Comet 3I/ATLAS Brightens Rapidly Near Perihelion

Comet 3I/ATLAS, just the third confirmed object to visit from beyond our Solar System, has been increasing in brightness much faster than anticipated while travelling towards perihelion, its nearest approach to the Sun.

For the last month, the comet has sat almost exactly behind the Sun as viewed from Earth, leaving ground-based astronomers largely unable to observe it at this vital stage. Space-based observatories have therefore taken over the watch.

Solar satellites track Comet 3I/ATLAS

An unexpected set of observers has stepped in: satellites built to monitor the Sun.

Qicheng Zhang of Lowell Observatory and Karl Battams of the US Naval Research Laboratory recognised that spacecraft intended to observe the Sun's corona could follow the comet while it was close to conjunction with our star.

They used instruments on STEREO-A, SOHO and GOES-19 to record the comet's striking change.

Between mid-September and late October, 3I/ATLAS moved inwards from approximately 2 astronomical units (around twice the Earth-Sun distance) to only 1.36 AU. Over that interval, its brightness rose sharply.

The researchers determined that its brightness grew in proportion to the inverse of heliocentric distance raised to the 7.5 power. This is substantially steeper than the brightening rate measured earlier, when the comet was more distant.

Unusually rapid brightening and blue colour

Most comets brighten steadily on approaching the Sun, as their ice changes into gas. This interstellar visitor is becoming brighter at about double the usual rate, indicating that something unusual may be occurring on its surface.

The data further showed that the comet looks noticeably bluer than sunlight, an indication that gas, rather than dust alone, is making a major contribution to its visible brightness.

Previous observations had indicated that its dust was reddish, so this move towards blue is especially notable. The team believes emissions from molecules such as cyanogen, and potentially ammonia, may account for the unusual colour.

GOES-19 coronagraph images showed the comet as an extended object, with a visible atmosphere, known as a coma, spanning roughly four arc-minutes in the sky. This luminous cloud of gas and dust around the nucleus confirms that 3I/ATLAS is actively losing material as heating from the Sun strengthens.

Perihelion and forthcoming observations

The comet passed perihelion on 29 October. The researchers' calculations indicate that it could have reached about magnitude 9, sufficiently bright to be seen with smaller amateur telescopes.

As it now comes out from behind the Sun and returns to dark skies during November and December, ground-based observers will at last be able to examine this exceptional interstellar traveller in detail.

The reason for the exceptionally rapid brightening is still unresolved.

The researchers suggest that the comet's unusual behaviour could result from its composition, its fast incoming speed, or characteristics gained during its lengthy voyage through interstellar space.

A preprint of the study is available on arXiv.

This article was originally published by Universe Today. Read the original article.