Astro Bob: Comet PanSTARRS takes a dip in the dipper bowl

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

Time exposure of Comet PanSTARRS (C/2017 T2) taken through an 8-inch telescope on May 29, 2020. North is up and west to the right. Alfons Diepvens

Half the problem in observing astronomical objects is finding them. Often a comet or asteroid will lie off the beaten track with no good guide stars to point the way. For beginners it’s next to impossible to know just where to look. That’s why skywatchers are grateful when bright stars, planets or even the moon can provide guidance.

For the next eight nights Comet PanSTARRS (C/2017 T2) will crawl across the Bowl of the Big Dipper, one of the easiest star patterns to identify in the sky. At the end of evening twilight — around 10:30 p.m. for central U.S. and 45 minutes to an hour later for the northern border states — the Big Dipper hangs like a tool on a workbench pegboard high in the northwestern sky.

The comet slices diagonally across the Bowl of the Big Dipper this week and part of next. Stars are shown to about magnitude 8 with positions marked every two nights. PanSTARRS will look like a faint fuzzy patch (fainter than shown here) through 50mm or larger binoculars from a dark, rural sky. Stellarium with additions by Bob King

The Bowl measures roughly 10° x 5° and for the next eight nights it will be the comet’s home. I plotted PanSTARRS’s position on the map (above) for every other night starting June 8. For in-between just interpolate. Stars are plotted down to magnitude 8, all of which are visible in a typical pair of binoculars. PanSTARRS is currently about magnitude 8.5 and faintly visible under a dark sky in 50mm or larger binoculars. To find it, note the comet’s position in relation to stars around it. Connect it into a triangle or line with two other stars and then play your gaze around that spot, looking for a small, fuzzy patch of light resembling a puff of fog.


If you have a telescope you have a much better chance of seeing it. A 4-inch will show a bigger, fuzzy blob and also reveal its brighter center. Larger telescopes 8 inches and up will show a faint, westward-pointing tail. PanSTARRS will remain between 8th and 9th magnitude throughout June as it travels from the Big Dipper into neighboring Canes Venatici the hunting dogs.

When far from the sun in the deeps of space comets are inert, but at approximately Jupiter’s distance (5x the Earth-sun distance) they begin to vaporize in the sun’s heat and develop a thin coma or atmosphere. More ice and dust get boiled off the closer to the sun the comet gets. Some of that material is blown back to form a tail. The comet shown here is 67P/Churyumov-Gerasimenko along with the planets out to Jupiter. ESA

The tail is composed of dust embedded in the comet itself, an irregularly round, icy object typically a kilometer or two across. Solar heating vaporizes the ice, releasing the dust to form a hazy envelope around the comet’s nucleus called a coma. Think of it as a temporary atmosphere. Sunlight, made of individual packets of energy called photons, pushes the particles back behind the coma to form a tail. Some comets trace orbits that only take them as far out as Jupiter but PanSTARRS comes from the Oort Cloud , a roughly spherical repository of comets centered on the sun and located between 2,000 and 10,000 times the Earth-sun distance.

Comets twiddle their thumbs there in cold storage until the gravity of passing stars nudges one toward the inner solar system on a journey that take tens of thousands of years. When you look at Comet PanSTARRS consider its patient voyage.

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