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I read some news about this new comet and some unreliable sources said it is twice as big as Jupiter. I tried to find a reliable estimation of its size or mass, but I couldn't find anything. Would you please provide me a general way to distinguish reliable facts from unreliable facts about comets? Like their size and mass range, how much mass they lose, etc. Thank you in advance.
The confusion comes from the difference between the nucleus and the coma.
The nucleus is a small icy body, only a few km across.
The coma is the cloud of gas and dust released from the nucleus as it warms up. With not much gravity, the coma spreads out into space, and it can be hard to say exactly where the edge of the coma lies, however, a coma "the size of Jupiter" would not be unusual. Active comets can have comas that are larger than the sun.
But the mass of the coma is tiny. It is made of a very low-pressure gas. It is nearly all empty space, most of the mass of the comet remains locked in the nucleus.
The shattered heart of comet C/2019 Y4 ATLAS
After observations suggesting the presence of fragments in the inner coma of C/2019 Y4 ATLAS, we imaged this comet every night. In particular, tonight we observed at least three fragments, telling that the comet really experienced a breakup event. Here it is in our image.
The image above comes from the average of 63, 60-seconds exposures, remotely taken with the “Elena” robotic unit (PlaneWave 17″+Paramount ME+SBIG STL-6303E) available at Virtual Telescope in Rome. The telescope tracked the apparent motion of the comet and images were stacked using the orbit of the comet, to provide the best accuracy. Image scale is 0.63″/pixel. No image processing was performed, to preserve the reliability of the visible features. The signal-to-noise ratio is quite good, so it is possible to do, carefully, some image processing.
In the upper left insert, you can see the central region, this time after applying an unsharp masking filtering: there are at least four fragments there, telling us the comet broke up for sure, this causing the evident fading trend of the object.
Below, our previously released image, made using a subset of available images:
C/2019 Y4 ATLAS and its fragments on April 11, 2020, via the Virtual Telescope Project.
In the upper left insert, we present a 2x re-scaled detail, processed via unsharp masking. In short: the same situation as in the first image, which is of course better, using more sub-frames: consider it as your reference image.
We have much more data from all the past nights and we are processing them, to provide a detailed analysis, but we wanted to share right away this latest, amazing view of comet C/2019 Y4 ATLAS.
View at EarthSky Community Photos. | Vlad Dumitrescu in Prahova County, Romania captured the comet Saturday night, too. He wrote: ” There are at least 3 fragments visible. Shot last night from my backyard … It is amazing to see this happening right now.” Thank you, Vlad!
Bottom line: As seen by Gianluca Masi at the Virtual Telescope Project in Rome on April 11, 2020, the nucleus of comet C/2019 Y4 ATLAS has broken into three or four fragments. Vlad Dumitrescu in Romania also captured the comet this weekend he saw three fragments.
C/2019 Y4 (ATLAS): The bright comet of 2020?
It’s not every day, or every year for that matter, that we get to see a really bright comet. Many of us, including the author of this article, remember comet Hale-Bopp that adorned Earth’s skies in the first months of 1997. Since then we’ve had several comets bright enough to be plainly visible to the eye, for instance Ikeya-Zhang in 2002 and McNaught in 2007, though arguably none as spectacular as H-B. Indeed, there have been no new bright comets since comet Lovejoy back in 2011 (pictured).
Now, there is a serious contender for the proverbial “next bright comet”: C/2019 Y4 (ATLAS), or just ATLAS, discovered just before the New Year and steadily getting brighter since then. Though comets are notoriously difficult to predict, it is now quite possible the comet will become bright enough to rival Sirius, the brightest star in the sky – which, incidentally, can currently be seen low in the south after sunset.
