There\u2019s been plenty in the news about 3I\/ATLAS over the course of the past 8 months. Our third confirmed interstellar visitor went behind the Sun during its closest approach, but reemerged in December with plenty of eyes watching it. Papers describing what it looks like following its closest brush with the power of a star in probably billions of years are starting to come out, including a new one available in pre-print on arXiv from Carey Lisse of Johns Hopkins University and his co-authors, which shows how much the comet – and it is definitely a comet – has changed in the matter of only a few months.<\/p>\n
Their observational tool of choice is NASA\u2019s SPHEREx observatory, which launched in March last year only a few months before 3I\/ATLAS\u2019s discovery. Its primary mission is to map the entire sky in near-infrared every six months, but it has a secondary mission of tracking \u201ctargets of opportunity\u201d. 3I\/ATLAS is one such object – and a great test of SPHEREx\u2019s observational capability of a fast moving target.<\/p>\n
During a preliminary observational run in August last year, SPHEREx was watching 3I\/ATLAS closely – and not seeing much. The interstellar visitor showed very little activity, with only minor outgassing and very little to no water being released at all. The paper mentions that is most likely due to the cosmic rays that had been bombarding the comet in interstellar space for the last few billion years. They would have created a \u201cshell\u201d of process material devoid of volatile ices that would easily sublimate.<\/p>\n
Fraser goes into all the latest science on 3I\/ATLAS – as of October 2025<\/p>\n
But the Sun had begun blasting this rock with more than just cosmic rays, and as it seems to have finally cracked it open. During SPHEREx\u2019s next observational run at the comet, around December of last year, it showed significant changes that proved the thermal wave of energy from our star broken through its protective crust.<\/p>\n
One of the most notable features was water – it skyrocketed to nearly 40 times what the water production level was when it was observed in August. Carbon dioxide was another outlier, with production jumping to 80 times the original reading. Even the ratio of carbon monoxide to carbon dioxide changed to 2.5 – a more typical reading of the carbon-monoxide dominant comets we see in our own solar system. <\/p>\n
But it wasn\u2019t just the regular water and carbon\/oxygen combinations. There were other volatiles too – most notably a rich soup of organic molecules, which likely included methanol, formaldehyde, methane, and ethane, which are hard to distinguish in the spectrographic signature of SPHEREx\u2019s data. A new feature, that wasn\u2019t present at all in the August data, was a spectral line at 0.925um – which shows that the comet is also outgassing cyanide. <\/p>\n
Fraser discusses 3I\/ATLAS\u2019s perihelion, and what it means for the comet.<\/p>\n
The levels of this outgassing are such that the authors think the comet is evaporating before our eyes as it passes as close to as star as it has, possibly in its billions of years of existence. The shape of the outgassing appears :\u201dpear-shaped\u201d – literally in this sense rather than figuratively. The \u201cstem\u201d of the pear is pointing toward the Sun, but there is no anti-sunward tail, which normally forms when small particles are pushed via radiation pressure. The authors believe this is because the particles that form the comet\u2019s halo aren\u2019t actually small enough to be pushed around by radiation – and are likely on the order of centimeter or even decimeter size.<\/p>\n
So this really is what it\u2019s like watching a mult-billion year old comet die. It doesn\u2019t seem likely the 3I\/ATLAS will completely disintegrate before it leaves the solar system to carry on its interstellar journey. It is, at this point, traveling away from the Sun, and our star\u2019s energy directed toward it will slowly diminish. It should escape intact, but not before SPHEREx itself will have one final observational window to watch our retreating visitor in April. There\u2019s sure to be plenty more papers to come about this unique observational opportunity.<\/p>\n
Learn More:<\/p>\n
C.M. Lisse et al. – SPHEREx Re-Observation of Interstellar Object 3I\/ATLAS in December 2025: Detection of Increased Post-Perihelion Activity, Refractory Coma Dust, and New Coma Gas Species<\/a><\/p>\n UT – 3I\/ATLAS’s Coma Is Largely Carbon Dioxide<\/a><\/p>\n UT – Hubble Catches Another Glimpse of 3I\/ATLAS<\/a><\/p>\n