An interstellar comet can seem to be silent when it is the hottest and so at the same instant it will suddenly unleash its energy on its return journey. This is the nature of such a difference between heating and reaction as an engineering fact that SPHEREx was constructed to sense: the faint infrared signals that inform us about what ices and gases are up to when sunlight finally gets to peep beneath one of the processed surfaces.
NASA SPHEREx (2025) then in December 2025 turned its 102 band infrared vision to comet 3I/ATLAS and saw the object glowing in the sky in an enormous manner long after it had passed through perihelion. It was found to have the organic molecules like methanol, cyanide and methane, a coma that was more complex compared to what earlier readings had revealed. The time was the significant clue: the maximum heating around the nearest contact was not correlated with the maximum outgassing, and this fact proved that it took time to transport heat to the deeper layers until the volatile-rich material began to escape.
The head scientist of the experiment, Carey Lisse, explained the event by merely saying the following: Comet 3I/ATLAS was at full blast out in space in December 2025, when it was in close proximity to the Sun, making it glow significantly brighter. The water ice was even evaporating rapidly in the interstellar space to gas. The coma at the time of that eruption had accumulated blocks of ice body familiarity, forming a water body, and carbon bearing compounds; it was also a chemical record of matter which had been accrued about another star. Gases were not the only ones to be blown off, SPHEREx observations even indicated the existence of rocky material that was expelled from the nucleus, and a dust tail was being formed as a consequence of the solar radiation pressure and an outgassing of relatively large grains and BB-size particles, which is hard to push away the comet.
The fact that reaction is late is subject to a mere physical image. The fibre of the crust long since altered by cosmic ray may then serve to insulate the lower strata, and the ice locked in the ground does not instantly start warming when the sun radiation strikes it. The example of this process was in the case of Phil Korngut: The comet has been moving through the interstellar space over many years and it is open to very high energy cosmic rays, and most likely a crust has been exposed to such radiations and has been subjected to them. In the recent days however, as the energy of the Sun has amassed deep into the comet, the ice at the core of the comet begins to warm and burst, releasing a cocktail of chemicals that never saw the light of day in the space environment in the last billions of years.
The number of these visitors confirmed has been very few and is interesting in that there exist a number of space-based assets which could follow the behavior of a geometry changing. It was made to move on a hyperbolic orbit, not slow enough to orbit the Sun, but passed through the inner solar system on a one-way orbit. It had independent constraints, which helped to frame the target: the Hubble observational estimate was 440 meters up to 5.6 kilometers in diameter of the nucleus, and acceleration of the comet rose to only some 246,000 km/h at perihelion, and cannot fall below 1.8 astronomical units the Earth.
It was not expected that one of the longest tracking was made. In one of the projects of Southwest Research Institute, data retrieved by NASA/PUNCH spacecrafts were used to track 3I/ATLAS across a whole period of weeks when it was virtually covered by the glare of the Sun through the use of conventional telescopes on the ground. PUNCH (a coronavirus digester) was designed to seize the solar corona and the solar wind, and its wide field of view with a high frequency, about one image a minute during this campaign, enabled it to follow the variations in the brightness and the tail development through an otherwise empty expanse of the comet.
In each of these observations, it is not drama at the perihelion which is to come about afterwards but the bookkeeping of composition. A separate analysis of 25 comets found systematic variations in the relative abundances of water, carbon dioxide and carbon monoxide with the orbital history, distant or less-processed comets favoring high CO2 over CO and repeat inner-solar-system visitors over outer ones. That model gives additional meaning to SPHEREx observations of 3I/ATLAS: an outburst at the season end is not merely a difference in brightness, but a late sampling of what had already been covered up by thermal diffusion until it was given a hole by diffusion.
It is based on the work of the all-sky survey SPHEREx, but it is in the background of a truth of general interest in modern astrophysics: a general-purpose mapper can be converted to a high-precision comet laboratory with the right geometry and timing.
