On November 28, 1961, an 11 1/2-hour launch preparation count began for MA-5. The count stopped at T minus 390 minutes, to be resumed the next day. Some 11 hours before the launch, Enos, the 39-pound chimpanzee, underwent his final physical examination, stood still as his medical sensors were taped on, allowed himself to be secured in the specially constructed primate couch, and rode in the transfer van to the gantry. About 5 hours before launch the couch  was inserted in the spacecraft. Thereafter Enos' condition was monitored by lines connected to his couch in the Mercury capsule and by radio telemetry. He was relaxed during countdown. His temperature ranged from 97.8 to 98.4 degrees, normal for the suit inlet temperature of about 65 degrees; his respiration averaged 14; and his pulse rate was 94. The only time Enos displayed agitation was when he was roused by the opening of the hatch during a countdown hold caused by a telemetry link failure at T minus 30 minutes. The gantry was hauled back to the spacecraft, the hatch was opened, and an off-and-on switch was correctly positioned. This hold lasted 85 minutes. Some members in the control center joked that Enos had turned the switch off because he had talked to Ham and did not want to go.52
In the Mercury Control Center the flight control monitors had manned their stations and were busily checking out their consoles. Tecwyn Roberts, serving as flight dynamics officer, noted the intermittent problems cropping up in the data-gathering and translating computers. A faulty transistor in the direct data receiver caused one hold, and when the replacement was also faulty, several more minutes were lost in repairing the computer. Morton Schler, the capsule environment monitor, reported that the environmental control system was working smoothly. The Freon flow rate, he reported, leveled at a comfortable 20 pounds per hour in the prelaunch period. From the oxygen partial-pressure transducer some erratic readings proved erroneous; Mercury Control teletyped the tracking stations to discount these readings as the spacecraft passed over.53
Holds during the countdown amounted to almost 2 hours and 38 minutes. Shortly after the hatch was bolted at T minus 90 minutes, the technicians discovered that they had failed to install some hatch cover heat insulation material. They took a little more than an hour to correct this oversight. Then, at T minus 30 minutes, the discovery of an improperly positioned switch necessitated the 85-minute hatch-opening hold. And finally, at T minus 15 minutes, a 4-minute hold was called to correct a data-link problem between Mercury Control and the General Electric ground command guidance equipment.
Walter Williams, the mission director, listened as the various difficulties arose and became somewhat agitated at the chain of events. Although his usual position during such times was at a console in the mission control center, he left the building and quickly drove the distance to pad 14 to personally express his expectations that things would proceed in a more orderly manner. As a member of Convair later said, "Williams was a master in imparting a need for orderly urgency."
Despite these holds, weather conditions remained favorable. Only a few thin cirrus clouds hung in the sky, visibility was 10 miles, and the surface wind velocity was at a moderate 11 miles per hour from the northwest. In the landing area the weather was even better.54
The Goddard computers received the liftoff signal 13 seconds before the booster actually rose from the pad, an error apparently caused by feedback between two recorders. Nor was this the last incorrect signal.  The Goddard computers registered sustainer engine cutoff twice before that event happened, once shortly after liftoff and again two minutes after launch. In each case the Mercury Control Center had to switch to override the signal until the panel indicator cleared.55 Liftoff came at 10:08 a.m. The powered phase of the flight went well, although there were minor discrepancies. Between liftoff and staging, the horizon-scanner signal was lost briefly. All spacecraft systems nevertheless appeared to be working normally, with the guidance system of the Atlas keeping the booster on an almost perfect insertion trajectory. Guidance system noise was only about half that recorded during MA-4, and vehicular vibration also was much lower. Four and a half minutes after launch, Christopher C. Kraft, Jr., unhesitatingly made the go-for-orbit decision. At sustainer engine cutoff, the velocity, flight angle, and altitude were nearly perfect. The Atlas hurled the spacecraft into an orbit with a perigee of about 99 miles and an apogee of 147 miles, .5 and 5.4 miles low, respectively.56
Spacecraft and booster separation occurred precisely as planned, while the turnaround maneuver took less than 30 seconds. The capsule's position excursions were very slight, which contrasted sharply with the erratic turnaround of MA-4. The spacecraft quickly dropped into its 34-degree orbit mode and began streaking over the oceans and continents. Of the 61.5 pounds of control fuel aboard, turnaround and damping had consumed 6 pounds, as opposed to 9.5 for MA-4. From that point and on through the first orbit the thrusters used only 1.5 pounds to maintain a correct position, with the automatic stabilization and control system functioning perfectly.
