Catch Comet R4 ATLAS as it Nears Earth


The coming weeks are a great time to catch Comet C / 2020 R4 ATLAS … while you can.

Do you fancy spring astronomy? With temperatures in the northern hemisphere warming up from April to May, the galaxy season is just around the corner. At dusk, the area in the asterism of the Bowl of the Virgo rising to the east is full of clusters of galaxies that merge into the adjacent constellations of Coma Berenices and Boötes …

But be on the lookout for a non-galaxy fuzzball invader this May: Comet C / 2020 R4 ATLAS.

R4 ATLAS was discovered on the night of September 12, 2020 by the productive automated sky survey of the Asteroid Terrestrial Impact Last Alert System (ATLAS) in Haleakala (ALTAS-HKO) and Mauna Loa (ATLAS-MLO) Hawaii and has proven to be over – PERFORMANCE. In fact, it was never originally predicted that it would exceed the 10th magnitude … until a surprise outbreak in late December 2020 raised the brightness 100 times from +18 to +13. At the time of writing, it is around + 8th magnitude ‘with a ball’ and could top + 7th magnitude this week. This is clearly a busy, dynamic comet … and another well-placed breakout this week could propel it into the realm of the naked eye.

The comet reached perihelion in its 957-year orbit on March 1, 2021 at 1.029 astronomical units (AU) (154 million kilometers) from the Sun. Had the comet passed in August, its passage against Earth would have been four times closer.

The path of comet C / 2020 R4 ATLAS through the inner solar system. NASA / JPL

This once a millennium orbit also means that the comet last passed in the middle of the 11th century, around 1064 AD. Although R4 ATLAS – like most of the weaker comets of the pre-telescopic era – was not recorded, another famous inhabitant of the solar system emerged historically a few years later: Comet 1 / P Halley, 1066 AD. This appearance became the most famous of all comets not only captured for posterity on the Bayeux Tapestry, but appeared around the same time as the death of Harold II during the Battle of Hastings and the rise of William the Conqueror (a bad omen for Harold, but good for William) .

The Bayeux Tapestry with Comet 1P Halley. Public domain.

The comet ends in April in the constellation Hercules the Hero. Comet R4 ATLAS is now well suited for observation and ends high in the east in the ‘Spring Kingdom of the Galaxies’ from April to early May, away from the plane of the Milky Way and the ecliptic plane at dusk, and grows higher as the night progresses. The comet can be seen high in the south at dusk at the end of May.

Comet C / 2020 R4 ATLAS animation on April 17th

Here is a short list of ‘Heavenly Fate Dates’ for the comet, month after month:


23 – passes closest to the earth and 0.464 AU (69.5 million kilometers) away.

23 – crosses in the constellation Corona Borealis, the northern crown.

25 – Passes less than one degree from the + 4th star Theta Coronae Borealis.

26 – Crosses in the constellation Boötes the Shepherd.

27 – Reaches resistance at 180 degrees from the Sun.

30 – Crosses in the constellation Canes Venatici of the hunting dogs.

The heavenly path of comet C / 2020 R4 ATLAS from April 20th to June 1st. Photo credit: Starry Night / Dave Dickinson


1 – Photo-Op: Passes only five degrees from the globular cluster Messier 3 of the + 7th magnitude.

7 – Passes into the constellation Coma Berenices (Berenice’s hair).

11-12 – Nicks the corner of the Ursa Major (the Great Bear) constellation.

13 – Crosses in the constellation Leo the Leo.


1- May fall below + 10th size again.

From there comet C / 2020 R4 ATLAS returns in AD 2500 from the inner solar system to a distant aphelion 193 AU (29 billion kilometers) from the Sun. That’s almost five times the average distance from the Sun to Pluto. The comet won’t visit us again until 2978 AD.

The brightness curve of comet C / 2020 R4 ATLAS. The black dots are based on actual observations. Adapted from Seiichi Yoshida’s weekly information on bright comets.

Will R4 ATLAS brighten or hiss? With comets, you just never know. While +8 Magnitude R4 ATLAS placed in the binoculars area, we also got the waxing moon reaching full on April 27th to deal with. Look for the R4 atlas like you would a moderately bright globular cluster … sweep the suspicious field with binoculars or a telescopic wide field until a blurry spot becomes visible. In contrast to a globular cluster, however, a comet like R4 ATLAS will stubbornly refuse to break up into individual stars when it is focused. This contrasting comparison was one of the legendary inspirations for eighteenth-century comet hunter Charles Messier to create his first catalog of deep sky objects. as a way to mark the “false comets” of the sky.

When the skies are clear, be sure to check out Comet C / 2020 R4 ATLAS, one of the first good comets of 2021.

Main picture: Comet C / 2020 R4 ATLAS from April 11, 2021 with the kind permission of Michael Jäger.

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