Comet Lovejoy, seen from Santiago, Chile on 22 December 2011. Source: Wikipedia Commons
Comet ATLAS currently resides in the outskirts of the constellation of Ursa Major aka the Plough. It appears as a tiny smudge of light shining at 8th magnitude, somewhat fainter than the faintest stars visible with the naked eye. It is heading towards Perseus, which it will reach in mid-May, and should then be marginally visible to the eye as an evening object in the north-west. From mid to late May, it should be possible to pick up the comet in the evening but also the morning sky its closest approach to our planet occurs on 23 May at a distance of 70 million miles, about three quarters of the Earth’s distance from the Sun. The comet should reach maximum brightness in late May or early June and be lost in the Sun’s glare. It may, however, still be possible to make out a sufficiently long & prominent tail – provided one develops – and presenting a similar spectacle to comet Lovejoy back in 2011 in the skies Down Under.
Astronomers Observe Disintegrating Comet ATLAS
This image of C/2019 Y4 (ATLAS) was captured on March 14, 2020. Image credit: Martin Gembec / CC BY-SA 4.0.
C/2019 Y4 (ATLAS) is a comet with a near-parabolic orbit and an orbital period of about 6,000 years.
Also known as comet ATLAS, the object was discovered on December 28, 2019 by a reflecting telescope atop Mauna Loa in Hawaii as part of the Asteroid Terrestrial-impact Last Alert System (ATLAS).
At the time of its discovery, the comet was about 3 AU from the Sun and shone at magnitude of 19.6 in the constellation of Ursa Major.
In April 2020, it moved into Camelopardalis, and next month it will move into the constellation of Perseus.
These images of C/2019 Y4 (ATLAS) were captured on April 2 and 5, 2020. Image credit: Quanzhi Ye & Qicheng Zhang.
On April 6, University of Maryland astronomer Quanzhi Ye and Caltech astronomer Qicheng Zhang reported on the possible disintegration of comet ATLAS.
“Images taken on April 5 from the 0.6-m Ningbo Education Xinjiang Telescope (NEXT) showed an elongated pseudo-nucleus measuring about 3 arcsec in length and aligned with the axis of the tail, a morphology consistent with a sudden decline or cessation of dust production, as would be expected from a major disruption of the nucleus,” they wrote in the Astronomer’s Telegram.
“A disruption event could also potentially explain the large non-gravitational forces acting on the comet.”
This image from the Lulin 1-m telescope shows at least two fragments of C/2019 Y4 (ATLAS). The separation distance between nucleus and 1 is about 3,400 km and that of fragments 1 and 2 is about 1,600 km. Image credit: Lin et al.
Images captured on April 12 by the 1-m telescope at Lulin Observatory, Taiwan, showed the presence of at least two fragments of the comet.
On April 18, a large team of astronomers confirmed that the comet fragmented into several pieces, designated A to D.
This image, taken on April 13, 2020, shows the relative position of four fragments of C/2019 Y4 (ATLAS). Image credit: P. Birtwhistle.
“Images taken during the past week display clear evidence of the fragmentation of the comet,” they said.
“Greater than expected astrometric residuals, with respect to the computed orbit of the comet, also suggest unusual activity in the nucleus and significant non-gravitational forces acting over the past several weeks.”
This image, taken on April 20, 2020, shows fragments of C/2019 Y4 (ATLAS). Image credit: Quanzhi Ye & Man-To Hui.
University of Maryland’s Dr. Ye and University of Hawaii’s Dr. Man-To Hui then used Hubble to observe fragments of comet ATLAS.
“Single-epoch observation with Hubble on April 20 showed only two remaining major fragments, likely A and B, each consists of 1 to 2 brighter components and a couple more fainter components, situated 4 arcsec from each other along their common orbit,” they said.
“Fragments C and D seem to have been reduced into a handful of fainter fragments.”
“At the time of the observation, the comet was at a heliocentric distance of 1.10 AU and a geocentric distance of 0.98 AU.”
Quanzhi Ye & Qicheng Zhang. 2020. Possible Disintegration of Comet C/2019 Y4 (ATLAS). ATel #13620
Zhong-Yi Lin et al. 2020. The Fragmentation of Comet C/2019 Y4 (ATLAS) observed at Lulin observatory. ATel #13629
I. Eglitis et al. COMET C/2019 Y4 (ATLAS). MPEC 2020-H28
Quanzhi Ye & Man-To Hui. 2020. Continuing Fragmentation of C/2019 Y4 (ATLAS). ATel #13651
Hubble Space Telescope Witnesses Breakup of Comet ATLAS
New images from the NASA/ESA Hubble Space Telescope of the long-period comet C/2019 Y4 (ATLAS), taken on April 20 and 23, 2020, provide the sharpest views yet that the comet’s nucleus is breaking apart into as many as 30 fragments.
