SKYTOP PROJECT - INFRARED MAPPING OF THE MOON

The SKYTOP instrument was a fully automatic balloon-borne astronomical observation system developed by Dr. Alvin H. Howell's team at Tufts University in the early 1960s. Equipped with interchangeable instruments at its focal plane, it was primarily employed for spectroscopic studies of celestial bodies such as the Moon, Mars, and Venus.

In the image at left we can see a picture of the system after landing during a test flight (click for more details). The core of the Skytop system was a 12-inch telescope mounted on a precision platform. This telescope was coupled to an interferometer or a spectrometer and supported by a sophisticated orientation and tracking system. Initial azimuthal alignment was provided by a north-seeking gyroscope. This alignment was refined and maintained using three gimbals: one for coarse azimuth and two for fine azimuth and elevation control. The fine controls were managed through a photoelectric guide telescope.

The coarse azimuth gimbal employed a simple gearmotor, while the azimuth and elevation gimbals used dual contra-rotating free gyroscopes. These gyroscopes, custom-built by Tufts for this application, combined reaction wheel and torque generation into one unit. Each spun at 12,000 revolutions per minute and generated 3.0 SI units of angular momentum. Gyroscope rotor precession generated the torque required to adjust telescope orientation. Although this method limited the system's operational time, it was extended using mechanical trimming: the elevation gyroscope was adjusted by a motor-driven traveling weight, and the azimuth gyroscope by twisting the parachute's shroud lines relative to the balloon using the coarse azimuth drive.

Vertical-axis torque was generated by two motor-driven wheels spinning at 12,000 rpm. Mounted on a common axis within a hinged horizontal shaft, they produced precession torque when the hinge bent in response to an amplified signal from the north-seeking gyro, introducing a vertical angular momentum component. Energy consumption for this payload rotation was minimal—only one-eighth watt-second per radian.

To mitigate balloon-induced rotation that could twist the 100-foot parachute train, a motor-driven coupling was used between the balloon and parachute. This was roughly aligned to magnetic north via a magnetic sensor, stabilizing the system's orientation.

For autonomous tracking, Skytop used a dual-tape guidance mechanism: one tape encoded azimuthal coordinate changes relative to north, the other for elevation. These incremental signals were amplified and sent to the telescope's drive motors, which assumed the gondola stayed vertically aligned. Ground-based corrections could be transmitted to adjust for discrepancies, such as gyro error due to north-south balloon drift.

The telescope's optical path was split evenly: one half of the beam fed the tracking system, while the other was directed into the interferometer. Tracking was maintained using photosensors that monitored light from the shared beam.

The total payload weight of the Skytop system, excluding ballast and parachute, was 450 pounds. The system achieved a tracking accuracy of approximately ±1 minute of arc.

Balloon launched on: 2/4/1966 at  
Launch site: Holloman Air Force Base, Alamogordo, New Mexico, US  
Balloon launched by: Air Force Cambridge Research Laboratories (AFCRL)
Balloon manufacturer/size/composition: Zero Pressure Balloon 5.000.000 cuft (1.0 Mil. Stratofilm)
End of flight (L for landing time, W for last contact, otherwise termination time): 2/4/1966
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 11 h 30 m
Landing site: 25 miles N of Mineral Wells, Texas, US
Payload weight: 1610 lbs

This mission obtained the first lunar infrared spectrum taken from above most of the Earth's atmosphere

• A Lunar Infrared Spectrum from a Balloon-Borne System Icarus, Volume 8, Issue 1, p. 486
• Balloon Flight Record Facilities for Atmospheric Research Nº 1, Fall 1966, Pag. 18
• Mid-Infrared Spectroscopic Observations of the Moon Philosophical Transactions of the Royal Society of London. Vol. 264, No. 1150 p.109
• Summary of Balloon Flights launched from H.A.F.B. NM (1962 thru 1987) U.S. Gov. National Archives