Observers on Earth looking up at the night sky could be treated to a rare sighting in the fall of 2024. Comet C/2023 A3 Tsuchinshan-ATLAS, which likely traveled from the outer solar system, made its closest pass to the Sun in September. The comet was expected to come within a range of about 70 million kilometers (44 million miles) of Earth on October 27. The comet was mainly visible to people in the Southern Hemisphere and the tropics until about October 8. Viewers in the Northern Hemisphere would have more opportunities. to catch a glimpse in the coming days.

The crew aboard the International Space Station also observed Tsuchinshan-ATLAS as it traveled through the inner solar system. An astronaut took this photo of the comet on September 19, 2024. At that time, the mass of dust, ice and rock was approaching its closest point to the sun in its highly elliptical orbit. The photo also provides a cross-section of Earth’s bright horizon, or edge, and the planet’s colorful atmospheric layers.

When a comet approaches the sun, it becomes warmer. The heat sublimates its ice into gas, and these gases and dust become a glowing coma and tail that can stretch for millions of miles. The Tsuchinshan-ATLAS dust tail is prominent in this photo, extending toward the top of the image. A second type of tail, the ion tail, is faintly visible and points down and to the right, noted astronomer Bill Cooke, who directs the Meteoroid Environment Office at NASA’s Marshall Space Flight Center.

The sun affects the two types of tails in different ways, often sending them in different directions. The heat and pressure of sunlight push the particles in the dust tail away from the sun, although the tail may bend slightly in the direction the comet came from. Likewise, the solar wind removes ions from the comet’s surface, creating the ion tail, which may extend at a different angle.

Some comets do not survive a close encounter with the sun. If they get too close, radiation and gravity can disintegrate them completely. Tsuchinshan-ATLAS did not suffer this fate, but another comet that astronomers were monitoring, C/2024 S1 ATLAS, may have. Recent data suggests that this comet, which was expected to be visible from Earth later next year, recently broke into fragments, Cooke said.

Given their extremely long orbits, both ancient sky travelers likely originated in the Oort Cloud, a large spherical shell of icy debris at the outer reaches of our solar system. C/2023 A3 Tsuchinshan-ATLAS was discovered in 2023 and identified by observers from China’s Tsuchinshan (or “Purple Mountain”) Observatory and an ATLAS (Asteroid Terrestrial-impact Last Alert System) telescope in South Africa. It was officially named in honor of both observatories.

Editor’s note: The orbits of comets are continually revised as new observational data becomes available. An earlier version of this article cited a period of 80,000 years for C/2023 A3 Tsuchinshan-ATLAS, which is no longer accurate based on newly available data. Starting October 14, 2024, the comet’s orbit may remove it from the solar system altogether.

Astronaut photo ISS071-E-676484 was captured on September 19, 2024 with a Nikon Z9 digital camera with a focal length of 200 millimeters. It is provided by the ISS Crew Earth Observations Facility and the Earth Science and Remote Sensing Unit, Johnson Space Center. The photo was taken by members of the Expedition 71 crew. The image has been cropped and enhanced to improve contrast, and lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab to help astronauts take images of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the Internet. Additional photos taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Story by Lindsey Doermann.