In 2017 a series of stellar occultations by asteroid 2014 MU69 "Ultima Thule" now officially named 486958 Arrokoth were timed in order to obtain better orbital information before the New Horizons flyby and to look for additional large chunks of debris that might be of scientific interest and pose a danger of collision thereby warranting a more distant flyby.

There was a pleasant surprise in the shape of the asteroid determined from an array of portable telescopes deployed across the occultation paths. The asteroid appeared to be binary, which was puzzling because there were no obvious oscillations in its light curve, but that's a different story.

Below is an assemblage of three such occultation measurements. We can see the asteroid's double circle shape nicely. But what interests me here is the single trace for July 10th. It seems to show an occulting object clearly distinct from yet incredibly close to Arrokoth. A pure coincidence would be more than extremely unlikely as the time window is seconds and the distance less than 100 km at ~45 AU!

Question: But I don't remember reading about a separate secondary body orbiting Arrokoth. So what caused this apparent occultation on July 10, 2017?

From this answer to Will the upcoming observations of occultation by Arrokoth (2014 MU69) be of a single object, or two?

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There were three attempts to measure Arrokoth by occultation, and the June 3rd attempt didn't detect anything. The July 10th attempt had a tiny blip, that appeared to be in the "wrong place", well away from the location that astrometry had predicted. The July 17th occultation was successful, it determined the shape and location well.

Some thought that the July 10th "blip" might have been evidence of a moon.

It was not, it turned out that there was an error in the software that was used to produce the astronometry (the author of the software describes one as "a case of me failing to read my own documentation, another one a sign error") If those bugs are fixed, then the blip on July 10th is moved to the same place as the blips on July 17th. That blip is Arrokoth.


According to Wikipedia:

A preliminary analysis of all collected data suggested that Arrokoth was accompanied by an orbiting moonlet about 200–300 km (120–190 mi) away from the primary. It was later realized, however, that an error with the data processing software resulted in a shift in the apparent location of the target. After accounting for the bug, the short dip observed on 10 July was considered to be a detection of the primary body.

So this “mysterious stellar occultation” is nothing else than an error in calibrating the data.


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