Comet 3I/ATLAS Brightens Rapidly Near Perihelion

Solar satellites track Comet 3I/ATLAS

Comet 3I/ATLAS, just the third recognised object to visit from outside our Solar System, has been increasing in brightness much faster than anticipated while nearing perihelion – its point of closest approach to the Sun.

For the previous month, the comet has sat almost exactly behind the Sun from Earth's viewpoint, leaving ground-based astronomers virtually unable to observe it during this vital interval. Space-based observatories have instead provided the means to follow it.

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

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

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

Rapid brightening near perihelion

Their results were notable. From mid-September to late October, 3I/ATLAS moved in from approximately 2 astronomical units – around twice the Earth–Sun distance – to only 1.36 AU, and its brightness rose sharply.

The researchers determined that the comet's brightness rose in proportion to the inverse of its heliocentric distance raised to the 7.5 power. That represents a substantially steeper increase than the rate measured earlier, when the comet was more distant.

Most comets steadily become brighter on their approach to the Sun as their ice changes into gas. This interstellar visitor is brightening at about twice that usual rate, indicating that an unusual process may be occurring on its surface.

Blue colour and an active coma

The observations further showed that the comet is noticeably bluer than sunlight, an indication that gases – not dust alone – are making a major contribution to the light it emits visibly.

Previous measurements had shown that the comet's dust was reddish, so this move towards blue is especially significant. The researchers believe emissions from molecules such as cyanogen, and potentially ammonia, may account for the unusual colour.

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

The comet passed perihelion on 29 October. The team's estimates indicate that it may have reached approximately magnitude 9, sufficiently bright to be seen using smaller amateur telescopes.

Ground-based observations return

As it comes out from behind the Sun and re-enters dark skies during November and December, observers on the ground will at last have an opportunity to examine this extraordinary interstellar traveller in detail.

The reason for the exceptionally swift brightening is still unknown.

The team suggests that the comet's distinctive activity could result from its composition, its fast approach velocity, or characteristics it may have developed over its lengthy passage through interstellar space.

A preprint of the study is available on arXiv.

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