The environmental control system and the tracking and communications network performed in a satisfactory fashion. On this mission, for the first time, the primary and secondary oxygen bottles were pressurized at 7,500 pounds per square inch (the design specification) rather than at 3,000, as on previous flights. A functioning water separator also was used for the first time. Each tracking station's range on the ultra-high-frequency band lasted for about six minutes; on high frequency, overlap communications between stations were similar to that experienced during MA-4. The Goddard computers received valid telemetry data from all stations except Woomera, but there were instances when communications were momentarily lost at particular stations. Just before retrofire, for example, Point Arguello, the site giving the firing command, lost contact with the Cape. In each instance, as Walter Williams would point out at the postflight press conference, communications were reestablished whenever that particular station was needed.57
Enos, the orbiting chimpanzee, fared well. He withstood a peak of 6.8 g during booster-engine acceleration and 7.6 g with the rush of the sustainer engine. He had been performing his lever-pulling duties for some two minutes before the Atlas roared and rose from the pad. During his two orbits he made 29 pulls (divided among four sessions) on the continuous-avoidance and discrete-avoidance levers, receiving only one shock in each category. On his second problem, which  required at least a 20-second delayed response to receive a drink of water, his average delay was about 33.8 seconds. For these labors he was rewarded with a total of 47 measures of water, or about a pint during the three-hour mission. For the fixed-ratio task, problem three, Enos pumped his lever and received 13 banana pellets during his four opportunities. On the first session of problem Four, Enos was correct for 18 out of 28 symbol presentations (64.2 percent), thus receiving 10 shocks as a result of his miscues. On the second session of problem four, however, the center lever malfunctioned, causing shocks even if he pulled the correct lever. He received 36 and 43 successive shocks on the third and fourth sessions, respectively, because a manmade device had failed. The shocked and frustrated chimpanzee nevertheless kept pulling the levers. As he was also trained to do, Enos remained at rest during the six-minute intervals between problems.58
Near the end of the first orbit, the tracking monitors noted that the capsule clock was about 18 seconds too fast and as it passed over the Cape a corrective command was dispatched to and accepted by the clock. At that time the Mercury Control Center display panels indicated that all spacecraft systems were in good order. Suddenly the Atlantic tracking ship reported that inverter temperatures were rising. The Canary Island trackers confirmed the environmental control system malfunction. Since abnormal heating had occurred on earlier flights and the inverters had continued working or had switched to standby, there was no alarm among members at Mercury Control. Then, across the world from the Cape, Muchea, Australia, detected high thruster signals and capsule motion excursions, although other data indicated that the 34-degree orbit mode was being maintained. The Woomera, Australia, tracking station failed to confirm this report, and it was discounted.59
By the time the MA-5 capsule reached the vicinity of the Canton Island station, the operations team realized that the attitude control system was allowing the vehicle to go out of its proper orbital mode. A metal chip in a fuel supply line (the postflight inspection would reveal) had cut off the propellant flow to one of the clockwise roll thrusters. This inactive thruster allowed the spacecraft to drift minus 30 degrees from its normal attitude, at which point the automatic stabilization and control system brought the spacecraft to zero in a normal roll-turn maneuver. Then the spacecraft swung briefly back into the nominal 34-degree orbital attitude, and the sequence started again. The spacecraft repeated this process of drift and correction nine times before retrofire and once more between retrofire and the receipt of the .05-g light telemetry signal. The still-active thrusters used about 9.5 pounds of control fuel working to keep the capsule properly aligned. Each loss of orbit mode cost a little over a pound of fuel compared with a first-orbit expenditure of only 1.5 pounds.60
The cooling equipment in the environmental control system also began to give trouble during the second orbit. Between the Canary Islands and Kano, the suit circuit temperature rose rapidly from 65 to 80 degrees, indicating a freezing condition in the heat exchanger. As water in the felt evaporator pad of the  exchanger turned to ice, Enos' body temperature climbed to 99, then to 100 degrees. The medical observers began to worry, especially about the chimp's ability to handle his psychomotor test problems. Then, at 100.5 degrees, his body temperature appeared to stabilize, suggesting that the environmental system was ceasing to overheat. Their fears relieved, the physicians felt that the mission could continue. Although the cooling system had seemed to correct itself, Kraft, the flight director, later remarked that a deicing unit should be added to warm the troublesome unit, which had also caused a freeze-up on MA-4.61 Although the medical monitors were willing to allow the mission to proceed through its scheduled third pass around the world, the operations team believed that the problem of the spacecraft's erratic attitude was too grave to live with. The engineers felt that there simply was not enough attitude fuel left to complete the circuit and then go through the reentry phase, in which, even under normal circumstances, fuel usage would be high.
