When an object from outside our star system zooms through our neighborhood in the solar system, it provides a rare opportunity to study the raw material from which distant worlds are made. The interstellar comet known as 3I/ATLAS has become just such a laboratory and is exhibiting a series of behaviors that are pushing the boundaries of current cometary theory.
At perihelion, the closest point to the Sun, the comet’s size has been estimated to be 1.36 astronomical units, or roughly 203 million kilometers. Analysis by NASA’s Jet Propulsion Laboratory has also shown that the comet has a radial component of acceleration at 135 kilometers per day² and a transverse component of 60 kilometers per day². These measurements far surpass expectations based on models for outgassing, even when taking into consideration the effects of gravity. Based on the sublimation of ice alone, the comet would lose a tenth of its mass in a month, resulting in a clearly visible plume.
Spectroscopy has further contributed to the enigma surrounding the comet’s origin. With the Very Large Telescope’s X-shooter and UVES, nickel vapor was discovered at 3.88 AU, which is much colder than the usual conditions for metal vaporization and in much larger quantities than iron vapor. Together with the ratio of carbon dioxide to water vapor, which the James Webb Space Telescope indicates to be about 8:1, the results suggest an environment conducive to the formation of the comet with high concentrations of CO₂, perhaps an ice line in its native planetary system. The comet’s distinctive blue color, perhaps from ionized carbon monoxide, was maintained through perihelion.
Other observations have verified more typical characteristics of comets. Hubble observations have put constraints on the size of the nucleus to be between 320 meters and 5.6 kilometers, while dust plumes are consistent with active comets in the solar system. Hydroxyl radicals, which result from ice decomposition, were discovered at distances at which most comets are dormant. This water loss of about 40 kg per second indicates icy grains are vaporizing at distances away from the Sun, which are consistent with layered ices.
However, the acceleration itself has also been the subject of debate. A thermophysical and Monte Carlo analysis carried out by Florian Neukart suggests that jets of CO and CO2 driven by volatile substances were sufficient to explain the acceleration with less than one percent of the surface active. This is consistent with known physics of cometary bodies and eliminates the requirement for exotic acceleration mechanisms. A small number of jets may explain the acceleration with the retained volatile material being tens of centimeters from the surface.
Certain scientists, such as Avi Loeb, have listed a set of “anomalies,” from the peculiar polarization to the rapid brightening, and have suggested that these should not be attributed to any technological cause. Planetary scientists argue that these properties are not unusual when considering the large variability of cometary phenomena, especially when comets are created under alien stellar environments and are moving at a speed of 130,000 miles per hour after interstellar travel for thousands of years.
With the arrival of the PUNCH mission by NASA, a new aspect of the comet 3I/ATLAS is being unveiled, as it is being monitored minute by minute as it is hidden behind the Sun as far as the Earth is concerned. This provides information on the interaction between the tail of the comet and the solar wind.
Future telescopes such as the Vera Rubin Observatory hold the key to increasing the rate at which such interstellar objects are discovered. Missions studied at the Southwest Research Institute have shown that reconnaissance missions are possible with current propulsion technology, which means direct sampling missions could become a reality in the future. At least for now, 3I/ATLAS is a unique case study. What exactly is fueling its acceleration: exotic chemistry, complex jet geometry, or a combination of both? Its trajectory is changing the way researchers model icy bodies from beyond our solar neighborhood and reminding us that planetary systems come in far more variety than our own.
