Did you see the comet? Visible just after sunset for the last week from the northern hemisphere — and now receding from Earth along its orbital path — comet Tsuchinshan-ATLAS (also called C/2023 A3 and Comet A3) became the brightest comet for over a decade.
First discovered by China’s Purple Mountain Observatory and the Asteroid Terrestrial-impact Last Alert System (ATLAS) in January and February 2023, respectively, comet Tsuchinshan-ATLAS has captivated observers with its bright tail streaking away from the sunset.
A long-period comet from the Oort Cloud, comet Tsuchinshan-ATLAS is a sphere around our solar system home to millions of comets and is on an incredibly long orbit lasting around 80,000 years. Some astronomers doubt it will ever return to the inner solar system.
The comet reached its perihelion — the closest it gets to the sun — at about 36 million miles (58 million km) on Sept. 27. That’s about the distance that Mercury orbits the sun from. It was visible for around a week before sunrise, though mainly in the Southern Hemisphere. It wasn’t exceptionally bright and remained a challenging observation, but plenty of photographers imagined the icy space rock in the days before it cruised into the sun’s glare.
Thankfully for skywatchers, it survived its passage close to the sun — the riskiest part of any snowball’s journey close to a star — to become brighter than comet NEOWISE (also known as the “lockdown comet” because it appeared in July 2020, during the Covid pandemic).
It got closest to Earth on Oct. 12 when it passed about 44 million miles (71 million km) from Earth, with peak post-sunset viewing for the entire planet beginning Oct. 10. Come Oct. 14, Earth passed through the comet’s orbital plane, creating the best views of the comet and, crucially, its tail, of its apparition.
One unique image came from the LASCO instrument on NASA and ESA’s SOHO Observatory, which orbits Earth. In the days before the comet became visible above the horizon, it was lost in the sun’s glare but not to LASCO, which imaged it very close to the sun using its coronagraph disk to occult the sun’s disk.
However, on Oct. 10, it snapped comet Tsuchinshan-ATLAS as a strong X2.1 solar flare (extreme ultraviolet flashes of electromagnetic radiation) and a pair of coronal mass ejections (clouds of charged particles) left the sun. A video was also published of the event, with Tsuchinshan-ATLAS claimed to be the second brightest comet SOHO has ever seen (below).
As one comet fades, another could brighten. Comets visible to the naked eye are very rare, but remarkably, comet Tsuchinshan-ATLAS could be the first of two bright comets this month. C/2024 S1 (Atlas) will make its closest approach to Earth on Oct. 23 and have its perihelion with the sun — its closest pass — just five days later on Oct. 28. It could be extraordinarily bright, or it could break apart and be virtually invisible — only time will tell.
Wishing you clear skies and wide eyes.
