What does it take to pursue a cosmic bullet? This week, scientists are on the case as interstellar comet 3I/ATLAS makes its final passage by our planet on its way out to the distant reaches of space. The third confirmed interstellar visitor to be discovered, comet 3I/ATLAS provides a brief window of opportunity to examine the chemistry, dynamics, and composition of a cosmic bullet produced elsewhere in another star system.
On December 19th, the comet will be 167 million miles away, close enough to see the fuzzy star in the constellation Leo, but only for those with a large backyard telescope. “It’s like glimpsing a rifle bullet for a thousandth of a second,” says David Jewitt, a comet observer who uses the Hubble Space Telescope. At a speed of 60 km/s, it is impossible to trace the comet’s course. This comet’s orbital eccentricity of 6.145 and velocity of infinity of about 60 km/s means it is not bound to the Sun.
In an effort coordinated by the United Nations’ International Asteroid Warning Network (IAWN), over 80 observatories and citizen scientists worldwide have been tasked with monitoring the passing stellar object, dubbed 3I/ATLAS. This object marks the first comet to be tracked from beyond the solar system since the monitoring campaigns launched by IAWN in 2017, and it is essentially a testing ground for cutting-edge astrometry tools intended for use with spacecraft interception missions in the future. The problem is made more difficult by the comet’s variable coma.
High-resolution imaging by the Hubble Space Telescope indicates that the optical cross section is dominated by the sunward puffed-out fan of comet ejecta, rather than the tail following radiation pressure. Cometary dust ejection velocities are estimated at 50 m/s toward the Sun and 7.5 m/s orthogonal to the orbital plane, which is much too low compared with the thermal velocities of sublimating gases such as water or carbon dioxide. This indicates poor gas-dust coupling, which may have a number of causes, such as the presence of large particles with sizes ranging from tens to hundreds of microns, or the sublimation of ice below the surface porous mantle. The coma’s enhanced surface brightness gradient indicates the continuous destruction of ice particles while being transported toward the Sun.
The mass-loss rates are suggested to range from 12 to 120 kg/s, depending on the size range, with the estimated dust production by the sublimation of ice H₂O, CO₂, or CO ices from regions as small as 0.04 km² for the most volatile body. The mass-loss rates, in units of 1 AU, are similar to or even higher than those for 2I/Borisov and are much higher than the rates observed in the dormant 1I/’Oumuamua. The actual nucleus itself has not been observed yet, although Hubble Space Telescope photometry limits its size to 2.8 km or smaller, and it could potentially be below 1 km.
In terms of celestial mechanics, 3I/ATLAS’ orbit can be considered to be an extreme case of a hyperbolic orbit. These orbits occur when either a body’s velocity overcomes solar escape velocity, either through ejection from another planetary system or through scattering in the Oort Cloud by a massive perturber. Models of hyperbolic orbits indicate that orbits with large values of eccentricity and small values of perihelion distances, like that of 3I/ATLAS, are much more probably genuine interstellar objects than Oort Cloud comets that were scattered on to escape orbits. The velocity with which this comet struck our solar system’s planetary region implies that it has been roaming our galaxy for approximately several billion years, with possible alteration of its exterior through exposure to cosmic rays to form either a refractory exterior layer or “mantle,” which prevents sublimation aside from areas penetrable to solar heating.
Finder Chart: Since the comet is faint, it will not impress the naked eye, but it should be visible as a faint smear through an 8-inch or larger telescope when the sky is dark enough. While a GoTo or smart telescope can guide the observer to the coordinates adjacent to the star Regulus and then to the comet itself, the star-hop approach would involve guiding the telescope to the star Rho Leonis and then to the comet’s location. Gianluca Masi’s Virtual Telescope Project will webcast the flyby of this rare visitor from another star system.
By March 2026, the 3I/ATLAS will fly by the orbit of Jupiter and continue through the orbits of Saturn, Uranus, and Neptune before escaping the solar system as a whole. After passing through the orbit of Pluto in 2029, 3I/ATLAS will become subtle in transitioning into interstellar space as the Sun becomes no more than a star behind this object. Today’s event marks a scientific nadir for astronomers and amateur star gazers alike as 3I/ATLAS reminds us of the massive exchange between star systems that this comet allows us to see in action.
