Blame space weather. CMEs hitting comets can cause magnetic reconnection in comet tails, sometimes ripping them off entirely. NASA's STEREO-A spacecraft watched this happen to Comet Encke in April 2007: movie.
This month, multiple CMEs have swept past Comet ZTF as a result of surging solar activity. One of them is responsible for the disconnection event.
Soon, we'll get a better look. Comet ZTF is approaching Earth for a close encounter (0.28 AU) on Feb. 1st. Between now and then, the comet's brightness will cross the threshold of naked-eye visibility, possibly peaking at magnitude +5. CME effects will be increasingly visible as the comet approaches. Stay tuned!
Video update: A new animation shows the disconnected fragment in motion.
Observing Tips: from Sky & Telescope
DIAMOND DUST SKI HALO
Ski resorts are great places to see sun halos--rings of light that surround the sun when ice crystals fill the air. Spaceweather.com reader Christian Schartner was skiing at Reiteralm ski resort in Austria yesterday morning when he witnessed this specimen:
Snow cannons were key to this display. Ordinary sun halos are caused by ice crystals forming naturally in high cirrus clouds. Snow making machines create a spcial type of crystal called "diamond dust." These man-made crystals tend to be more optically perfect than natural crystals in clouds, producing extra-bright, extra-sharp halos.
A close look at Schartner's picture shows specks of light in the air. Those are the glittering crystals of diamond dust which make ski halos so beautiful.
Nobody looks to the skies anymore, and the wondrous sights, if one looks at the universe, how one could think we are alone.
I read somewhere that stars and planets represent people, living in such such a wondrous starlight night, how one be alone, against all the activity traversing the night sky, following the movements of celestial progression.
So sad because, civilizations, thousands of years ago, built ancient monuments, with encoded messages, to look to the skies, a warning from the past.