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The asteroid 486958 Arrokoth was discovered to be binary by astronomers lugging a bunch of telescopes around (I think) camping with them. The timing offered additional astrometry for the orbit determination before New Horizon's flyby, and it also served to put an upper limit on any dust or debris cloud around the object that might suggest a farther flyby distance be considered.

Their hard work certainly paid off and an outline of the object was reconstructed, showing that it was likely a binary; two circular objects. This posed a bit of a mystery because there wasn't any periodic variation in brightness detected that would suggest rotation, but that's a different story.

Is this the only solar system object that was discovered to be binary (or higher) via occultation?


enter image description here

above: reconstructed occultation timing from several nearby sites from this answer below: series of images from New Horizons during flyby from this question

1.3 MB GIF: https://i.stack.imgur.com/Ci213.gif also viewable in YouTube

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    $\begingroup$ I'd suggested removing the animation. It does little for the question and it's a megabyte in size, which is a bit rough for people without good internet connections (and lots of people still have poor internet bandwidth). $\endgroup$ – StephenG Mar 10 '20 at 16:09
  • $\begingroup$ @StephenG okay doing so now. $\endgroup$ – uhoh Mar 10 '20 at 16:14
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    $\begingroup$ Buie et al. 2020 give lots of detail on the Arrokoth campaign but don't say much about other targets. $\endgroup$ – Mike G Mar 10 '20 at 17:40
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    $\begingroup$ Presumably not the only one, but KBO occultations are a fairly new trick only possible thanks to the Gaia DRs. $\endgroup$ – AtmosphericPrisonEscape Mar 12 '20 at 11:10
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    $\begingroup$ How are you defining “binary” here? All evidence that I’ve seen is that the two lobes of that object are in contact with each other, i.e. it’s a single object. $\endgroup$ – Eric Jensen Jun 30 '20 at 1:30
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Actually, the very first binary asteroid was discovered by stellar occultation in 1980. The evidence was rejected by the IAU, and wikipedia credits the first binary asteroid discovery to the Galileo spacecraft finding the moon of 243 Ida in 1993.

The first moon of the well known asteroid 216 Kleopatra was observed through stellar occultation in 1980. From this article:

As occultation czar David Dunham would later report in Sky & Telescope, nine different observing stations visually timed blinkouts of the 9th-magnitude star SAO 128066. Collectively these timings traced chords across an irregular, elongated silhouette measuring roughly 60 by 80 miles (95 by 130 km). "But there was an unexpected secondary occultation too!" Dunham wrote. "Gerald Rattley and Bill Cooke, observing from sites 2,000 feet apart near Loma Prieta, California, independently and nearly simultaneously saw the star disappear for about one second."

Dunham concluded that they had discovered a satellite of 216 Kleopatra. At that time, however, it was unknown if asteroids had satellites, and their evidence was not accepted. There was some stigma associated with the occultation methods. In 2008, the Keck II telescope finally verified the moons of 216 Kleopatra:

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In 2011, one of Kleopatra's moon's size and orbit was shown to be consistent with the original 1980 occultation observations, referenced in this paper, and reconstructed here with the original chords:

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Even though there were two independent chords that occulted the star, the IAU's former Minor Planet Director Center called the evidence "shoddy", even decades later. In 1980, he had refused to accept that asteroidal satellites existed.

Interestingly, even after vindication 30 years later, stellar occultation evidence for binary asteroid systems are still met with skepticism. For example, the wikipedia page for the asteroid 911 Agamemnon says a 2012 occultation is merely 'suggestive' of the Asteroid having a satellite. To me, the stellar occultation of the light intensity measurements from this paper, is far more than just suggestive, but instead clear evidence of a satellite of 911 Agamemnon:

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The large steady dip is the main body of the asteroid passing over the V magnitude 8.0 star HIP 41337 = SAO 60804 = BD= +37° 1857 = HD 70920, spectral type K0. The small dip is the satellite passing over the star.

Note: It's strange to me that the discovery of binary asteroid systems is met with skepticism since the method is essentially the same method often used to detect extra-solar planets (the transit method explained nicely in this answer).

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    $\begingroup$ Wow, what an amazing story! Slightly related: Who first reported the Crab pulsar's pulsing but was dismissed by an astronomer? Hmm, Loma Prieta sounds familliar, I was in an earthquake of the same name. In 1980 I think amateur astronomers timing occultations in the field had to either use visual timing or invest in and battery-power a photomultiplier tube and a tape recorder in order to record time resolved photometry, IAU should not have rejected the observation. $\endgroup$ – uhoh Dec 9 '20 at 3:42
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    $\begingroup$ @uhoh There sure are a lot of cases in math and science where a discovery actually happened much earlier than the attributed date by someone other than the attributed discoverer. I would be quite curious what the timing method and video method was used in 1980. The 2012 Agamemnon occultation observations employed these systems: scottysmightymini.com . I bet some similar systems were used for Arrokoth. $\endgroup$ – Connor Garcia Dec 9 '20 at 4:15
  • $\begingroup$ Yes indeed. That's a great question and I have a hunch that its asking will be forthcoming... $\endgroup$ – uhoh Dec 9 '20 at 5:12

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