Interstellar Comet 3I/ATLAS’s Surprising Blue Glow
Comet 3I/ATLAS, only the third known interstellar object to visit our Solar System, has begun brightening far faster than astronomers anticipated while nearing its perihelion—the closest point to the Sun.
Over the past month, the comet was positioned almost directly behind the Sun from Earth's point of view, making it nearly impossible to observe from the ground during this key period. Instead, scientists turned to space-based observatories for continuous monitoring.
Unexpected Help from Solar Satellites
An unexpected group of observers stepped in: solar-monitoring spacecraft. Researchers Qicheng Zhang of Lowell Observatory and Karl Battams of the US Naval Research Laboratory identified that satellites designed to monitor the Sun’s corona could also follow the comet during its near alignment with our star.
Using instruments on board STEREO-A, SOHO, and GOES-19, they documented the comet’s remarkable transformation.
Rapid Brightening Observed
Between mid-September and late October, as Comet 3I/ATLAS approached from about 2 astronomical units (AU) to roughly 1.36 AU—nearly half that distance from Earth to the Sun—its brightness surged sharply.
The team found that the comet’s brightness rose in proportion to the inverse of its heliocentric distance raised to the 7.5 power, a much steeper increase than was earlier recorded when it was farther from the Sun.
For comparison, most comets brighten gradually as warming sunlight converts surface ice into gas, but 3I/ATLAS appears to be behaving far more energetically.
Author’s Summary
The interstellar comet 3I/ATLAS is astonishing scientists with an exceptionally fast brightening rate, revealing new insights into how these rare visitors interact with solar radiation.