Spectroscopy of Dynamically New and Long Period Comets from TelescopioNazionale Galileo

Comets are some of the oldest objects in our solar system and are often seen as remnants from its formation. The nuclei of comets are thought to have formed inward in the solar system, in the giant planets’ region, where ice can easily form. Later, the giant planets changed the paths of smaller bodies and pushed many into areas where comets are now found (Ceccarelli et al. 2022). The Kuiper Belt contains the precursors of short-period comets, which have low-inclination orbits, while the Oort Cloud is a distant region with long-period comets that are considered to be among the least altered objects since the formation of the solar system. The outside of a comet’s nucleus is exposed to cosmic and interstellar radiation, which creates a protective layer of tough material. This layer helps to keep the ices inside safe (Prialnik & Bar-nun 1988).

The upcoming Comet Interceptor (CI) mission by ESA (Jones & et. al 2024) represents the initial attempt to strategically meet an Oort Cloud comet as it travels toward the inner Solar System. The mission aims to intercept a Dynamically New Comet (DNC) or an interstellar object (Meech et al. 2017; de León et al. 2019) before its perihelion passage, offering a unique chance to collect data on its composition, activity, and the surrounding dust and gas. Given the brief timeframe between a comet’s detection and its approach to perihelion, the CI mission is set to launch without a specific target. Instead, it will depend on identifying a suitable target post-launch. This strategy requires a solid understanding of the expected behaviour and activity of DNCs. Understanding these comets’ evolution is vital to optimize the mission’s scientific results.
Spectroscopic analysis plays a crucial role in defining the composition of cometary ices, providing insights about a wide range of molecules detectable across various wavelengths. In the visible range, cometary spectra are dominated by molecular bands of daughter species such as OH, CN, C2, C3, CH,
and NH2, which result from the photolysis of parent molecules. These daughter species can either sublimate directly from the nucleus or be ejected into the coma, with their distribution acting as a key indicator of the comet’s internal composition. In the infrared range, spectra display rovibrational transitions of primary parent molecules, such as H2O, CO2, CH4, HCN, NH3, and NH2 (Bockelée-Morvan & Biver 2017). Additionally, atomic lines from alkali metals like Na and K are observed in the coma, providing essential data for studying the solar system’s original elemental abundances (Fulle et al. 2013). 

Observing comets from a high inbound distance from perihelion is crucial to understand their behaviour as they approach the Sun for the first time. One of the main objectives of current research, in anticipation of the Comet Interceptor mission, is to identify the primary drivers of cometary activity.
The purpose is to determine how and when the transition from one activity driver to another occurs. For instance, several comets with well-documented high-distance activity, such as C/2017 K2 (active at rh = 23.7 AU) (Jewitt et al. 2017), C/2010 U3 Boattini (active at 25.8 AU) (Hui et al. 2019), and
C/2014 UN271 Bernardinelli-Bernstein (active at 23.8 AU) (Farnham et al. 2021), are believed to have their activity driven by volatile species other than water ice. These comets are thought to be primarily influenced by sublimation of supervolatiles like CO or CO2 (A’Hearn et al. 2011). 
In preparation for the Comet Interceptor mission, a detailed series of spectroscopic observations of Oort Cloud comets has been carried out using the Telescopio Nazionale Galileo with the DOLORes and NICS instruments, covering a total wavelength range of 3800 - 25000 Å. This initiative involved six programs (AOT 37-41-42-43-44-45), through which 31 comets were observed, of which 11 are long-period comets and 20 are Hyperbolic. Notably, four of the hyperbolic comets meet the criteria for classification as DNC. These programs encompass heliocentric distances ranging from 1.4 to 9.2 AU, offering an extensive overview of the general behaviour of long-period and hyperbolic comets along various segments of their inbound trajectories.

Preliminary results indicate that there are few signs of activity in terms of gas emissions. At great distances from the Sun, the primary indicator of a comet’s activity is its dust environment. In addition to spectral data, imaging frames of the targets have been collected to monitor the shape and development of the coma alongside gas emission observations. In these data, beyond 5 AU, the coma appears condensed, and the continuum shape in the spectrum is very narrow. As we get closer, the thickness of the continuum increases, the diameter of the coma begins to expand, and only below 3 AU, certain
spectroscopic features start to emerge, such as CN blue band and C2 Swan bands.

This work presents the preliminary results from the analysis of this large campaign of observations with TNG. Also, one of the newest discoveries of ideal Comet Interceptor virtual targets is covered by this presentation.
C/2024 E1 (Wierzchos) is one of the most recently discovered long-period comet currently in its inbound arc and very little is still known about this target. Discovered in March 2024, C/2024 E1 was located 8 AU from the Sun and is expected to pass perihelion on January, 20, 2026 within Earth’s orbit. For this
target, at the time of this writing, two spectra with MODS at LBT and three spectra from AFOSC at INAF Copernico telescope in the visual range have been collected. The results of these spectra will be compared with JWST observation of the comet (PI: C. Snodgrass, Snodgrass et al. (2025), preprint).