Eventually their efforts enabled Dr. Buie to piece together hints of MU69’s shape — not circular but perhaps elongated like a potato or two separate objects in close orbit around each other.
But the story still was not quite right.
During an attempt in July to view an occultation from NASA’s Sofia observatory, a modified 747 jet that carries a 100-inch telescope, the astronomers did not see anything definitive, but there was a suggestive blip. Perhaps Sofia’s path just grazed the shadow.
But whatever Sofia observed was not quite in the same position as where MU69 turned out to be during successful observations in Argentina a week later. Even in the successful observations, MU69 was slightly off from the predicted location.
That might have been a consequence of uncertainties in the positions of the background stars. When Dr. Buie received an update of the star catalog — a compendium of Milky Way stars produced by a European Space Agency mission called Gaia — he reran his calculations.
The discrepancy was even larger, too large for him to reconcile. Then he realized that Sofia may have detected a second object: a moon, which could explain the discrepancies.
That could be the first of the surprises to come.
If there were just one moon, said S. Alan Stern, the principal investigator for New Horizons, “then it’s kind of a needle in a haystack, and it would be unlikely that Sofia would just happen to trip over it. So this might be the harbinger. It might be a hint there’s actually a swarm of satellites around MU69.”
The presence of a moon would also add complications; its gravitational pull would cause MU69 to wobble, and the mission managers will have to adjust the observations to make sure the instruments are pointed in the right direction. But that will all have to be done in the days just before the flyby.
“That’s going to make the last few days of 2018 very exciting,” said John Spencer, the mission’s deputy project scientist.
MU69 will pass in front of a star one more time, in August next year, allowing the New Horizons team one last chance before the flyby to catch a glimpse of the object and what could be its moon.