NRL coronagraph captures unique images of a dusty comet

The comet, discovered in 2023, traversed the field of view of NRL's Large Angle and Spectrometric Coronagraph (LASCO) telescope between Oct. 7-11. After the nucleus itself had left the scene, its massive dust trail remained visible for several days.

LASCO has operated on the joint NASA and European Space Agency (ESA) Solar and Heliospheric Observatory (SOHO) spacecraft since the mission launch in 1995. From its vantage point at the Lagrange (L1) position in space, the comet passed almost directly between the Sun and the spacecraft. This resulted in a substantial boost in its apparent brightness as sunlight illuminated the dust surrounding the comet's nucleus was illuminated from behind.

Despite having seen thousands of comets in its almost 29-years of solar observing, comet Tsuchinshan-ATLAS' passage revealed a rare facet of comet dust that LASCO has never seen before.

For a brief period on Oct. 14, the extensive dusty trail of the comet coalesced into a narrow, dense trail spanning the entire field of view. This unique density enhancement was the result of the SOHO spacecraft crossing the comet's orbital plane and observing the comet's vast dust sheet edge-on. The dust plane then moved into the lower-half of the data, where it could be seen globally illuminating half of the field of view.

"LASCO has seen many beautiful comet transits during its decades of operations, but seeing a side-on view of a comet's dust plane absolutely ranks as one of the most spectacular that we have seen," said NRL scientist and LASCO Principal Investigator Karl Battams, Ph.D.

Scientists were curious if it would be the brightest comet ever observed by LASCO, surpassing Comet McNaught in 2007. It did not quite reach the same peak, but did get brighter than a visual magnitude of -4.0[KB1] , earning it the title of the second brightest comet observed.

"We knew this comet was going to be bright, but the extremely fortuitous viewing geometry here has led to a truly spectacular sequence of images," added Battams.

LASCO also plays a significant role in the NASA-funded and NRL-based Sungrazer Project. Since 2003, the Sungrazer Project has been a citizen science program that enables the discovery and reporting of previously unknown comets in heliophysics imaging data, primarily the LASCO and NASA Solar Terrestrial Relations Observatory-A (STEREO-A) observations.

The project is responsible for discovering over half of all officially documented comets and has led to numerous scientific publications looking at comet dynamics, evolution, composition, and more. Through these studies of the interaction between comets and the sun, scientists have gained new insights into the nature of the near-sun environment and solar outflows that drive space weather.

A number of CMEs were also observed during this transit, some of which likely interacted with the comet's dust and gas tails. Observing the reaction of comet tails under such circumstances has historically led to unique insights about the near-Sun environment.

LASCO was not the only NRL instrumentation to observe the comet. NRL's HI-1 heliospheric imager, operating on the NASA STEREO-A spacecraft since 2006, also observed it from Oct. 4-9, as did the recently-launched Compact Coronagraph (CCOR-1) instrument on National Oceanic and Atmospheric Administration (NOAA) GOES-19.

Comet Tsuchinshan is now moving into the early evening skies for northern hemisphere observers, and is currently visible to the naked eye shortly after sunset for observers with a clear view of the horizon. As it moves higher into the evening skies it will gradually fade but should still be a binocular object for several weeks. It will ultimately return to the Oort Cloud -- a vast and distant reservoir of comets at the farthest reaches of our solar system -- following a trajectory that will likely eventually see it ejected from our solar system entirely.