Saturday, May 18, 2024

Comet Tsuchinshan-ATLAS Brightens, Grows a Tail – Sky & Telescope

AstronomyComet Tsuchinshan-ATLAS Brightens, Grows a Tail - Sky & Telescope

Even my little Seestar S50 revealed C/2023 A3’s bright coma and curved tail in this 5-minute stacked exposure made on May 4, 2024. The bit of fuzz in the tail is the faint galaxy UGC 8262.
Bob King

On April 25th I pointed my 15-inch Dob toward the much-anticipated Comet Tsuchinshan-ATLAS (C/2023 A3) and was thrilled to see how much brighter it had become since mid-winter — up to magnitude 9.7. Even at low magnification (67×) the small but strongly condensed coma was immediately apparent. I could even trace a 2′ tail curving to the east-southeast. Increasing the magnification and using averted vision I detected a slight twist in the tail that gave the object a comma-like appearance clearly depicted in images.

Comet Tsuchinshan-ATLAS on the move
The comet crosses northern Virgo this month and is well-placed for observation as soon it gets dark. Barring an outburst, expect it to brighten to around magnitude 9.5 by month’s end. Positions are shown daily with the date marked every five days. Stars are plotted to magnitude 10 and north is up. Click here for a large map.
MegaStar, courtesy of Emil Bonanno

I reobserved it on May 4th, and although the coma’s overall brightness had diminished to 10.0 the tail had accordioned to 3′. My initial observation of a brighter coma in April had much to do with the comet’s orientation relative to Earth. Only a week before, on April 18th, the object lay nearly opposite the Sun in Earth’s sky like the Moon does at full phase. With the Sun at our backs, cometary dust particles hide their shadows, so we only see their bright, sunlit surfaces, leading to a temporary brightness surge called the opposition effect. The same alignment, with a helping hand from coherent backscattering, bumps up the full Moon’s brightness, intensifies Saturn’s rings at opposition, and gives rise to the gegenschein.

C/2023 A3 sketch
I made this sketch of Comet Tsuchinshan-ATLAS through a 15-inch scope at 145× the night of May 5th. While small, it’s a remarkably beautiful object with a surprisingly bright tail and a prominent pseudo-nucleus displaced to the west side of the coma. Star background is from MegaStar and south is up.
Bob King

Qicheng Zhang, a postdoctoral fellow working on small-body astronomy at Lowell Observatory (Arizona), noted that the viewing geometry in mid- to late April also meant that the tail was tucked behind the coma which further boosted the comet’s apparent brightness. In short, the spike in light has nothing to do with intrinsic activity. Comet Tsuchinshan-ATLAS’s magnitude has since leveled out. Barring future outbursts — never a guarantee! — it should gradually brighten. Observers with access to dark skies will be able to track down C/2023 A3 in a 6-inch telescope as it glides northwest across Virgo this month.

Comet C/2023 A3 light curve
The light curve charts the estimated development of C/2023 A3’s brightness. Blue and black dots are visual and photometric CCD observations, respectively, from the COBS (Comet Observation database) or MPC (Minor Planet Center). The gray curve is based on data from the original MPEC (Minor Planet Electronic Circular) or the latest values from the MPC. The red curve is based on fresh observations from MPC / COBS. The green curve shows the potential effect of forward scattering. Under ideal circumstances the comet could reach magnitude –4 in early October when very near the Sun.
Gideon van Buitenen

We expect a slow and steady rise from magnitude 10 in early May to magnitude 8.5 by early July before observers at mid-northern latitudes lose sight of it in the solar glare. Equatorial and Southern Hemisphere skywatchers will be able to hold on to the comet until mid-August when it could reach magnitude 6.5.

When it returns to view in mid- to late September for both hemispheres, C/2023 A3 will hang just a few degrees above the eastern horizon in a twilight-soaked morning sky and shine around 1st magnitude or brighter. When October opens the comet is already dashing back in the Sun’s direction and briefly disappears from view before it slingshots into the evening sky at mid-month.

If the object releases a lot of dust as it rounds the Sun, forward scattering (discussed below) could briefly raise its magnitude into negative territory, suggesting the possibility of seeing this rare visitor in the daytime sky with a small telescope.

Comet phase angle
Forward scattering by comet dust increases as the phase angle (β) increases, leading to a surge in the object’s brightness. The more closely the comet is aligned with the Sun the greater the potential increase. At high phase angles, when a comet appears in nearly the same direction as the Sun, we see its dusty tail glow brightly the same way your frosty breath on a cold morning lights up when you face the Sun.
Bob King

Reasons for optimism

Mid- to late October will likely be the most fortuitous time for observation. As C/2023 A3 swiftly distances itself from the Sun it may exhibit both a prominent dust tail and anti-tail provided it survives perihelion and releases oodles of dust during its solar passage. Here’s why:

  • C/2023 A3 will enter the evening sky at a high phase angle around 145°. Forward scattering of sunlight from the dust could greatly enhance the brightness of the tail.
  • Earth passes through the comet’s orbital plane on October 14th. Dust released prior to this time will be stacked along our line of sight and could greatly amplify the tail’s surface brightness, according to Zhang. 
  • During orbital plane crossing, not only will the material be “superimposed on top of the normal tail,” said Zhang, “it should also extend far in the opposite direction” to form a prominent anti-tail, similar to that observed in Comet Arend-Roland (C/1956 R1) in 1956-7.
Comet Arend-Roland
Comet Arend-Roland and its spiky anti-tail photographed with the 48-inch Schmidt telescope at Palomar Observatory on April 27, 1957.
Public domain

Recent observations reveal yellow and pink hues in C/2023 A3’s developing coma suggesting the presence of dust. That’s a good sign. The wild card is that the comet is dynamically new, a fresh arrival from the Oort Cloud making its first trip around the Sun. Many of its kind are infamous for crumbling apart on approach to perihelion like what happened to Comet ISON (C/2012 S1) in late November 2013. Still, I remain optimistic. Let’s see where Comet Tsuchinshan-ATLAS will take us.

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