One of the things that has been a big challenge for #UltimaFlyby had been figuring out #UltimaThule’s spin period. It’s bounced everywhere between 4 and 30 hours. Currently we think it’s either 15 or 30 hours. @NewHorizons2015 @NASANewHorizons pic.twitter.com/7mwsH4FR70

— James Tuttle Keane (@jtuttlekeane) January 1, 2019