[Illustration: _Enlarged portion of a typical pen record of flashes of sunlight from Telstar mirrors, showing a burst of 21 glints from the 68° mirror recorded at 03:40:58 Greenwich Mean Time on August 9, 1962. Synchronizing vibration mark seven lines above the recorded burst indicates the time 02:59:00. Measuring the horizontal distance between consecutive sawtooth marks tells us that the spin rate is between 163 and 164 revolutions per minute. (Precise measurements of the oscilloscope traces fixed the exact spin rate at the time of this burst at 163.64 revolutions per minute.)_] Results Telstar I was launched on July 10, 1962. That evening, beginning on the satellite’s seventh pass, we were able to detect trains of flashes from the mirrors. We assumed that, since Telstar had been launched almost exactly according to plan, its spin axis would be perpendicular to the plane of the earth’s orbit, and we calculated when we should see the flashes. And, each time, we actually saw them within two minutes of the times we had predicted—so we knew that the spin axis was almost exactly where it should be. Our measurements have continued whenever the weather and other conditions permitted. Combining readings from the bursts of flashes and telemetry data from the solar aspect cells, we have accurately plotted Telstar’s spin axis; it has continued to precess very much as we predicted it would. We have also seen what happens to the spin axis when the satellite’s torque coil is turned on. And, by measuring the intervals between flashes, we have made very precise measurements of the spin rate, which is gradually decreasing mostly according to schedule. However, the plot is showing some small unexplained variations of spin decay rate, and a study of them will, we hope, throw light on some of the variations of the earth’s magnetic field. For future communications work, particularly with satellites at longer ranges, it would seem to be preferable to use stiffer, flatter mirrors and to make them from beryllium rather than aluminum alloy. More accurate tracking means, more observatory sites, and more powerful telescopes will also be needed. But for this first experimental use our little mirrors have worked very well. Jeofry S. Courtney-Pratt _was born in Hobart, Tasmania, Australia, and received a Bachelor of Engineering degree from the University of Tasmania in 1942 and a Ph.D. from Cambridge University in 1949. He was also awarded an Sc.D. by Cambridge in 1958. He joined Bell Telephone Laboratories in 1958, and has done research in high-speed photography, optics, optical masers, the properties of materials, and the physics of the contact of solids._ CASE HISTORY NO. 4 How Do We Keep Solar Cell Power Plants Working in Space? Kenneth D. Smith _Electronics Engineer—Member of Staff, Semiconductor Device Laboratory_ THE PROBLEM