Hubble resolved roughly 30 fragments of comet C/2019 Y4 (ATLAS) on April 20, 2020. Image credit: NASA / ESA / Hubble / D. Jewitt, University of California, Los Angeles / Q. Ye, University of Maryland.
Also known as comet ATLAS, C/2019 Y4 (ATLAS) is a comet with a near-parabolic orbit and an orbital period of about 6,000 years.
The comet was discovered on December 28, 2019 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Hawaii.
It brightened quickly until mid-March, and some astronomers initially anticipated that it might be visible to the naked eye in May to become one of the most spectacular comets seen in the last two decades.
However, the object abruptly began to get dimmer, leading astronomers to speculate that its nucleus may be fragmenting, or even disintegrating.
Comet ATLAS is currently located inside the orbit of Mars, at a distance of approximately 145 million km (90.1 million miles) from Earth.
The new Hubble observations of its breakup on April 20 and 23 reveal that the broken fragments are all enveloped in a sunlight-swept tail of cometary dust.
Hubble resolved roughly 25 fragments of comet C/2019 Y4 (ATLAS) on April 23, 2020. Image credit: NASA / ESA / Hubble / D. Jewitt, University of California, Los Angeles / Q. Ye, University of Maryland.
“Their appearance changes substantially between the two days, so much so that it’s quite difficult to connect the dots,” said University of California, Los Angeles astronomer Dr. David Jewitt, leader of one of two teams who imaged the comet with Hubble.
“I don’t know whether this is because the individual pieces are flashing on and off as they reflect sunlight, acting like twinkling lights on a Christmas tree, or because different fragments appear on different days.”
“This is really exciting — both because such events are super cool to watch and because they do not happen very often,” added University of Maryland astronomer Dr. Quanzhi Ye, the leader of the second Hubble observing team.
“Most comets that fragment are too dim to see. Events at such scale only happen once or twice a decade.”
C/2019 Y4 (ATLAS) live position and data
This page shows Comet ATLAS (C/2019 Y4) location and other relevant astronomical data in real time. The celestial coordinates, magnitude, distances and speed are updated in real time and are computed using high quality data sets provided by the JPL Horizons ephemeris service (see acknowledgements for details). The sky map shown below represents a rectangular portion of the sky 60x40 arcminutes wide. By comparison the diameter of the full Moon is about 30 arcmins, so the full horizontal extent of the map is approximately 2 full Moons wide. Depending on the device you are using, the map can be dragged horizondally or vertically using the mouse or touchscreen. The deep sky image in the background is provided by the Digitized Sky Survey (acknowledgements).
Breakup of comet ATLAS
View larger. | Image via NASA/ ESA/ D. Jewitt (UCLA), Quanzhi Ye (University of Maryland).
On Tuesday (April 28, 2020), the NASA/ESA Hubble Space Telescope released the sharpest views yet of the breakup of comet C/2019 Y4 (ATLAS). The space telescope images reveal roughly 30 fragments of the comet on April 20 – each about the size of a house – and 25 pieces on April 23. Prior to breaking up, the comet had been brightening, and many had hoped it would become bright enough to be seen with the eye alone. Those hopes are now dashed.
Astronomers using the ATLAS (Asteroid Terrestrial-impact Last Alert System) in Hawaii discovered Comet C/2019 Y4 (ATLAS) on December 28, 2019. It brightened quickly until mid-March, and some astronomers initially anticipated that it might be visible to the naked eye in May to become one of the most spectacular comets seen in the last two decades. However, the comet began to get fainter, leading astronomers to speculate that the icy core may be fragmenting, or even disintegrating. By April 11, ATLAS’s fragmentation was confirmed by amateur astronomer Jose de Queiroz, who photographed several pieces of the comet.
