Astro Bob: Hubble photographs a shattered comet

"Astro" Bob King is a freelance writer for the News Tribune. Read more of his work at

Closeup of the roughly 30 fragments of the comet resolved by the Hubble on April 20. When a comet breaks up freshly-exposed ice on each of the fragments vaporizes in sunlight and carries away gas and embedded dust. NASA, ESA, D. Jewitt (UCLA), Q. Ye (University of Maryland)

The NASA/ESA Hubble Space Telescope has provided astronomers with the sharpest view yet of the breakup of Comet C/2019 Y4 (ATLAS). The telescope resolved roughly 30 fragments of the fragile comet on 20 April and 25 pieces on 23 April. NASA, ESA, D. Jewitt (UCLA), Q. Ye (University of Maryland)

Poor Comet ATLAS. It just couldn’t hold it together. We talked about the breakup of this comet in an earlier blog , but I thought you’d enjoy seeing the recent high-resolution photos taken by the Hubble Space Telescope. The comet was discovered in December 2019 and quickly brightened until mid-March with a forecast to reach naked-eye brightness by mid-May. But it quickly stalled and abruptly began to fade at the same amateur photographs revealed that the nucleus — the solid, icy heart of the beast — was in pieces. Astronomers speculated that the core may be fragmenting or even disintegrating.

The Hubble Space Telescope’s new observations of the comet on April 20 and 23 confirm the break up and reveal that the broken fragments are all enveloped in a sunlight-swept tail of cometary dust. These images provide further evidence that comet fragmentation is probably common and might even be the dominant mechanism by which comets die.

Every time a comet passes near the sun it loses mass through sublimation, a process where ice transforms directly into vapor when heated. Over time, repeated heating and cooling of a comet can structurally weaken it until it’s liable to crack along stress lines and break apart into pieces. Jets of gas and dust that shoot like the comet like newborn geysers and push and torment the nucleus as well.


“Their appearance changes substantially between the two days, so much so that it’s quite difficult to connect the dots,” said David Jewitt of UCLA, leader of one of two teams who imaged the doomed comet with Hubble. “I don’t know whether this is because the individual pieces are flashing on and off as they reflect sunlight, acting like twinkling lights on a Christmas tree, or because different fragments appear on different days.”

Jets of carbon dioxide laden with dust and water ice blast from beneath the surface of Comet Hartley 2 in this photo taken during the flyby by NASA’s EPOXI/Deep Impact spacecraft in 2010. The jets carry water ice in the form of large snowballs (white dots) and dust particles. Jets push against and stress comets. (NASA)

Hubble’s crisp images may yield new clues to the breakup. The telescope has distinguished pieces as small as the size of a house. Before the breakup, the entire nucleus may have been no more than the length of two football fields!

The disintegrating comet is currently located inside the orbit of Mars and make its closest approach to Earth on May 23 at a distance of approximately 71.5 million miles (115 million km). Eight days later it will zip within 23 million miles (37 million km) of the Sun. I last saw it about 5 nights ago when it looked soft and diffuse but still readily visible in my telescope. It’s doubtful whether any of the multitude of fragments will survive. Every newly-broken chunk exposes fresh ice to sunlight which sublimates rapidly in cascades of dissipation.

Lunar explorations Part 2

The moon tonight (May 3) through a small telescope. Once again, we’ll find many of the best-lit and most interesting lunar features along the terminator, line separating day and night on the moon. (Virtual Moon Atlas)

The moon continues to fill out in its gibbous phase tonight with several new prominent craters coming into view for small telescopes and even steadily-held binoculars. I’ve marked several of them on tonight’s moon map — Kepler, a beautiful, 20-mile-wide (32 km) ring surrounded by a nimbus of bright, star-like rays and Gassendi, a 68-mile-wide (110 km) beaten-down crater with a shallow floor, several central mountain peaks and a broken rim. In the distant past, lava breached Gassendi’s rim, filling and smoothing the crater’s interior like pudding. Sinus Iridum (Bay of Rainbows) is also a crater and a large one at that, but it looks more like a bay because lava completely overcame its southern wall and partially filled the crater’s interior.


The crater Plato (left) and the craggy “arm” of Sinus Iridum (Bay of Rainbows) at far right. The low-angle lighting emphasizes individual wave-like lava flows that filled the Bay long ago. (Bob King for the News Tribune)

Copernicus, which we visited several nights ago, now wears a glorious crown of rays that extends across 500 miles (800 km) of the moon’s surface, big enough to easily discern with the naked eye. The rays are secondary impact craters created when material blasted from the moon during the impact that excavated Copernicus rained back down on the surface. Tycho, located in the moon’s southern hemisphere, also possesses an extensive system of rays.

Have fun exploring these and other sights on the moon tonight. Don’t have an instrument? Take a moonlit stroll instead.

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