Observers on Earth looking into the night sky could see a rare sight in the fall of 2024. Comet C/2023 A3 Tsuchinshan-ATLAS, which likely traveled from the outer reaches of our solar system, made its next flyby of the Sun 27 in September and was expected to come within about 70 million kilometers (44 million miles) of Earth on October 12 . The comet was primarily visible to people in the southern hemisphere and the tropics until around October 8th. Viewers in the northern hemisphere would have more opportunities to take a look at it in the next few days.

Crew members on board 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 this point, the mass of dust, ice and rock was approaching the closest point to the Sun in its highly elliptical orbit. The photo also provides a cross-sectional view of Earth's bright horizon, or edge, and the planet's colorful atmospheric layers.

When a comet approaches the sun, it becomes warmer. Heat causes the ice to sublimate into gas, and these gases and dust become a glowing coma and tail that can stretch for millions of kilometers. Tsuchinshan-ATLAS's dust tail is clearly visible in this photo and extends to the top of the image. A second type of tail, the ion tail, is faintly visible and points downward and to the right, noted astronomer Bill Cooke, who leads 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 pushes the particles in the dust tail away from the sun, although the tail may bend slightly in the direction from which the comet came. Likewise, the solar wind draws ions from the comet's surface, creating the ion tail, which may extend at a different angle.

Some comets do not survive close encounters with the Sun. If they get too close, radiation and gravitational forces can completely disintegrate them. Tsuchinshan-ATLAS did not suffer this fate, but another comet that astronomers observed, C/2024 S1 ATLAS, did. Current data suggests that this comet, which should be visible from Earth later next year, may have recently broken into fragments, Cooke said.

Because of their extremely long orbits, these two ancient celestial travelers likely came from the Oort Cloud, a large spherical shell of icy debris at the outer edge of our solar system. C/2023 A3 Tsuchinshan-ATLAS was discovered in 2023 and identified by observers at China's Tsuchinshan — or “Purple Mountain” — observatory and an Asteroid Terrestrial-Impact Last Alert System (ATLAS) telescope in South Africa. It was officially named in honor of both observatories.

Editor's Note: Comet orbits are continually revised as new observational data becomes available. An earlier version of this article stated a period of 80,000 years for C/2023 A3 Tsuchinshan-ATLAS, which is no longer correct due to newly available data. From October 14, 2024, the comet's orbit could be taken completely out of the solar system.

Astronaut photo ISS071-E-676484 was taken 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 Johnson Space Center's Earth Science and Remote Sensing Unit. The image 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 capture images of Earth that are of greatest value to scientists and the public and to make these images freely available on the Internet. Additional images captured by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Story by Lindsey Doermann.