Friday, July 21, 2017

The #mu69occ campaign: Occam's razor wins again ...

On July 17 the third and final attempt to observe a stellar occultation by the next target of the New Horizons mission in the Kuiper Belt, 2014 MU69, succeeded with several telescopes seeing the star blink out briefly - in contrast to a similarly effortful attempt on June 3. In spite of placing two 'fences' of portable telescopes perpendicular to the expected shadow track, none of them saw anything. When this was finally acknowledged over a month later, the respective press release from NASA and the JHU APL was full of wild speculations about how MU69's shadow could have slipped somehow through the telescope fences and tried to spin the campaign's negative outcome as a success. Of course it wasn't: you can only claim success when at least one telescope has seen the target star blink out.

Furthermore occulation observers know from decades of experience - especially from the past century - that by far the most likely explanation for a total failure is that the shadow track had not been where it had been calculated: this could be due to either bad astrometry of the star or a bad orbit of the occulter. This, however, wasn't even mentioned as a possibility in the official word - and when I asked scientists involved in the campaign about it, I got either no answer at all or a snarky one at best. Apparently the use of unpublished Gaia astrometry of the star and HST astrometry of MU69 to determine its orbit had made it all but impossible in their minds that the June 3 shadow track could have shifted beyond all telescope fences.

Alas, this is exactly what had happened, New Horizons' Alan Stern has acknowledged now: "we [...] didn't put telescopes in the right place because back then we didn't have the MU69 orbit prediction well enough in hand. Subsequent HST June-July data helped with that." So there, the - by far - most likely explanation for the June 3 failure was the right one indeed and all the speculations were wrong! Fortunately the negative observations from back then (and whatever SOFIA saw or didn't on July 10; that's still kept secret) can now be put in context as the offset of the telescopes from the true shadow track can now be calculated and secondary bodies or debris or even a ring can be excluded at that relative location from 2014 MU69. A happy ending for the campaign and for New Horizons - and a lesson for press release writers trying to make a failed observation look good ...