as backups. Ninety minutes before the planned launch time, he was inserted into the Mercury spacecraft inside his pressurized “primate capsule.” Launch day was trying, with two-and-a-half hours of delays in part caused by the pad crew’s mistakes; NASA mission director Walt Williams, not known for his even temper, became so angry he left the Mercury Control building and drove to the launch complex to impress on the crew the need for care as well as speed. Finally, at 10:08 am, the Atlas lifted off, and five minutes later injected the MA-5 capsule into an orbit of 99 miles (160 km) by 147 miles (237 km). Enos experienced “G” forces of nearly eight times Earth’s gravity before the rocket burned out. Now he was weightless, but strapped tightly inside his box.

Launch of Mercury-Atlas 5 (MA-5) carrying Enos. (NASA)

In front of Enos were three levers and several lights. Because he was to fly a three-orbit mission lasting nearly five hours—the prime objective for an astronaut in Project Mercury—he had a more elaborate set of cognitive tests than Ham, whose suborbital flight lasted 18 minutes. Enos had four different problems to solve, such as pulling levers to turn off lights. The problem cycle lasted 12 minutes, followed by six minutes of rest. One task provided a chance, if performed correctly, to get a drink of water, another, banana pellets. The other two problems had a punishment for mistakes: a mild (at least that’s how the trainers described it) electrical shock on his left foot. But that system began to malfunction on the fourth problem, with the result that he was shocked every time he pulled on the central lever, whether he was correct or not. He ended up being shocked dozens of times in a row, yet kept working as trained.

Meanwhile, problems began to pile up with the Mercury capsule after a smooth first orbit. The environmental control system began to overheat; eventually Enos’s body temperature rose to 100.5 F (38 C) before stabilizing. Ground controllers also began to notice excessive fuel usage in the attitude control system. Due to a failed thruster, the capsule would drift until the automated system would fire other jets to put the spacecraft back into the programmed attitude. The cycle kept repeating. With the end of the second orbit approaching and only seconds to spare to get the spacecraft down in the Atlantic Ocean landing area, flight director Christopher Kraft told the Point Arguello, California, ground station to fire the retrorockets. A recovery aircraft spotted the capsule descending on its parachute and circled it until a Navy ship came to pick it up. After another delay, U.S. Navy seamen finally got aboard, fired the explosive hatch, and extracted Enos from his box. But he had become angry and frustrated at the three-hour wait and had broken out of his restraints inside the box, ripped off his medical sensors, and pulled out his urinary catheter. Afterwards he returned to his usual calm and enjoyed some fresh fruit after many hours of only water and banana pellets, but his experience had been bad enough that NASA was not very frank with the press about what had happened.

Enos’s celebrity was brief. At the post-flight news conference, Bob Gilruth announced that John Glenn was the prime pilot for Mercury-Atlas 6 (MA-6), with Scott Carpenter as his backup. Media attention quickly became focused on when Glenn would launch. Enos went home to the Holloman facility, but died a year later of a severe form of dysentery unresponsive to antibiotics. After an autopsy, his body was supposedly sent to the Smithsonian, but no trace of what happened to it has been found. He was a pioneer for human space travel, but one might well question the necessity of his sacrifice today.


Michael J. Neufeld is a senior curator in the Museum’s Space History Department and is responsible for Mercury and Gemini spacecraft, among other collections.

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