3I/ATLAS reappears after sunset with high brightness and a rare blue hue; STEREO, SOHO, and GOES-19 confirm an exceptional photometric jump

Interstellar comet 3I/ATLAS has been detected again after its conjunction with the Sun, exhibiting a greater-than-expected increase in brightness and an unusual blue color. Data from solar observatories indicate photometric variations outside the pattern recorded in previous comets.

The interstellar comet 3I/ATLAS It was detected again after the conjunction with the Sun, showing... brightness increase above what was expected e bluer color than sunlight.

Data obtained by coronagraphs and solar imagers between September and October indicate a surge in brightness as we approach the perihelion of October 29th, followed now by the reappearance of the object in the morning sky throughout November and December.

Ground-based telescopes are expected to resume detailed measurements during this period.

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Solar probe records

The most comprehensive analysis to date has been published by the researchers. Qicheng Zhang and Karl Battams, gathering data from four cameras mounted on three platforms: HI1 and COR2, from the STEREO-A probe.the coronagraph LASCO C3, from SOHO; it's the CCOR-1, aboard GOES-19.

According to the authors, the brightness of 3I/ATLAS increased in a manner consistent with a curve. r^−7,5 (r is the heliocentric distance), a more pronounced behavior than that recorded in many comets of the inner solar system.

O CCOR-1 also recorded the comet as extended font, with an apparent coma of about 4 minutes of arc, a parameter that provides a direct estimate of the gas and dust envelope as it passes through the instrument's field.

High-resolution image of the interstellar object 3I/ATLAS captured by the Hubble Space Telescope in July 2025, showing coma and hyperbolic trajectory. (Image: NASA/ESA)

Blue color and origin of the glow

Measurements in different filters of LASCO and the spectral response of CCOR-1 They indicate that, in the vicinity of perihelion, 3I/ATLAS presented bluer shade than the solar reference used in the calibration.

According to the technical analysis, the color deviation suggests gaseous emissions as the main source of visible light, and not just Solar reflection in dust grains, as is the case with most observed comets.

This conclusion is supported by comparisons between photometric bands and by measurements calibrated with reference stars in each field.

Experts say the observed pattern is consistent with the presence of excited molecules near the cometary nucleus.

Observations during the solar conjunction

The perihelion phase coincided with the conjunction of the comet with the Sun, which made optical observation from Earth impossible.

During this period, space-based coronagraphs — which block the solar disk — continued to track the object.

The series obtained by STEREO-A, SOHO and GOES-19between mid-September and the end of October, they presented consistent light curves, confirming the rapid and continuous increase in brightness up to perihelion.

Return to the morning sky

As the sun moves away, 3I/ATLAS becomes visible again in morning twilight.

Projections released by observation guides indicate that the comet should be accessible to telescopes starting from... mid-November, initially with a glow around magnitude 11, gradually decreasing in the following weeks.

According to astronomers, the phenomenon will not be visible to the naked eye, but it can be observed by those who have access to [equipment/equipment]. medium-sized optical equipment and cameras capable of photometric and spectroscopic measurements.

The scientific community awaits the comet's return to the dark sky to confirm the data collected by space probes and to monitor the... evolution of coloration and cometary activity after perihelion.

Photo of the discovery of 3I/ATLAS by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Chile on July 1, 2025, the third recorded interstellar visitor. (Image: ATLAS/Univ. of Hawaii/NASA)

Maximum brilliance and open questions.

Independent studies based on the same datasets indicate that the peak brightness It happened near magnitude 9.

This estimate is consistent with the curve. r^−7,5 described by the researchers.

According to practical astronomy publications, the authors of the space analyses predict that 3I/ATLAS "It must emerge from the conjunction brighter than when it entered.".

This expectation is based on values measured before and after the closest approach to the sun.

So far, the exact cause of the accelerated increase in brightness It was not determined.

Researchers claim that new observations post-perihelion These will be essential to assess whether the gaseous component remains predominant, whether the dust emission increases as the comet moves away from the Sun and as the coma It evolves in size and density.

CCOR-1 and the instrumental contribution

O CCOR-1, the first operational coronagraph in orbit geostationary, was developed by United States Navy in partnership with NOAA and embedded in the satellite GOES-19.

The instrument operates in visible (470–740 nm), with a typical cadence of 15 minutes, and it records continuous images of the solar corona and objects that cross its field of view.

According to NOAA, the equipment was designed to assist in space weather forecast, complementing the work of scientific coronagraphers such as LASCO, from business center, and the cameras of STEREO.

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The combination of these instruments made it possible to track the 3I/ATLAS during the period in which it was... invisible to terrestrial observatories, offering a complete sequence of data on the photometric behavior of the comet.

Discovery and trajectory

O 3I/ATLAS was discovered in July 1, 2025 through the project telescope ATLAS, in Chile.

Its nature interstellar was confirmed by Minor Planet Center the next day.

O perihelion took place in October 29, Ea closest approach to Earth is scheduled for December, without risk to the planet.

The acronym "3I” indicates the third interstellar object known, after 1I/ʻOumuamua e 2I / Borisov.

According to astronomers, the fact that the comet presents unusual characteristics in brightness and coloration This justifies the interest in new observation campaigns.

Next observation steps

For Brazilian research groups and observatories, the reappearance of 3I/ATLAS represents an opportunity to bring together... multiband light curves e low and medium resolution spectra, allowing direct comparison with the data from STEREO-A, SOHO and GOES-19.

Experts emphasize that monitoring should focus on verifying if the pattern "more blue" It remains unclear what happens in the coming weeks, and at what rate the brightness stabilizes or decays after the sun passes.

According to international circulars, teams are planning coordinated observations as the comet gains momentum. height on the eastern horizon before dawn, which will facilitate continuous measurements and cross-referencing of data between terrestrial and space-based observatories.

Registration

3I/ATLAS reappears after sunset with high brightness and a rare blue hue; STEREO, SOHO, and GOES-19 confirm an exceptional photometric leap.