Hubble Space Telescope image of comet C/2019 Y4 (ATLAS) on April 20, 2020. Image via NASA/ ESA/ STScI/ D. Jewitt (UCLA). Hubble Space Telescope image of comet C/2019 Y4 (ATLAS) on April 23, 2020. Image via NASA/ ESA/ STScI/ D. Jewitt (UCLA).
Hubble’s April 20 and 23 observations of the comet’s breakup show that the broken fragments enveloped in a sunlight-swept tail of cometary dust. These images provide further evidence that comet fragmentation is probably common and might even be the dominant mechanism by which the solid, icy nuclei of comets die. UCLA’s David Jewitt is leader of one of two Hubble teams who imaged the doomed comet. He said in a statement:
Their appearance changes substantially between the two days, so much so that it’s quite difficult to connect the dots. I don’t know whether this is because the individual pieces are flashing on and off as they reflect sunlight, acting like twinkling lights on a Christmas tree, or because different fragments appear on different days.
Because comet fragmentation happens quickly and unpredictably, reliable observations are rare, and astronomers remain largely uncertain about the cause of fragmentation. University of Maryland’s Quanzhi Ye, leader of the second Hubble observing team, said:
This is really exciting, both because such events are super cool to watch and because they do not happen very often. Most comets that fragment are too dim to see. Events at such scale only happen once or twice a decade.
Hubble’s crisp images may yield new clues to the breakup. The telescope has distinguished pieces as small as the size of a house. Before the breakup, the entire nucleus may have been no more than the length of two football fields (180 meters).
The disintegrating comet was approximately 91 million miles (146 million km) from Earth when the latest Hubble observations were taken. If any of it survives, the comet will make its closest approach to Earth on May 23 at a distance of about 72 million miles (116 million km), and eight days later it will skirt past the sun at 25 million miles (40 million km).
Comet Atlas: A comet for all of us?
Finally! After several years of waiting, we may soon have, in Comet ATLAS, a bright comet passing by Earth that will be visible with the naked eye. The last visitor to do this was Comet 46P/Wirtanen, which was barely visible in December 2018 from highly polluted areas. Current models predict that Comet ATLAS could be brighter and easy to spot in the sky, even for novice citizen astronomers. Of course, anyone who has a telescope can already witness brightness and morphological changes in the comet as it approaches the Sun. Many of us are sheltering at home now. So, it’s the ideal time to look up with your binoculars or telescope and enjoy your date with a comet.
Officially known as Comet C/2019 Y4 ATLAS, ATLAS was discovered on Dec. 28, 2019, by the robotic survey Asteroid Terrestrial-impact Last Alert System, which detects near-Earth objects. At the time of its discovery, the comet was a very faint fuzzy blob with a small tail whose orbit seemed to mirror that of the legendary Great Comet of 1844. Astronomers hypothesized that Comet ATLAS could be a fragment of the same parent body.
Last year, astronomers discovered 94 new comets, which may make you wonder why this one is particularly interesting. In truth, we’re still not sure if it is special because comets are like cats— highly unpredictable—but current models show that it could reach a magnitude in visible between 0 and 2 by the end of May 2020, meaning that it will even be visible from cities.
If you’re lucky and own a telescope, you’ll be able to observe its tails and see it changing over the upcoming weeks. Several astronomers have already started following the comet, including citizen astronomers in the Unistellar network. Once they receive the comet’s coordinates, owners of Unistellar’s eVscopes will be able to use their device’s Enhanced Vision capabilities to reveal the greenish color of the comet as well as its coma and tail, which are made up of dust particles ejected from the nucleus.
As the comet gets closer to the Sun (minimum distance 0.25 AU, so one-quarter of the distance from the Earth to the Sun on May 31) and closer to Earth as well, it will become more active and hence brighter. It will also show some fascinating morphological changes such as the development of a longer tail, a second tail connected to the magnetic field of our Sun, and jets of activity in the coma.
