A spectacular celestial event—the perihelion (closest approach to the Sun) of the interstellar comet 3I/ATLAS—is occurring around October 30, 2025. However, this moment is obscured from Earth-based observers because the comet is in solar conjunction, hidden behind the Sun's intense glare. Despite this invisibility to us, a select fleet of space-based instruments is strategically positioned to capture valuable data about this rare visitor from beyond our solar system.
🛰️ The Spacecraft and Observatories on the Job
Since the comet is too fast and its closest approach window too short for a dedicated flyby mission, scientists are relying on remote observation from powerful space telescopes and spacecraft already positioned throughout the solar system.
1. Flagship Space Telescopes
These orbiting observatories provide the clearest, most detailed views, even when objects are near the Sun.
Hubble Space Telescope (HST): The Hubble has already provided striking images of the comet's development, revealing a dust plume and a teardrop-shaped cocoon of dust being released from the nucleus. Its continuing observations help astronomers accurately estimate the comet's size and monitor its activity as it interacts with solar radiation.
James Webb Space Telescope (JWST): Webb's infrared capabilities are crucial for a detailed spectroscopic analysis of the comet's composition. It has already detected frozen compounds like carbon dioxide and water, offering scientists a chemical blueprint of its birthplace in a distant star system.
2. Deep Space Missions
Several spacecraft, on their way to other destinations, have been tasked with turning their instruments toward the comet, offering unique viewing angles.
Jupiter Icy Moons Explorer (JUICE): The European Space Agency (ESA) spacecraft, currently cruising toward Jupiter, is expected to turn its instruments toward 3I/ATLAS for remote observation as the comet heads out of the inner solar system, starting around early November.
Mars Orbiters (ExoMars and Mars Express): In the weeks leading up to perihelion (around early October 2025), ESA's ExoMars Trace Gas Orbiter (TGO) and Mars Express had the closest views of the comet. They captured images and attempted spectral measurements as 3I/ATLAS passed close to Mars, providing an essential, up-close dataset that ground telescopes could not achieve.
3. Solar Observatories
A few solar-monitoring spacecraft have also been able to track the comet as it approached the Sun.
Solar and Heliospheric Observatory (SOHO): This ESA/NASA mission, along with other coronagraphs like the one aboard NOAA's GOES-19 satellite, has been monitoring 3I/ATLAS as it edges closer to the Sun's glare. Coronagraphs are designed to block the Sun's bright disk, allowing them to see faint objects in the Sun's immediate vicinity.
☄️ The Enigmatic Interstellar Comet 3I/ATLAS
3I/ATLAS is only the third confirmed object detected passing through our solar system that definitively originated from another star system, following 1I/ʻOumuamua (2017) and 2I/Borisov (2019). The "3I" in its name stands for "third interstellar."
Key Characteristics
Interstellar Origin: Its trajectory is hyperbolic (eccentricity greater than 1), meaning its speed is too high for the Sun's gravity to bind it into a closed orbit. It will pass through our system once and then continue its journey back into the vast interstellar space.
Closest Approach: The comet reaches perihelion on or around October 30, 2025, at a distance of approximately $1.4$ Astronomical Units (AU) from the Sun—just inside the orbit of Mars.
Compositional Clues: Spectroscopic studies have identified the presence of typical cometary materials like water and carbon dioxide, but also the surprising detection of atomic nickel vapor far from the Sun. This unusual makeup suggests it formed in an environment vastly different from our own solar system's comets.
Age and Speed: The comet is moving at an exceptional speed, one of the fastest of the three known interstellar objects, which, along with its calculated trajectory, suggests it is likely an ancient object, possibly billions of years older than our $4.6$-billion-year-old solar system.
Why the Perihelion is a Crucial Test
The event on October 30 is a critical turning point. As the comet gets blasted by solar heat and radiation, scientists will be watching to see how the comet behaves.
Activity: They expect an accelerated rate of sublimation (frozen gases turning directly into vapor), potentially resulting in a larger and brighter coma (the gaseous halo) and tail.
Integrity: The intense heat is the "acid test." If the nucleus remains intact, it confirms its structural integrity. If it fragments, as comets sometimes do, it would suggest a more fragile composition.
Unusual Dynamics: Some theories, though speculative, point to unusual features in its observed trajectory or outgassing, prompting scientists to look for any signs that defy standard cometary physics.
By pooling the data collected from these various space assets and ground telescopes after the comet re-emerges from the Sun's glare around late November or early December, astronomers will unlock a wealth of information about the composition, evolution, and potential chemical diversity of planetary building blocks across the Milky Way.