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Understanding Orbits Assessment Questions 11-10-25 Morehead State University Morehead, KY Prof. Bob Twiggs [email protected] 1 Understanding Orbits Sun – Earth System Earth’s rotation axis Equator Eclipse Sun’s rays Spacecraft orbit plane Orbiting Spacecraft 1. Why are the suns rays assumed parallel when reaching the earth? Sun is far away and large that the rays can be assumed parallel. SSE-120 Understanding Orbits Earth System Orbit plane Orbit inclination q Ascending node crossing the equator r Earth radius h Orbit altitude 2. SSE-120 How much is the earth tilted on it’s axis? 23.5 degrees Understanding Orbits Earth System Orbit plane Orbit inclination q Ascending node crossing the equator r Earth radius h Orbit altitude 3. SSE-120 Yes Does the earth have the axis tilted all year long? Understanding Orbits Earth System Orbit plane Orbit inclination q Ascending node crossing the equator r Earth radius h Orbit altitude 4. SSE-120 What is the nadir point of a spacecraft? The point directly below the SC toward the center of gravity. Understanding Orbits Earth System Orbit plane Orbit inclination q Ascending node crossing the equator r Earth radius h Orbit altitude 5. How is the inclination of a LEO spacecraft orbit defined? Angle between equator and orbit plane for an ascending node. SSE-120 Understanding Orbits Earth System Orbit plane Orbit inclination q Ascending node crossing the equator r Earth radius h Orbit altitude 6. SSE-120 Why is inclination defined only for an ascending node (nadir point crossing the equator? Need to have a unique reference. Ascending node make it unique. Understanding Orbits Earth System Orbit plane Orbit inclination q Ascending node crossing the equator r Earth radius h Orbit altitude What is the approx. inclination of the orbit shown? 30 degrees SSE-120 Understanding Orbits Earth System Orbit plane Orbit inclination q Ascending node crossing the equator r Earth radius h Orbit altitude 7. SSE-120 If a spacecraft could change directions of flight by 180 degrees, how would that change the inclination? Inclination would now be 180 degrees minus original inclination = 150 Understanding Orbits Earth System Orbit plane Orbit inclination q Ascending node crossing the equator r Earth radius h Orbit altitude 8. SSE-120 Will the inclination change with increase in altitude? No no no no. Understanding Orbits Earth System Orbit plane Orbit inclination q Ascending node crossing the equator r Earth radius h Orbit altitude 9. No SSE-120 Will the inclination change if the orbit were elliptical? Understanding Orbits Earth’s rotation around the sun Equinox Winter Summer Equinox 10. If there are no perturbations to the orbit, can you draw a terminator orbit for each season? Super bunch!!!!!!!!!!!!!!!!!!!!!!!!!!!!! SSE-120 Understanding Orbits Earth’s rotation around the sun Equinox Winter Summer Equinox 11. Does the eclipse of the earth change with seasons? Changes directions, but not shape or size. SSE-120 Understanding Orbits All orbits go thru the center of the earth 12. Why must all earth orbits rotate around the earth’s center of gravity? Forces of gravity make it that way. That is just the way it is. Nature is in control. SSE-120 Understanding Orbits Earth’s rotation around the sun Equinox Winter Summer Equinox 13. Does the terminator or noon-midnight orbit differ for each season? Both change if no perturbations. SSE-120 Understanding Orbits Noon – Midnight Orbit Spacecraft enters eclipse Spacecraft Leaves eclipse 14. Will the eclipse time of the spacecraft change with an increase in altitude? Why? Because the time in eclipse is almost constant, but orbit time will increases. SSE-120 Understanding Orbits Noon – Midnight Orbit 15. What would be the percentage of eclipse time if a spacecraft could orbit at zero altitude? SSE-120 Half the period or 50%. Understanding Orbits Calculating eclipse time for a noon-midnight orbit r h r+h q f f q Solution Q = cos-1 [r/(r+h)] f = 90 - Q Eclipse time = [(2* f)/360] * orbit time 16. What is the eclipse time for an altitude of 750 km for the noon-midnight orbit? Make a Excel (spread sheet) chart for eclipse time from 300 km to 800 km by steps of 50 km. SSE-120 Understanding Orbits Calculating eclipse time for a noon-midnight orbit r h r+h q f f q Solution Q = cos-1 [r/(r+h)] f = 180 - Q Eclipse time = [(2* f)/360] * orbit time 17. What would be the maximum eclipse time for a 750 km altitude orbit if the inclination was 90 degrees? Do you need more information? Need orbit orientation toward sun --- noon-midnight, terminator, or in between? SSE-120 Understanding Orbits Changing orbit inclination. T A Change of eclipse time. B N-M C 18. How does the eclipse vary with inclination? SSE-120 From a terminator orbit, decrease in inclination will increase eclipse time after entering the eclipse. Understanding Orbits Earth’s rotation around the sun for a GEO stationary spacecraft. Equinox Winter Summer Equinox 19. Does an eclipse occur with a GEO stationary spacecraft? Yes, during equinoxes. SSE-120 Understanding Orbits Earth’s rotation around the sun for a GEO stationary spacecraft. Equinox Winter Summer Equinox 20. How often during the year will there be an eclipse of a GEO stationary spacecraft? Twice – each equinox. SSE-120 Understanding Orbits GEO stationary spacecraft. Start here Thursday for Menifee Equinox What is the max eclipse time? 21. How would you calculate the maximum time of the GEO eclipse? Same way as for the LEO spacecraft. Only altitude of GEO is larger. SSE-120 Understanding Orbits GEO stationary spacecraft. Equinox After Equinox Before Equinox http://www.satellite-calculations.com/Satellite/satellite_suneclipse_info.htm 22. How long would be the period that the GEO would continue to have some eclipse twice a year? SSE-120 Find when time between entrance to eclipse to exit from eclipse. (days) Understanding Orbits Done SSE-120