Unfortunately, we don’t know how this comet will evolve. Comets approaching our Sun for the first time become extremely active and can retain that activity for a time, or suddenly lose it. Those fizzing comets trick us, and the history of modern astronomy is full of big disappointments. One of the more famous was Comet Kohoutek, which was promoted by the media in 1973 as the “comet of the century” and reached only magnitude -3. Today we think that its early explosive activity was related to the fact that it was on its first visit to the inner solar system, and its nucleus partially disintegrated when it came close to the Sun.
Cometary astronomers discussing the case of Comet ATLAS all seem to agree that we can’t predict the activity of this comet. Its current morphology is abnormal, and it seems to brighten every day because its coma is growing.
I think it’s great that comets still surprise us. Because they come from far away and their composition and evolution seem to be very complex, their behavior when they come close to our Sun is unpredictable. Like cats, they are beautiful and mysterious objects.
In the meantime, let’s do what we can to observe Comet ATLAS from our homes. By observing it, you can collect data for scientists whose largest telescopes are shut down due to the COVID-19. And while observing from your backyard, you can also enjoy the beauty and erratic behavior of Comet ATLAS, and remember that this body formed on the outskirts of our solar system and comes back to visit us every 6,025 years. What will be on Earth next time it visits? That’s something we can’t predict either.
Comet C/2019 Y4 ATLAS
Click image for full size version
April 12, 2020
This image shows Comet C/2019 Y4 ATLAS, high in the northern sky on the evening of April 10. There are other interesting objects in the field, including LBN 702, the large brownish dust cloud dominating the lower left of the image. I couldn’t bring it out much more without revealing noise from the very short acquisition times. There are also many galaxies, some of which are marked in the annotated image. Note edge-on spiral galaxy PGC19652, towards upper left. Its disk appears slightly bent, with the tips going in different directions.
Acquisition, focusing and control of Paramount MX mount with TheSkyX. Focus with Optec DirectSync motor and controller. Automation with CCDCommander. Equipment control with PrimaLuce Labs Eagle 3 Pro computer. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Minimal moonlight, above average transparency and average or better seeing. Acquired April 10, 2020.
Data Reduction and Processing
Preprocessing: The WeightedBatchPreProcessing script was used to create a Lightness master frame (from the mono camera) and a RGB master frame (from the one-shot colour camera). A second Lightness master was made from just three 2m subs, centred on the same time of acquisition as the larger stack. MURE Denoise was applied to both Lightness masters, and the comet from the smaller stack was cloned into the larger stack, tightening up the comet.
Gradient Removal: DBE was applied to Lightness and RGB masters using Subtraction to remove the gradient that remained after integration.
Channel Registration: To improve channel registration, the colour channels of the RGB master were extracted and aligned with StarAlignment, using Thin Plate Splines with Distortion Correction and the green channel as the reference frame. The registered colour channels were recombined with ChannelCombination.
Colour Balancing: Colour was balanced with PhotometricColorCalibration.
Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the background areas, using an internal mask to protect bright structures. Layer settings for threshold and strength: Layer 1: 5.0 0.85, 2 iterations Layer 2: 3.5, 0.75, 2 iterations Layer 3: 3.0, 0.5, 1 iteration Layer 4: 1.0, 0.25, 1 iteration.
Stretching: HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.10.
Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the background areas, using an internal mask to protect bright stars. Layer settings for threshold and strength: Layer 1: 3.0 0.85, 1 iterations Layer 2: 2.0, 0.75, 1 iterations Layer 3: 1.0, 0.5, 1 iteration.
Stretching: HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.10.
Combining Lightness and Colour Images
Registration of L and RGB: The Colour image was rotated and cropped similar to the Lightness master, and then registered to the Lightness master using StarAlignment.
LRGB Combination: The Lightness image was applied to the RGB image using LRGBCombination with default settings.
Nonlinear Noise Reduction: TGVDenoise was used in L*a*b* mode to reduce noise with a mask used to target nebula and background and protect the comet and the stars (max. 1000 iterations and convergence selected for both lightness and chrominance).
