Satgen364 Sunspots and Satellites by GM4IHJ 16th March 1996 Way back in the ancient history of amateur satellites, the then current pair of Amsats, orbited the earth at about 1700 kms altitude. Orbital timing was so stable that everyone used a printed orbit calendar. Each year, Project Oscar produced an excellent 365 day record of all the satellite equator crossing times for every day of the year. All would be users had to do, was get out their Oscar Locator, turn the orbit track to EQX time, and read off the AZ and EL. Then in 1978, the first signs of a problem began to appear. By September the calendar was 2 minutes late. A situation which was worrying but presented little operational difficulty. By mid 1979 hower things were much worse . The error was over 7 minutes and getting larger, rapidly. Clearly something was perturbing the Oscar 7 orbit, but no one had much of a clue what it was. With hindsight however, it is easy to see where the problem lay. During the first (1968/69) occurence of Solar Sunspot maximum in the satellite era, there were very few users and few if any had accurate satellite tracking equipment. But as the next solar cycle peak approached in 1979 , amsat users were numerous and they had got used to getting stable long term orbit predictions during the previous solar quiet decade. So it came as something of a shock to find that the atmosphere up at 1700 kms could periodically, become dense enough to affect a satellites orbit. Where had this extra density come from ? It had of course come from the Sun. In fact, the earths atmosphere at 1700 kms is almost entirely the product of the Solar Wind, the stream of particles ( mostly Hydrogen) which is constantly being thrown out by the Sun. Equally important the density of the Solar wind is highly variable, being consistently higher at the Sunspot cycle peak. Fortunately for amsat users help would shortly be at hand , with the arrival in the ham shack of the first micro computers. Initially primed with equatorial Crossing data, but very soon going over to Keplerian orbital elements, as computers and software became more sophisticated. Such that by the next Solar cycle maximum around 1990 no one noticed that there had been a problem. Recent events in 1996 however , suggest that we are capable of forgetting our history too quickly. Listening to both the Amateur and the Professional forecasts of the expected re entry date of the errant Chinese satellite FSW1-5 Jianbing, suggested that frequent reference was being made to the previous examples of satellite orbit decay such as Salyut 7 and Cosmos 1686. Readers may remember that Salyut 7 was deliberately elevated up to a " supposedly safe parking orbit around 480 kms altitude " , but it then positively refused to stay there and quickly descended re entered the lower atmosphere and mostly burnt up. Unlike Salyut however Jianbing was not coming down anywhere near Solar cycle maximum. Its decay rate was slower because the atmosphere was much less dense than it had been at the time of Salyuts demise. Not surprisingly therefore, there were a lot of Jianbing re entry predictions which proved to be far too early. The moral being - Do not forget your history, even for this youngest of sciences.