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An Interesting Case Study - Asteroid Cacus
The large asteroid Cacus (161989) will make a close approach (22.38LD) on Sept 1, 2022. Suspected of being ~1Km in size and listed as a Possible Hazardous Asteroid, I thought I might consider observing its close approach as it may reach a magnitude of 13.5 and be within reach of my 13" scope visually.

I did a bit of research and discovered it has been known since early 1978 and was provisionally designated 1978 CA when discovered by the German astronomer Hans-Emil Schuster at the European Southern Observatory, La Silla. Its orbital uncertainty is 0!

This rock has a high inclination of 26° so passage near our home planet is somewhat rare as they can only happen when near ascending or descending node. It's descending node is close to the Earth's orbit, but it's ascending node is quite distant, which is why the close approaches appear to be all in the first week of September, with none in the first week of March. I went looking for future passes under 22LD and only found 7 in the next 300 years.

I created a stereographic chart using the ecliptical coordinate system and plotted those passes positions in the sky. Pretty interesting stuff.
Turns out of the 2,275 PHAs, 350 have inclinations higher than 25°. So not as rare as I thought with about 15% having high inclinations. Only a few recent discoveries of PHAs have similar high inclinations (2022 CN1, 2022 EX, 2022 BA, 2022 BJ3 with fairly good orbital uncertainties). BJ3 has the highest (28.8°) of 2022 PHA discoveries.


Forgot to mention, all the ephemeris data is for my home location in SE Texas. Should maybe have used geocentric instead.

Very interesting. Unless these events are very close, parallax shouldn't be too bad. In any event something for the grandkids & beyond to be concerned about without some unexpected medical breakthroughs  Big Grin.

Phil S.

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