Brightening comet’s nucleus has broken into fragments

Updated: A comet was brightening faster than expected earlier this year, giving hope that it would become a prominent object visible with the unaided eye in May.

An image of Comet C/2019 ATLAS taken from Walmer, Kent, UK, on March 12th, 2020. Image credit: Paul Sutherland

However, during early April, the comet’s nucleus was seen to break into fragments, suggesting that it will not become so prominent. We will just have to wait and see!

The celestial visitor is labelled C/2019 Y4 ATLAS, indicating that it was discovered in 2019 by a sky survey called the Asteroid Terrestrial-impact Last Alert System (ATLAS).

This robotic survey, based in Hawaii, searches for small objects to help defend Earth against possible future impacts, and has found several comets as well as asteroids.

Comet C/2019 Y4 was expected to be only around magnitude 15 at the time of writing (March 15th, 2020) but has surprised astronomers by climbing more rapidly to about magnitude 8. This brightening does not appear to be a sudden flare, but rather a steady steep increase.

This steep climb led comet experts to speculate that the comet might become easy to see without a telescope of binoculars in the north-western evening sky in May as it approaches the Sun.

If it becomes as bright as had been anticipated – magnitude 2 or better – then the comet will be easy to spot until it is lost in the twilit sky. Binoculars will certainly help you see it. Ideal size for hand-holding would be 10×50 or similar, where 10 is the magnifying power, and 50 is the width of each front lens in millimetres. You can find suitable binoculars from reputable makes, such as Celestron, Olympus and Orion, at Amazon. Here is a link to a choice of binoculars for readers in the USA, and here is a link to such binoculars if you want to buy in the UK.

Of course, comets are notoriously unpredictable in their behaviour, and this one could fizzle out and not provide us with any great spectacle. However, another interesting fact about this comet is that it has a very similar orbit to a Great Comet that became a bright object in the night sky in the 19th Century.

The track of Comet C/2019 ATLAS from mid March into April. Points are shown for 0h UT at 5-day intervals, in the format year-month-day.

They cannot be the same comet, because the orbital period has been calculated to be nearly 5,520 years, taking it far out into the edge of the Solar System. However, it might very well be related to it, as a fragment of a once much larger comet.

Catalogued as Comet 1844 III, the 19th Century object was discovered on December 17th of that year just four days after its closest approach to the Sun. According to a book called Comets, A Descriptive Catalog, by Gary Kronk, the comet developed a tail 10° long in the first days of January 1845, and had a nucleus around magnitude 2.5.

The track of Comet C/2019 ATLAS during April and May. Points are shown for 0h UT at 5-day intervals, in the format year-month-day.

Jonathan Shanklin, Director of the British Astronomical Association’s Comet Section, reported that the current comet, C/2019 Y4, brightened quite rapidly in mid February, and adds “as of March 11 there is no sign of a slowdown in the rate of brightening. It is already visible in large binoculars . . . The uncertainty in brightness at the time of perihelion is large, though the worst case indicator is 2nd magnitude. It will remain well placed for UK observers into May and could become a prominent object.”

You can also read more about the comet on this website.

In mid March, the comet is lying a little to the right of the bowl in the asterism known as the Big Dipper in the USA, and the Plough in the UK. This means it can be seen in dark skies, away from twilight, and is circumpolar from mid-northern latitudes and above.

We have charts here to show its track as it moves across the sky and into the evening twilight in May, when it should appear at its best, in the constellation of Perseus.

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