Comets are small entities in the Solar System that emit and expel gas over at least a portion of their orbits. This is due to volatile chemicals on or near their surfaces that sublimate when heated by solar radiation. They are believed to be the ice remains of the outer planets’ early phases of creation. They are among the most dynamically complicated objects in the Solar System. They have entered the inner planetary zone via a remote repository located approximately 50 000 au from the Sun or via chaotic diffusion from the trans-Neptunian belt. Cometary collisions with the planets may account for a fraction of the water on Earth, as well as the frozen reservoirs believed to reside at the bottom of permanently shadowed lunar craters.
Near-Earth Comets (NECs) are comets with a rotation period of fewer than 200 years and a perihelion distance of less than 1.3 au, allowing close encounters to the Earth. Their population is currently approximately 100 but is gradually expanding (1 or 2 entries per year). Small comets have a substantially lower likelihood of colliding with the Earth than near-Earth asteroids. The NEO Coordination Centre has chosen the comet catalog maintained by JPL and kindly made it available to ESA as the authoritative data source. The NEOCC search functionality allows for displaying information about each comet, such as orbital elements, the date of the following perihelion passage, the orbit type (i.e. periodic, non-periodic, disappeared, or historical apparition), and if the comet is a member of the NEC group. Visualization of the comet’s orbit is also possible from the comet overview page.
A new class of near-Earth objects has been discovered, expanding the already-large menagerie of space objects that may one day crash into our planet. These new comets are described as long-period comets, meaning they orbit the sun far out in space, often coming closer to Earth only once every several hundred years. Their elliptical orbits cause them to be classified as Near-Earth objects due to their close proximity to our planet as they pass by, but do not actually cause them to collide with Earth.
Extinct Comets: Dynamical and Physical Evidence
A strong approach to unraveling the enigma of how many extinct comets remain in the NEO population is to examine both dynamical and physical parameters to find potential candidates. For instance, numerical simulations of the trajectories of short-period comets can be used to predict their likelihood of becoming NEOs as a result of gravitational interactions with Jupiter and the other planets. IThousands of hypothetical comets can be watched for millions of years in these simulations, each with a slightly different initial orbit, to highlight how they are hurled around chaotically. Similarly, thousands of potential starting points for main-belt asteroid orbits can be modeled to determine the efficacy of resonances in directing asteroids toward near-Earth space. Alessandro Morbidelli, William Bottke, and colleagues determined the relative efficacy of various dynamical processes through complex computer simulations. According to their calculations, around 6% of NEOs with 250 m or more excellent dimensions are comets.
Spacecraft and telescopic observations of known comets provide insight into the traits to search for when determining if an asteroid-like NEO is an inactive comet nucleus. For instance, spacecraft data indicate that portions of the surface of comets Halley, Borrelly, Wild 2, and Tempel 1 that are not actively outgassing are very dark (low albedo) and grey in hue. Other comets experience periods of very low activity, allowing astronomers to measure the nucleus’s albedos and colors unobscured by the coma’s gas and dust. These measurements consistently reveal low albedos (the surface reflects around 4% or less of the incident light) and grey or reddish colors. Spectroscopic study of reflected sunlight reveals no significant absorption bands, implying that the minerals olivine and pyroxene, frequently detected in asteroid spectra, are not present on their surfaces.
