The U.S. Naval Research Laboratory (NRL) has utilized imaging equipment from three sun-orbiting observatories to track the movements of comet C/2023 A3, also referred to as Tsuchinshan-ATLAS, as it moved between Earth and the Sun in early October 2024.
First spotted in 2023, the comet was visible in the field of the NRL’s Large Angle and Spectrometric Coronagraph (LASCO) telescope from October 7 to 11. Although the comet’s nucleus moved out of sight, its large dust trail lingered for several days afterward.
LASCO has been operational on the NASA and European Space Agency’s Solar and Heliospheric Observatory (SOHO) since its launch in 1995. Positioned at the Lagrange (L1) point in space, it saw the comet passing almost directly between the Sun and its spacecraft. This alignment significantly enhanced the comet’s brightness, as sunlight illuminated the dust surrounding its nucleus from behind.
During its nearly 29 years of observing the Sun, LASCO has witnessed thousands of comets. However, Tsuchinshan-ATLAS showcased a distinct characteristic of comet dust that had never been observed before.
On October 14, the comet’s expansive dusty trail briefly formed a narrow, dense band that spanned the entire observation area. This unusual density enhancement occurred when the SOHO spacecraft crossed the comet’s orbital plane, allowing for an edge-on view of the dust sheet. Consequently, the dust plane moved to the lower half of the data, illuminating half of the field of view on a global scale.
“While LASCO has documented numerous stunning comet transits, witnessing a side-on perspective of a comet’s dust plane definitely ranks as one of the most extraordinary observations we’ve made,” stated Karl Battams, Ph.D., NRL scientist and LASCO Principal Investigator.
Scientists were eager to find out if this would be the brightest comet recorded by LASCO, potentially eclipsing Comet McNaught in 2007. Although it didn’t quite match that brightness, it exceeded a visual magnitude of -4.0, earning the title of the second brightest comet observed.
“We anticipated that this comet would be bright, but the excellent viewing conditions have produced a truly impressive series of images,” added Battams.
LASCO also contributes significantly to the NASA-funded Sungrazer Project, which NRL runs. Since its inception in 2003, this citizen science initiative has facilitated the discovery and reporting of previously unknown comets using heliophysics imaging data, particularly from LASCO and the NASA Solar Terrestrial Relations Observatory-A (STEREO-A).
This project has led to the discovery of more than half of all officially registered comets and has produced numerous scientific publications on various aspects of comets, including their dynamics, evolution, and composition. These studies enhance our understanding of the interactions between comets and the Sun, providing insights into the near-sun environment and solar outflows that influence space weather.
While the comet was in transit, several coronal mass ejections (CMEs) were recorded, some likely interacting with the comet’s dust and gas tails. Studying how comet tails react under such conditions has historically yielded unique insights into the environment near the Sun.
NRL’s instruments were not limited to LASCO; the HI-1 heliospheric imager, which has been operating on NASA’s STEREO-A spacecraft since 2006, observed the comet from October 4-9. The newly launched Compact Coronagraph (CCOR-1) on the National Oceanic and Atmospheric Administration (NOAA) GOES-19 satellite also captured data on it.
Now, comet Tsuchinshan is becoming visible in the early evening skies for observers in the northern hemisphere and can be seen with the naked eye shortly after sunset, provided the horizon is unobstructed. As it ascends into the evening skies, it will gradually dim but should remain a visible object through binoculars for several weeks. Eventually, it will return to the Oort Cloud—a vast and remote area in our solar system filled with comets—following a path that may lead to its total expulsion from the solar system.
