Say Goodbye to Comet 3I/ATLAS, the Interstellar Visitor That Briefly Called Our Solar System Home

Everyone’s favorite comet is about to fly away from Earth and continue on its interstellar journey.

3I/ATLAS recently passed by Earth on Friday, Dec. 19, and gave astronomers a fleeting but invaluable chance to study material forged around another star. Now, after its closest encounter, the visitor is already heading back into deep space, taking its secrets with it but leaving behind a trove of data that scientists will be studying for years.

3I/ATLAS Comes Close to Earth

On Dec. 19, the interstellar invader made its closest approach to Earth, coming within 168 million miles of our planet around 1 a.m. EST. 3I/ATLAS had already passed its closest point to the Sun, and this near-Earth flyby marked the beginning of its exit from the inner solar system.

From here, the comet will continue moving outward, slipping past the planets before eventually leaving the solar system altogether and resuming its long journey through the Milky Way.

First spotted on July 1, 2025, by NASA’s Asteroid Terrestrial-impact Last Alert System, 3I/ATLAS immediately stood out. Its trajectory showed that it did not originate anywhere near the Sun, making it just the third known object discovered passing through our solar system from interstellar space. Researchers have traced its path to a region of the galaxy older than our 6.4 billion-year-old solar system, meaning 3I/ATLAS formed around a star that predates the Sun itself.

But you don’t have to say goodbye just yet. Skywatchers hoping to catch a final glimpse can tune into a livestream hosted by the Virtual Telescope Project beginning at 11 p.m. EST, as 3I/ATLAS slowly fades from view.

Read More: New Images of Comet 3I/ATLAS Reveal "How Magical the Universe Could Be"

What Scientists Have Learned, and Still Hope to Learn

When 3I/ATLAS was first discovered, speculation spread quickly, including claims that it might be an alien spacecraft or probe. Careful observations soon put those ideas to rest. Within months, astronomers confirmed that 3I/ATLAS looked strikingly familiar to comets from our solar system, with its icy nucleus surrounded by a glowing coma of gas and dust that changes chemical states as the comet warms.

3I/ATLAS also offered researchers a rare opportunity to examine the raw ingredients that form comets, asteroids, and planets around stars other than the Sun. Observations across multiple wavelengths revealed a composition rich in carbon, oxygen, and nitrogen – elements also common in our solar system comets.

One particularly intriguing question involved X-rays. While comets from our solar system emit X-rays, astronomers had never confirmed whether interstellar comets behave the same way. Using Japan’s X-Ray Imaging and Spectroscopy Mission, scientists observed 3I/ATLAS in late November 2025, detecting X-rays extending nearly 248,000 miles from its core.

The data collected from 3I/ATLAS’ most recent close encounter with Earth will help determine how these emissions form and whether they mirror processes seen closer to home.

Why Have Comets Always Captivated Humanity?

Long before telescopes and space missions, comets inspired awe, fear, and fascination. Humans have probably been captivated by comets longer than any archaeological record can testify. According to the European Space Agency, ancient rock carvings resembling comets appear in Scotland and Italy, while Chinese astronomers meticulously tracked them for centuries, believing they revealed the “will of heaven.”

Elsewhere, comets were often seen as omens. Roman astrologers blamed them for disasters ranging from war to disease, and medieval fear is famously preserved in the Bayeux Tapestry’s depiction of Halley’s Comet.

Even as recently as 1910, the discovery of cyanogen in the tail of Halley’s Comet triggered mass panic, complete with gas masks, prayer vigil, and so-called comet protection pills.

Although 3I/ATLAS’ visit is coming to an end, the insights it has provided will continue to shape our understanding of worlds far beyond our own.

Read More: Other ATLAS Comet Appears Fragmented After Close Encounter with The Sun

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• This article references information from NASA: Comet 3I/ATLAS
• This article references information from X-ray Imaging and Spectroscopy Mission: XRISM observes a cometary interloper 3I/ATLAS from beyond the solar system
• This article references information from the European Space Agency (ESA): Comets: from suspicion to science