Interstellar comet 3I/ATLAS nears the Sun: what it is, what’s next, and why Oct. 29 matters

The third confirmed visitor from another star system—interstellar comet 3I/ATLAS—is racing toward its closest approach to the Sun in the final days of October. As of today (Oct. 23), the comet is deep in solar conjunction, tucked so close to the Sun in our sky that ground-based telescopes can’t safely observe it. That blackout won’t last long: the days around Oct. 29–30 are pivotal for science, with multiple spacecraft lining up to watch, and the comet expected to reappear in dawn skies by early December.

What is interstellar comet 3I/ATLAS?

3I/ATLAS is only the third object ever confirmed to enter our solar system from interstellar space, after 1I/ʻOumuamua (2017) and 2I/Borisov (2019). Discovered on July 1, 2025 by the ATLAS survey in Chile, it follows a hyperbolic path—too fast to be bound by the Sun’s gravity—which is the hallmark of an interstellar origin. Unlike typical short-period comets, it will visit once and leave forever.

Crucially, its trajectory does not intersect Earth’s orbit. Even at its nearest to our planet later this year, it remains far beyond any hazard range. That’s why agencies classify 3I/ATLAS as a scientific opportunity, not a threat.

The latest on 3I/ATLAS: perihelion window and spacecraft watch

• Perihelion (closest to the Sun): around Oct. 29–30 (UTC) at roughly 1.35–1.4 AU from the Sun—outside Earth’s orbit and nearer to Mars’s distance.

• Why you can’t see it now: During solar conjunction, the comet is lost in the Sun’s glare. Professional observatories stand down until it emerges on the far side.

• Spacecraft vantage points: Teams plan opportunistic observations from multiple missions. Mars orbiters have geometric leverage near the ecliptic plane, and a Jupiter-bound probe could be brushed by ions from the comet’s tail between Oct. 30 and Nov. 6, effectively “sampling” material carried outward by the solar wind. Such detections would offer rare, indirect measurements of an interstellar comet’s composition.

• From “anti-tail” to tail: In recent weeks, imagery and modeling indicate a shift from a sunward “anti-tail” (a perspective effect) to a more familiar tail pointing away from the Sun as viewing geometry changes approaching perihelion.

Key dates and distances for interstellar comet 3I/ATLAS

Notes: AU = astronomical unit (Earth–Sun distance). Dates can shift slightly as orbit solutions refine.

What scientists are watching as 3I/ATLAS peaks

• Outgassing chemistry. Early datasets point to vigorous water vapor release and a CO₂-rich coma, valuable clues to how icy bodies formed around other stars. Comparing these ratios to solar-system comets may reveal whether 3I/ATLAS condensed in a colder or more carbon-rich nursery.

• Dust dynamics. The apparent “anti-tail” earlier this month likely arose from dust sheet geometry relative to the Sun and observer. As the angle changes, the dust and ion tails should settle into the classic, anti-solar orientation, offering a natural experiment in how radiation pressure sculpts comet tails.

• Solar-wind interaction. If a spacecraft intercepts packets of charged particles from the ion tail, it could constrain the comet’s gas species and outflow rates—even without an on-site flyby. That would be a first for an interstellar comet.

• Age and origin. Trajectory and composition hints suggest 3I/ATLAS may be ancient, potentially older than our solar system. Pinning down isotopes and volatiles helps test formation-zone theories in distant protoplanetary disks.

Visibility and safety: what skywatchers should expect

• No threat to Earth. All credible orbit solutions keep 3I/ATLAS at safe distances from our planet throughout its passage.

• When it returns to view. After conjunction, the comet should reappear in the morning sky by early December, low on the horizon before sunrise. It will remain a telescope target rather than a naked-eye spectacle; brightness depends on dust production after perihelion.

• How to follow along. Expect fresh images from space missions and large observatories once the comet clears the Sun, along with updated brightness estimates and refined orbital elements.

Cutting through the noise around 3I/ATLAS

Viral speculation has tried to recast 3I/ATLAS as something other than a comet. The evidence so far supports a natural, icy body undergoing familiar cometary physics—sublimation, dust release, and tail formation—with the twist that its materials were forged around another star. As more data arrive around Oct. 29–Nov. 6, the story should sharpen: not a mystery ship, but a messenger from a different planetary nursery, letting us test how “normal” our own comets really are.

This is a developing space-science story. Observation plans and timing may be adjusted based on operational constraints and geometry. Expect updates as the comet rounds the Sun and reenters view.