After the attitude aberrations were first noted, Kraft had alerted the tracking unit in Hawaii for a possible clock change to initiate retrofire during the second orbit. Then he decided to continue the flight toward California and notified Gordon Cooper at Point Arguello that that station might have to initiate a ground command for retrofire. Meanwhile, the capsule continued to drift and swing in and out of the orbital mode, demonstrating that the attitude control system, unlike the environmental control system, would not solve its own problems. Twelve seconds before the retrofire point was reached for the normal second-orbit primary recovery point, Kraft decided to bring Enos back to Earth. Arnold Aldrich, MSC's chief flight controller at Point Arguello, correctly executed the command.62
With the exception of the one repeated variation in attitude position, caused by the dead roll thruster, reentry went according to plan. The destroyers Stormes and Compton and a P5M airplane began preparing for spacecraft retrieval in Station 8, the predicted impact point. Three hours and 13 minutes after launch and about nine minutes from water impact, the P5M spotted the descending spacecraft at an altitude of about 5000 feet and radioed the Stormes and the Compton, 30 miles away. All spacecraft recovery aids except the sarah beacon functioned properly. During the spacecraft's descent, the airplane circled and reported landing events, then remained in the area until the Stormes arrived, an hour and 15 minutes after the landing, and hauled Enos and his spacecraft aboard. Shortly thereafter the hatch was explosively released from outside the capsule by a pull on its lanyard, causing the chimp's "picture" window to crack.63
Aboard the Stormes and later at Cape Canaveral, Manned Spacecraft Center and McDonnell engineers gave the capsule the usual close scrutiny and happily found that it had held up well. Except for a slight discoloration caused by aerodynamic heating, the exterior showed no buckling or warping. The interior was in good shape, too, although the inspectors did find a small amount of salt water. Activation of the explosive hatch caused minor damage in the form  of the cracked window, several bolts pulled from the skin, and a slight buckle. Thermodynamic effects on the ablation heatshield had produced several radial and circular cracks, none of which had been severe enough to threaten the capsule's structural integrity. The center plug of the heatshield was missing (it had only worked loose on MA-4 ), but close inspection of the opening showed that the plug had evidently been in place during reentry. Condition of the impact bag, which had survived its first orbital test, was fairly good, although several straps were broken and others were severely bent. Again the plastic bulkhead was pierced, probably by the heatshield, and the honeycomb material was crushed in several places. There was no damage to the tubing or wiring in this area.64
At the Cape postflight press conference the leaders of Project Mercury revealed no regrets over missing a third orbit. They seemed to regard the reprogramming operation, conducted in the middle of the mission, as a satisfying technical accomplishment. In view of the decisiveness with which the various potentially critical difficulties had been overcome or circumvented, MA-5 had to be termed an excellent operation, one that had achieved most of its objectives and that would become a milestone on the road into the unknown.
