Astro Bob: See the twin Messier craters and a curious lunar ‘comet’

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

Comet NEOWISE hangs low in the northwestern sky July 24 during a brief outburst of the aurora around 11:45 p.m. (Bob King for the News Tribune)

I’ve lived the comet this month. You, too? After so many fainter comets for so many years NEOWISE is like drinking from a fire hose. Last night, when hazy skies and tiredness dissuaded me from driving to a dark site to photograph the comet, I went out anyway, powerless to resist. Blame the aurora. On a late walk with my daughter we noticed the northern horizon aglow with northern lights, so I made a short drive to watch the show on the big screen.

And there it was when I arrived — Comet NEOWISE. How could I NOT include it in photos of the aurora?

This odd pair of craters, named Messier A (left) and Messier, may have formed during a single impact. A butterfly-wing pattern of rays above and below the crater surround Messier while twin-tails of debris lie downrange of Messier A. NASA / LRO / ACT-REACT

We’ve discussed how the moon’s entry into the evening sky the past few days will begin to affect the comet’s visibility. No lunar disrespect intended. The moon is a wonderful place to explore in binoculars and small telescopes in its own right. Unlike bright comets you don’t have to wait 10 or 20 years for the orb of night to show up. It cycles around the sky in under a month, its pockets stuffed with goodies. One of those bonbons is a pair of craters named Messier and Messier A.


Use this photo to find the Messier craters. The duo and “comet tail” are located in the diamond-shaped Sea of Fertility and are visible every clear night until past full moon — from July 25 to Aug. 5. Sight the moon at low magnification, navigate to the Sea of Fertility then increase the magnification to 75x – 150x for the best view. (Bob King for the News Tribune)

That name might already be familiar to you because it belongs to famous 18th century French comet hunter Charles Messier. Messier compiled a catalog of bright galaxies, star clusters and nebulae that includes many of the finest sights in the night sky for smaller telescopes. Each Messier crater is about 8 miles (12 km) across and located in the western half of the diamond-shaped Sea of Fertility (Mare Fecunditatis).

Artist view of the meteoroid impact on the moon. NASA

Point a telescope at the crater twins and you’ll immediately be struck by what looks like double tail shooting out of Messier A, which rather resembles the head of a comet. The streaks might also remind you of a jet contrail or the forked tail displayed by NEOWISE shortly after it passed closest to the sun in early July. What you’re really seeing are two rays composed of material ejected from the impact that created the two craters. Lunar rays like these form when rocks and boulders excavated by an impact fall back to the surface and dig countless secondary craters of their own, each “mini-impact” exposing fresh, lighter-toned lunar soil in the process.

The Messier craters (upper right) as they appear in a medium-sized telescope. They likely formed when a small asteroid struck the moon at a very low angle. Damian Peach


Ballistics experiments at NASA’s Ames Research Cente r have shown that when meteoroids and asteroids strike the moon they punch out circular craters until the angle of the impactor drops to about 15° or less. At these super-shallow angles speeding space debris digs out an oval or elliptical crater like the peculiarly elongated Messier.

Likewise with rays. Above 15° an impactor will create a circular pattern of debris, but a low-angle strike will spray material off to one side. One arriving at just a few degrees produces rays running sideways, creating a butterfly-wing pattern. As a result of these experiments, astronomers hypothesize that an asteroid or comet striking the moon at an angle of 1° to 5° at more than 35,000 miles per hour first excavated the trough-shaped Messier and produced faint butterfly wings . Its partner Messier A may have been carved out almost simultaneously by a piece of the impactor that broke off and ricocheted downrange. Debris from that strike formed dual plumes of debris in the direction opposite the impact.

These fascinating craters and their unique rays tell a fascinating story. You can see the crazy lunar comet from now until waning gibbous phase — more than 10 nights in a row. How generous of the moon to provide another “comet” as it steals our dear NEOWISE from view.

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