Orbiting Frog Otolith

The flight hardware unit, the FOEP, was a pressure-tight canister containing a water-filled centrifuge that housed the two frogs. The centrifuge was a cylindrical structure that rotated the frogs' heads at scheduled intervals.[3] The FOEP also contained a life support system which could maintain a regulated environment for the frogs. This system consisted of two closed loops, one containing liquid and the other containing gas. The interface between the two loops was a selectively permeable silicone rubber that acted as an artificial lung. Oxygen passed through the membrane from the gas to the liquid side, and carbon dioxide from the liquid to the gas side. The frogs were immersed in the liquid loop. A pump circulated oxygen through the gas-containing loop. Carbon dioxide entering the gas loop was removed by an absorbent and the purified oxygen returned to the pump for recirculation. A water evaporator and an electric heater maintained the water temperature at about 60 °F (16 °C). An amplifier system in the FOEP increased voltage output from the microelectrodes implanted in the animals to the level required by the telemetry apparatus.

A drawing of how a bullfrog fitted with electrodes was to sit inside the centrifuge of the Frog Otolith Experiment Package

Surgical preparation of the flight frogs was completed about 12 hours before launch, and the animals were sealed inside the FOEP. A backup FOEP was also prepared with similar specimens. The flight FOEP was installed in the satellite about three hours before launch.

The centrifuge was activated as soon as possible once the satellite was in orbit and stabilized at 10⁻³g (10 mm/s²). The centrifuge applied gravity stimuli in cycles. Each cycle lasted about 8 minutes, and consisted of the following: a 1-minute period without acceleration, an 8-second period when rotation slowly began, 14 seconds of constant 0.6 g (6 m/s²), an 8-second period when rotation slowly stopped, and a 6-minute period when aftereffects of rotation could be measured.[3] Cycles were performed every 30 minutes during the initial 3 hours in orbit, and less frequently during the rest of the flight.

The OFO experiment continued until the seventh day in orbit, at which time the onboard battery failed. Recovery of the OFO spacecraft and FOEP hardware were not required. The two frogs died as part of the experiment.

The experiment was successful. Electrocardiography (ECG) indices showed the flight frogs to be in good health during the entire flight. Vestibular recordings were made as expected. Two equipment malfunctions occurred during the flight: pressure in the canister increased to 11 pounds per square inch (76 kPa), and the temperature decreased to 55 °F (13 °C) for nine hours. However, control experiments performed on the ground showed that these malfunctions had little effect on the outcome of the flight experiment.

Several vestibular response changes were noted during the early period in weightlessness. All of the observed changes reverted to normal during the last 10 to 20 hours of the flight, suggesting acclimatization.[3]

The outer housing of the FOEP is a pressure-tight canister 18+1⁄16 inches (459 mm) in diameter and 18+1⁄2 inches (470 mm) long. The bottom closure and removable top lid are both slightly domed to prevent implosion should pressure reversals be encountered. The inner assembly structure is fastened to a support ring approximately 6 inches from the bottom of the canister and consists of upper and lower bulkheads joined by a cylinder. Cutouts in the cylinder permit access to the centrifuge, which houses the frogs. Near the top of the canister are two electrical feed-through receptacles for the power supply and data line.

The centrifuge is a hollow cylinder 6 inches in diameter and 13.5 inches long with both end caps in place. The cylinder is mounted perpendicular to the canister and supported by ball bearings housed in the upper and lower bulkheads. The rotational axis of the centrifuge is formed by shafts centrally located in the vertical plane at right angles to the cylinder, held in place by the ball bearings. Thin, shallow-domed end caps are bolted to each end of the centrifuge with intervening rubber gaskets to prevent leakage. In the center of each cap is a fitting which allows frog specimens to be fully instrumented and mounted directly to the end caps before insertion into the centrifuge, and immersion. The water serves as a cushion for the high accelerations and vibrations of launch and as a medium for gas exchange via the frogs' skin. The centrifuge is locked in position and not released until after the spacecraft orbit is fully stabilized. The motor which drives the centrifuge is mounted to the upper bulkhead. Signal amplifiers and an accelerometer are mounted on the centrifuge.

The micro-electrode consists of a probe of tungsten wire 50 µm in diameter, sharpened electrically to a point less than 1 µm in diameter and completely insulated to the tip. A bubble of air trapped in the polyethylene tubing which contains the probe adds buoyancy and makes the electrode the same density as the nerve in which it is implanted, thereby allowing the two to move together. A section of paraffin is used to connect the electrode to a handle which is used only during the implantation process, then removed. Nerve impulses detected by the microelectrodes are fed into a preamplifier directly attached to the frog's jaw, and passed on to a post-data amplifier for spacecraft telemetry.

The interface between loops occurs at a selectively permeable membrane of silicon rubber which separates the liquid and gas. This membrane, called the lung, passes oxygen from the gas loop to the liquid loop, and CO₂ from the liquid loop to the gas loop.

The frogs, housed in the centrifuge, are in the liquid loop. Moving from the lung to the frogs, the loop contains water and dissolved oxygen; moving from the frogs back to the lung, it contains water and free CO₂. A double layer of polyurethane foam lining the interior of the centrifuge prevents frog waste matter from fouling the water circulation system. Water is circulated through the liquid loop using a small pump and must pass through the filter before leaving the centrifuge.

The gas loop consists of a circuit in the lower bulkhead through which oxygen is circulated by a small pump. The pump delivers pure oxygen to the lung where some of it passes into the liquid loop, while the remainder becomes mixed with the CO₂ coming from the liquid loop. From the lung, the oxygen-CO₂ mixture is passed through a bed of Baralyme which absorbs the CO₂. Pure oxygen is returned from the Baralyme to the pump and recirculated. The oxygen supply is replenished by gas from the small oxygen tank.

Augmented by the thermal environment of the spacecraft, the water evaporator and 8 Watt electric heater will maintain water temperature at 60±5 °F (15.5±3 °C). The water supply for the evaporator is contained in a rubber bladder supported by a ring in the canister immediately above the lower dome. When water temperature exceeds the nominal 60 °F, a ground command actuates a timing circuit operating a valve. As a result of the ambient pressure inside the canister, water is forced from the bladder through the valve and into the evaporator. Internal heat loads are transferred through a heat exchanger to the evaporator and are dissipated in evaporating the water.