Enos had been weightless for 181 minutes and had performed his psychomotor duties with aplomb. Operations director Williams felt that an astronaut riding in the MA-5 spacecraft could have made the necessary corrections in flight to complete the three-orbit mission normally. On the spacecraft attitude control problem, for example, a man could simply have switched from automatic to manual mode, he said. At the same time, Williams was pleased that the automatic systems had worked well for over two hours. Equally significant, the vast network of NASA, military, and industry personnel had performed like veterans during the emergency. The spacecraft had reentered and landed without handing the Navy any unexpected recovery problems.
50 "Postlaunch Memorandum Report for Mercury-Atlas No. 5."
51 William Hines in the Washington Evening Star for Nov. 19, 1961, said it would be virtually impossible for the United States to make a manned orbital flight in 1961. On the other side, presenting an optimistic view, see Edward H. Kolcum, "Chimp Shot Raises Hope that U.S. Can Orbit Man Before Year's End," Aviation Week, LXXV (Dec. 4, 1961).
52 "Postlaunch Memorandum Report for Mercury-Atlas No. 5."
53 Memo, Roberts to Flight Dir., "Report on Test 1810 (MA-5)," Dec. 5, 1961; memo, Schler to Flight Dir., "Report on Test 1810 (MA-5)," Dec. 4, 1961.
54 "Postlaunch Memorandum Report for Mercury-Atlas No. 5." Communications with the tracking stations were very good during countdown, and there was little interference. Curiously, however, there was a brief period of interference from Radio Moscow just before liftoff. "Debriefing-Test 1810," anon., Nov. 29, 1961.
55 Roberts memo.
56 Memo, Christopher C. Kraft, Jr., "Flight Director's Report on Test 1810 (MA-5)," Nov. 30, 1961; "Postlaunch Memorandum Report for Mercury-Atlas No. 5"; "Debriefing - Test 1810." At his press conference in Washington, President Kennedy got a round of laughter when he said, "This chimpanzee who is flying in space took off at 10:08. He reports that everything is perfect and working well." Baltimore Sun, Nov. 30, 1961.
57 Ibid.; NASA News Release, "MA-5 News Conference," Nov. 29, 1961. Williams, in interview Aug. 23, 1965, recalled that communications with California had been disrupted momentarily by a tractor somewhere in Arizona that plowed up a telephone cable.
58 Stingely and Mosely, "MA-5 Operations," 41-50; "Postlaunch Memorandum Report for Mercury-Atlas No. 5." On the continuous avoidance, discrete-avoidance problem, Enos received his first shock of the first sessions about 15 minutes from launch and the second at the 201-minute point (after he had been weightless for 3 hours). He then pulled the lever correctly for the last 3 presentations before the psychomotor device turned off 207 minutes after launch.
During the first orbit, in the range of the Zanzibar tracking site, Mercury surgeon White noted that Enos' ventricular contractions had become more rapid. This White believed to be normal for the postacceleration period. The chimpanzee's respiration rate had risen with the onset of flight and the increase in his activity. His respiration rate was 21 and his pulse 122 during this phase, as compared with preflight rates of 14 and 94.
59 "Debriefing-Test 1810"; memo, Donald D. Arabian to Flight Dir., "Report on Test 1810 (MA-5)," Dec. 6, 1961; Kraft memo.
60 Ibid.; "Postlaunch Memorandum Report for Mercury-Atlas No. 5."
61 Ibid.; Schler memo.
62 Kraft memo; Arabian memo; "MA-5 News Conference." The spacecraft used 14.5 pounds of control fuel from retrofire to fuel jettison. Thirty pounds of fuel were dumped when the main parachute deployed.
63 Kraft memo; "Postlaunch Memorandum Report for Mercury-Atlas No. 5"; "Unofficial Record of Events - MA-5, November 28 [sic], 1961," anon. On Nov. 2, 1961, Low remarked to Purser that MA-5 should be announced as a one-orbit mission that might be allowed to go three orbits. (Purser, log for Gilruth, Nov. 7, 1961.) MA-5's total recovery force, for the support of aborts, primary, and contingency landing areas consisted of 17 ships and 13 airplanes.
64 "Postlaunch Memorandum Report for Mercury-Atlas No. 5." The drogue and main parachutes were not recovered, but the Earth-sky camera confirmed that they had functioned without damage. The drogue deployed at 21,000 feet and the main chute at 10,000.