A comet has become bright enough to be seen with binoculars in a dark sky as it makes one of its closest approaches on record to Earth.
Comet 21P/Giacobini-Zinner, which is the parent comet of the Draconid meteor shower, is currently crossing the constellation of Auriga and heading towards Gemini.
The Draconids are usually a very insignificant shower, and meteor watchers will be interested to see whether activity is enhanced by the comet’s close flyby.
The comet takes 6.54 years to orbit the Sun on a very elliptical, or oval, path. It will be at its closest to the Earth on September 10, 2018, when it also reaches perihelion, its closest point to the Sun.
When nearest, Comet Giacobini-Zinner will lie about 42 million km (36 million miles) from us, which is its closest approach since November 1959, nearly 60 years ago.
Where to find Comet Giacobini-Zinner
You can see the comet for yourself, with the aid of our star chart or an astronomy app, if you have binoculars or a small telescope. It is just too faint to see with the unaided eye, but a short time exposure with a camera should also record it.
Don’t expect to see anything too dramatic though. The comet looks like a fuzzy blob to the eye, and photographs will help show its short stubby tail.
Comet Giacobini-Zinner has been steadily brightening during August as it crossed Cassiopeia, Camelopardalis, and Perseus before entering Auriga.
On September 2nd and 3rd, it was easy to find as it passes close to Capella (α Aurigae), one of the brightest stars in the sky.
In early September, the comet had reached magnitude 7. During previous close encounters, the comet has occasionally brightened unexpectedly, due to sunlit bursts of gas from within its icy nucleus. It will be worth watching in case there is any similar brightening this time.
An observing highlight during its close encounter will come when Comet Giacobini-Zinner passes in front of a bright collection of stars called an open cluster. This cluster in Auriga is labelled M37 in Charles Messier’s famous catalogue, and is easy to see in binoculars. (Ironically, Messier was an 18th Century comet-hunter and drew up the catalogue simply to list fuzzy objects that he might otherwise mistake for comets!)
The comet will be seen to approach and then depart the cluster over several hours, so you will be able to see them appear close together from September 9th to 11th when the comet is above your local horizon.
Of course, they are nowhere near each other in reality, as the cluster lies about 4,500 light-years away. It contains around 500 stars.
From mid-northern latitudes, the comet rises during the evening and it will gradually get higher in the sky and become easier to see as the night progresses.
The comet will pass in front of a brighter cluster of stars in Gemini on September 15th and 16th. It closes in on the cluster M35 before twilight from Europe, and observers in North America will be best placed to see it pass in front of it during the hours before dawn. M35 lies about 2,800 light-years away and is dimly visible to the unaided eye.
The comet was discovered on December 20th, 1900, by Michel Giacobini, from Nice in southern France. Though astronomers recognised it had a short orbital period of just a few years, it was not recovered when it returned to the inner solar system in 2007.
Then on October 23rd, 2013, Ernst Zinner, from Bamberg, Germany, made his own discovery of a comet, and it was later recognized as being the return of Giacobini’s object. It has been given the name of the two astronomers to honour their discoveries. It is labelled 21P because it was the 21st comet found to be a regular, returning visitor.
Prospects for meteors
As already mentioned, Comet Giacobini-Zinner is the parent of the Draconid meteor shower. The meteors are produced from dust shed by the comet when warmed by the Sun.
Over many thousands of years, this dust is spread around the comet’s elliptical orbit. But the particles are not evenly distributed and are denser near the comet itself.
Usually, numbers seen are very low. In 2017, for example, the predicted rate for a single observer was just one an hour! However, rates have soared in previous years around perihelion – the comet’s closest approach to the Sun.
Meteor storms occurred in 1933 and 1946, and more recently there was strong activity in 1985, 1998 and 2011.
The shower is active from October 7th-10th, and is due to reach a peak around midnight on the night of October 8th/9th in 2018. Predictions for the zenithal hourly rate (ZHR) range from 15 to 50 an hour.
No one is predicting a storm in 2018, but anything could happen, so it is worth keeping an eye on the sky around maximum, particularly as there will be no moonlight to spoil the view.
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