Monday, 23 April 2018

Pinpointing the OTV 5 orbital manoeuvre on 19 April 2018

click map to enlarge

As related in a previous post, the X-37B robottic space plane OTV 5 made an orbital manoeuvre on the 19th, lowering its orbital altitude from ~355 km to ~315 km.

It has been observed in its new orbit enough by now (pass predictions for yesterday evening were spot on), to allow an analysis to reconstruct the time and location of the manoeuvre. This can be done by looking for a moment where the positions in the old orbit and the new orbit were close.

Using Mike's pre-manoeuvre OTV 5 orbit of epoch 18104.41928168 and my own post-manoeuvre orbit solution of epoch 18112.84880111, and feeding these into the COLA program written a long time ago by Rob Matson, the resulting time of coincidence is 19 April 2018 at 5:20 UT.

OTV 5 was near perigee and in its descending node at the time, over west Africa, as can be seen on the map above. Manoeuvres typically happen near the nodes and near either perigee or apogee, so that fits well with this reconstructed moment of manoeuvre.

Since the manoeuvre entailed both a lowering of the perigee and a lowering of the apogee, the time and location listed above is likely the second of two manoeuvre moments.

The first manoeuvre burn probably happened near 4:35 UT, near apogee and the ascending node of the original orbit, south of Hawaii. This burn lowered the perigee altitude of the orbit to 310 km. Next, a second burn lowering the apogee altitude to 323 km was conducted half an orbital revolution later at 5:20 UT, near perigee and the ascending node of the orbit over west Africa. The two points are depicted by red circles in the map above.

Past OTV missions frequently made such manoeuvres between different orbital altitudes. They probably are meant to be able to test experimental technology in the payload bay under various thermospheric density and irradiation regimes.

Meanwhile, we continue to track OTV 5 in its new orbit. My observations yesterday were hampered a bit by an untimely field of clouds, but I did get some astrometry. Here is some imagery from yesterday, showing OTV 5 ascending amidst a thin cloud cover (bright star in clouds at right is Capella):

click photograph to enlarge

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