A mysterious interstellar visitor has emerged from behind the sun, glowing in a way that has left scientists stunned.
The comet, which is not visible from terrestrial telescopes, appears to have unusual chemistry. It passed through solar conjunction with Earth – meaning it was hidden behind the sun – on October 21.
It is exciting for scientists because, unlike other comets, it does not orbit the sun.
The object, named as 3I/ATLAS, made its closest approach to it on Wednesday, observed by three NASA[1] spacecraft.
Data showed that it brightened at a rate about seven times faster than typical comets as it reached the sun.
Researchers who captured the observations noted: ‘The reason for 3I/ATLAS’s rapid brightening, which far exceeds the brightening rate of most Oort cloud comets at similar distances from the sun, remains unclear.’
Even more puzzling, the comet now appears distinctly bluer, a reversal from its previously reddish hue.
Scientists speculate that the blue glow likely comes from gas emissions, including carbon molecules (C2) and other compounds, rather than just sunlight reflecting off dust.
The object made its closest approach to the sun on Wednesday, as seen by NASA spacecraft
The study, conducted by researchers from the Lowell Observatory in Arizona and the US Naval Research Laboratory in Washington, DC, pulled data from the STEREO‑A (Solar TErrestrial RElations Observatory) and SOHO (Solar and Heliospheric Observatory).
They also used a weather satellite, called GOES‑19, carrying a coronagraph, a device which helped capture the comet’s brightness and color while it was hidden behind the Sun from Earth-based telescopes.
It detected a glowing plume extending nearly 186,000 miles around the object, comparable to the carbon dioxide halo observed months earlier.
The satellite revealed that 3I/ATLAS is enveloped in a huge, fuzzy coma, roughly half as wide as the full moon.
Ground-based telescopes had trouble observing 3I/ATLAS. At the same time, it was very close to the sun, but radio observations were able to detect emissions from the comet, showing it was producing huge amounts of water-related molecules.
The production rate of these molecules increased sharply as the comet approached the Sun, following a pattern similar to the rapid brightening observed in optical images.
The comet is expected to return to visibility in twilight and night skies during November and December, allowing ground-based telescopes to study it in much greater detail.
Early analysis suggests 3I/ATLAS will emerge considerably brighter than before.
Its light was increasing at an unusually rapid pace, which could be due to its chemical make-up
Scientists still do not fully understand why it brightened so fast, but it may be related to unusual activity from gases like carbon dioxide (CO2) and water (H2O), or unique properties of the comet’s nucleus, such as its composition, shape, or structure.
Observations from spacecraft also show that 3I/ATLAS has a large, fuzzy coma about half the size of the full moon, and its light is distinctly bluer than the sun, likely due to gas emissions from molecules like C2 and Amino groups, rather than just reflected sunlight from dust.
Its brightness increased much faster than typical comets, with a measured rate far exceeding the standard for Oort cloud comets, and the post-perihelion behavior remains uncertain; it could continue brightening, level off, or fade quickly.
Harvard professor Avi Loeb, who has been studying the object for months, said: ‘This unfavorable geometry, a possible hint of design, placed the comet within the fields of view of several space-based solar coronagraphs and heliospheric imagers, allowing continuous observation during its final approach to perihelion.’