Final Steps: Brightness, contrast, and colour saturation of various components of the image were adjusted in several iterations using CurvesTransformation with masks as required. ICCProfileTransformation (sRGB IEC61966-2.1 Relative Colorimetric with black point compensation) was applied prior to saving as a jpg.
Get ready for Comet ATLAS (C/2019 Y4) in the northern spring sky!Discovered at Mauna Loa in Hawaii on 28 December 2019 by the Asteroid Terrestrial-impact Last Alert System (ATLAS), Comet C/2019 Y4 brightened 6000-fold in just two months to attain magnitude +7.5 on 1 April. However, a major disruption of the comet’s nucleus observed on 5 April dashed hopes that it would be a naked-eye object in the high northern constellation of Camelopardalis in late April. C/2019 Y4 passes closest to Earth at 10h UT on 23 May at a distance of 0.781 astronomical units, or 117 million kilometres, when it lies in the constellation of Perseus. Comet ATLAS reaches perihelion on 31 May. Click the graphic to download a high-resolution PDF A4 finder chart suitable for printing. AN illustration by Ade Ashford.
Update: 9 April: Dr Gianluca Masi, Virtual Telescope Project
Update: 6 April: disintegration of Comet C/2019 Y4 (ATLAS)?
As if the mere mention of the word ‘bright’ in connection with a comet is tempting fate, astronomers Quanzhi Ye (University of Maryland) and Qicheng Zhang (Caltech) have reported a major disruption of the comet’s nucleus in observations made on 5 April with the 0.6-metre Ningbo Education Xinjiang Telescope (NEXT) at Mount Nanshan, Xinjiang, China. Follow-up observations by I. A. Steele, R. J. Smith and J. Marchant (Liverpool JMU) with the Liverpool 2.0-metre Telescope on La Palma confirm the nuclear elongation.
Any comet observer knows that these primordial, icy denizens of the deep Solar System are frequently faint and unpredictable. This is, of course, part of their appeal: you never know when a new one will catch you by surprise by appearing out of nowhere and suddenly surge in brightness as it approaches the Sun, hopefully developing a tail visible to the naked eye.
Northern Hemisphere skywatchers are getting excited about Comet C/2019 Y4, discovered at Mauna Loa in Hawaii on 28 December 2019 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) when it lay among the stars of Ursa Major. Now designated Comet ATLAS (C/2019 Y4), astronomers soon noted that its orbit is very similar to C/1844 Y1, the celebrated ‘Great Comet’ of 1844. If Comet ATLAS is a fragment of C/1844 Y1, we could be in for a treat.
Currently (2 April) shining with a greenish glow around magnitude +7.5 and traversing the somewhat obscure northern constellation of Camelopardalis (the Giraffe) at a rate of about two-thirds of a degree per day, C/2019 Y4 has brightened 6000-fold in the last two months. If it continues at the predicted rate, the comet could be a naked-eye object from rural locations around the time of new Moon on 23 April.
This extract of our scalable PDF finder chart shows the track of Comet ATLAS C/2019 Y4 at its closest and brightest before it reaches perihelion on 31 May 2020. Regrettably, the comet’s close approach to the Pleiades on 29 May occurs too close to the Sun to view in twilight. AN illustration by Ade Ashford.
Observing and astroimaging prospects
Note: the magnitudes quoted below were those estimated at the time the article was written on 2 April before the comet’s nucleus fragmented.
If you haven’t already done so, click here to view or download our scalable A4 PDF finder chart for C/2019 Y4, valid until early June. By mid-April the comet lies within 1 astronomical unit of Earth.
At 0h UT (1am BST) on 2 and 3 May, Comet ATLAS lies 1½ degrees southeast of fourth-magnitude star beta (β) Camelopardalis, which despite its Bayer designation is actually the brightest star in the constellation.
On 12 May, C/2019 Y4 crosses the constellation border into Perseus. At this time it could be the third magnitude and moving relative to the stars at a rate of a degree (or twice the width of a full Moon) per day.
