OSIRIS-REx's Special Dance

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OSIRIS-REx's Special Dance

Postby BurtJordaan on September 23rd, 2017, 2:15 am 

The ground track of OSIRIS-REx during its gravity assist flyby looks like a special dance for us to say goodbye...


This happened because it came in fast (above escape speed) and obviously from above the geosynchronous orbit height. Earth's angular speed due to rotation was then larger than the angular speed of the spacecraft and it seemed to move east-west relative to the ground. Its proper movement relative to a static Earth's center was actually west-east all the time.

Between 11:32 AM EDT and 2:11 pm EDT on Sept 22, it dipped below the geosynchronous height and the ground track gradually changed into its real angular movement, west-east. This was aided by the fact that the surface movement nearer the poles is much slower - above the poles one would only have seen its true angular speed.

The flyby was mainly for an orbital plane and course correction, as one can deduce directly from the diagram's time tags. The delta-time from dipping below geosynchronous height (11:32) to perigee (12:52) was 1h20m. The corresponding time to reach the same height again (12.52 to 2:11) was 1h19m. So only a tiny delta-v was needed. Delta-v depends on the difference between time 'falling in' and 'falling away' to the same altitude.
Last edited by BurtJordaan on September 23rd, 2017, 8:30 am, edited 2 times in total.
Reason: Typos
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