Satgen119 Oscar 13 Heading for a Fall by GM4IHJ 6th July 91 On 22nd of June 91, Oscar 13's perigee orbital low point nearest earth had dropped to 850 kms. In August 88 it had been 2546 kms. A decline of 1696 kms in under 35 months. If Oscar 13 continues to descend at 50 kms per month it will re enter and burn up in about 13 months time when it has got below 200 kms height. The decline has been remarkably regular up to now. In step with this loss of perigee height, the Apogee high point has increased in height by almost exactly the amount the perigee height has decreased. What can be causing this shift. Most elliptical orbiters are described as losing apogee height and keeping perigee height roughly constant, because the greater atmospheric drag at perigee inhibits the satellites ability to climb back up to apogee. BUT some Russian Molniyas have behaved as AO13 is doing , and have re entered the atmosphere much earlier than expected. It has been pointed out that AO13 is different from most other Molniya elliptical orbiters, because the Molniyas correct attitude using thruster mini rockets, whereas AO13 uses magnetic torqueing. But this does not explain why AO13 is being pulled DIRECTLY ALONG ITS SEMIMAJOR AXIS FROM PERIGEE TO APOGEE, a situation much more easily explained by blaming an unfortunate partial resonance which keeps the semi major axis looking towards the Sun along the line , Perigee - Earth - Apogee - Sun, thereby optimising Solar gravitational attraction. This being consequent on an unfortunate launch position which has resulted in the semimajor axis repeatedly pointing at the Sun (or even perhaps the Moon, or both). We do not have long to wait before theory is overtaken by practice. You will be able to check for yourself when ever you get new AO13 Keps by using the following gwbasic program :- 5 CLS 10 INPUT "mean motion ";MM 20 INPUT "eccentricity ";ECC 30 LET T=1440/MM 40 LET A=331.25 * T^(2/3) 50 LET APOGEE=A*(1+ECC)-6378 60 LET PERIGEE=A*(1-ECC)-6378 70 PRINT MM;" ";ECC;" apog ";APOGEE;" peri ";PERIGEE 80 stop Worked example 1991 day of year 173 , Ecc .7196447 , MM 2.09706085 gives apogee height 37958.6 kms, perigee height 850.237 kms. So in the next few weeks we should find out whether it is necessary for G3IOR to start a sweepstake on the demise of AO13. If it does re enter the last few months could have a very complex orbital geometry. One thing is sure ,we should all try to learn something , particularly if we want to avoid this happening to the next Phase III satellite ( now starting building). 73 de John GM4IHJ @ GB7SAN