No, no, no. Please don’t let it happen. It appears that the nucleus of Comet ATLAS, the one predicted to brighten to naked-eye visibility next month, may be breaking apart and breaking our hearts.
The comet had been brightening rapidly as it headed sunward the past couple months … until recently. In the past week it lost steam and plateaued. That’s not all. In an Astronomical Telegram astronomers Quanzhi Ye (University of Maryland) and Qicheng Zhang (Caltech) report that photographs on April 5th revealed that the comet’s false nucleus had changed from a compact, star-like object to an elongated, lentil-like appearance “as would be expected from a major disruption of the nucleus” as reported in an Astronomical Telegram. A second team of astronomers using the Liverpool Telescope in the Canary Islands confirmed the first team’s results.
The nucleus is the heart of the comet, the rotating ball of ice and dust that partially vaporizes in sunlight to create the comet’s fuzzy head and swept-back tail. Either a fragment broke off the nucleus or the entire body is crumbling, causing it to spread apart and appear elongated in photos. Other observers have noted a slight drop in ATLAS’s brightness as well as an unexpected shift in its direction of movement caused by “non-gravitational” forces. Fragmentation exposes fresh icy material to sunlight which then vaporizes rapidly. The expanding gases give the comet a push like a rocket engine and set it off on a slightly different path.
Astronomers warned us this might happen. When fresh comets that hail from the edge of the solar system approach the sun they’re often quick to brighten as pristine ices swiftly vaporize. But then as they draw closer to the sun they crack under the stresses of heating and jetting. That appears to be happening now with Comet ATLAS and could mean its swift demise. Those in the know expect the nucleus to expand and become more diffuse until the comet fades away.
Or not. Perhaps enough of the nucleus will survive to continue the journey and put on a modest show next month. We’ll just have to wait and see. Right now the full moon is out making the comet difficult to view, but a dark, moonless “window” opens up as soon as Thursday the 9th at the end of evening twilight. I’ll update you on the comet’s appearance and chances for survival later this week. Use this pdf map to spot it. Binoculars may still show it faintly but I recommend a 6-inch or larger telescope for a happy look. I will post a new map for you after full moon.
In 2011 Comet Elenin, another visitor from the distant deeps, self-destructed only a couple weeks before its closest passage to the sun. Comets are inherently fragile entities made of porous ice and dust. Although solar heating and gravitational tugs from the sun and planets can play a role in a comet’s breakup, spin might be the main culprit. Jets of gas blasting away from the comet can cause it to spin faster and faster, stressing the icy body until it develops cracks that lead to a breakup.
Comet ATLAS isn’t the only one suffering from internal fractures. On March 23rd, 28th and 30th, Hubble Space Telescope photos revealed that the interstellar comet 2I/Borisov’s core had also become elongated. Comet Borisov is the first comet ever discovered belonging to another solar system to pay a visit to our own. Instead of a single bright spot the photos show two, small fuzzy object about 112 miles (180 km) apart. A third tiny fragment was found later in the images. It’s unknown whether the entire nucleus is busting up or if it’s just a few fragments breaking off the main body.
Fragility is what makes comets so lovely to observe but it’s also their Achilles Heel.