At nautical dusk on Saturday, 16 May (around 11pm BST in the heart of the UK), the possibly second-magnitude comet lies just 12 arcminutes, or one-fifth of a degree from magnitude +4.1 star mu (μ) Persei, but the pair will be just 18 degrees high in the north-northwest.
New Moon occurs on 22 May. Comet ATLAS passes closest to Earth at 10h UT (11am BST) on Saturday, 23 May at a distance of 0.781 astronomical units, or 117 million kilometres, traversing the constellation of Perseus at a rate of 2 degrees per day.
C/2019 Y4 remains circumpolar for an observer in the heart of the British Isles up until about 24 May. Thereafter, UK observers will need to look for it in a rapidly brightening sky around the onset of civil twilight about 4am BST, some 50 minutes before sunrise. If it brightens according to predictions, the comet should have a total magnitude similar to Venus and located very low in the northeast in bright twilight.
C/2019 Y4 passes into the constellation of Taurus on 26 May and two days later a combination of distance and favourable viewing geometry means that it attains a peak rate of motion with respect to the stars of 2½ degrees per day.
Comet ATLAS and the Pleiades
C/2019 Y4 passes just 2 degrees from the Pleiades (Seven Sisters, or M45) open star cluster on the UK morning of 29 May. But before you get too excited about the prospect of getting a superb astrophotograph of the conjunction, consider that at this time M45 lies just 9 degrees from the Sun in Taurus, so the cluster and comet lie too close to the Sun to be seen together from Earth.
Perihelion and orbit
The comet passes closest to the Sun around 00:30 UT (1:30am BST) on Sunday, 31 May at a distance of 0.2528 astronomical units, or almost 38 million kilometres. Before this perihelion, C/2019 Y4 had a very eccentric orbit (e = 0.99923) inclined by 45.38 degrees to the ecliptic with a period of about 4,800 years. Due to gravitational perturbations, its modified outward orbit is likely to have a period of around 5,200 years.
This topocentric ephemeris of Comet ATLAS (C/2019 Y4) is computed for the centre of the British Isles (54°N, 2.5°W) at 0h UTC (1am BST) on the dates shown. The comet’s right ascension (R.A.) and declination (Dec.) are for the J2000.0 epoch Delta is its distance from Earth in astronomical units Mag. is the total predicted visual magnitude °/dy is the comet’s daily motion in degrees Cst is the constellation in which it resides. AN computation and graphic by Ade Ashford/Minor Planet Ephemeris Service/JPL’s HORIZONS system.
Size and mass of comet ATLAS (C/2019 Y4) - Astronomy
From Voronezh , C/2019 Y4 (ATLAS) is not readily observable since it is very close to the Sun, at a separation of only 17° from it.Current Position
|Magnitude:||0.00 (V)  |
|Absolute mag (H):||11.80 |
|Slope parameter (n):||6.00 |
|Right ascension:||06 h 37 m |
54.72 lightmin 
|Semi-major axis:||570.16 AU|
|Longitude ascending node:||120.58°|
|Argument of perihelion:||177.32°|
|Epoch of elements:||24 June 2021|
|Mean Anomaly at epoch:||0.03°|
All times shown in Voronezh local time.
The position of this comet was calculated from orbital elements published by the Minor Planet Center (MPC).
We estimate the brightnesses of comets from magnitude parameters published by the BAA Comet Section, where these are available. These are computed from the observations they receive from amateur astronomers.
Comets are intrinsically highly unpredictable objects, since their brightness depends on the scattering of sunlight from dust particles in the comet's coma and tail. This dust is continually streaming away from the comet's nucleus, and its density at any particular time is governed by the rate of sublimation of the ice in the comet's nucleus, as it is heated by the Sun's rays. It also depends on the amount of dust that is mixed in with that ice. This is very difficult to predict in advance, and can be highly variable even between successive apparitions of the same comet.
In consequence, while the future positions of comets are usually known with a high degree of confidence, their future brightnesses are not. For most comets, we do not publish any magnitude estimates at all. For the few comets where we do make estimates, we generally prefer the BAA's magnitude parameters to those published by the Minor Planet Center, since they are typically updated more often.