3I/ATLAS Interstellar Comet: UN Planetary‑Defence Campaign Begins as Mars Data Sharpens Its Path

On 27 November 2025, interstellar comet 3I/ATLAS hits a major milestone in its brief visit through our Solar System. The United Nations’ planetary‑defence network is officially turning the comet into a live global drill, while new data from spacecraft around Mars has tightened its predicted trajectory by a factor of ten. ^[1]

Here’s what’s happening today, why scientists are so excited — and why you absolutely don’t need to worry about a doomsday comet.

What is 3I/ATLAS, and why is it such a big deal?

3I/ATLAS is the third confirmed interstellar object ever seen passing through our Solar System, after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. It was discovered on 1 July 2025 by the Asteroid Terrestrial‑impact Last Alert System (ATLAS) survey in Chile and later confirmed to be on a hyperbolic (unbound) orbit, meaning it came from another star system and will never return. ^[2]

Key physical and orbital facts:

Official designations: 3I/ATLAS and C/2025 N1 (ATLAS) ^[3]
Size: nucleus estimated between roughly 0.3 and 5.6 km across — similar in scale to many typical comets ^[4]
Perihelion (closest to the Sun): ~30 October 2025, at about 1.4 AU (around 130 million miles / 210 million km) from the Sun ^[5]
Closest approach to Earth: 19 December 2025, at about 1.8 AU (≈170 million miles / 270 million km) — almost twice the distance between Earth and the Sun ^[6]
Future close pass to Jupiter: ~0.36 AU on 16 March 2026, close to Jupiter’s Hill sphere where its gravity dominates nearby space ^[7]

Studies of its motion in the Milky Way suggest 3I/ATLAS likely originated from an old star in the Galaxy’s “thick disk”, and could be older than the Solar System itself — somewhere between about 3 and 14 billion years old. ^[8]

Today’s headline: UN‑backed comet drill starts now

The biggest development on 27 November 2025 is that the International Asteroid Warning Network (IAWN) — operating under the United Nations planetary‑defence framework — is officially opening a two‑month observing campaign focused on 3I/ATLAS. ^[9]

According to the IAWN campaign page and UN‑linked coverage:

The “3I/ATLAS Comet Astrometry Campaign” runs from 27 November 2025 to 27 January 2026.
It’s described explicitly as a training and capability‑testing exercise, not an emergency response.
The campaign is the 8th official IAWN observing exercise since 2017 and is coordinated with the Space Mission Planning Advisory Group (SMPAG) and the UN Office for Outer Space Affairs (UNOOSA). ^[10]

IAWN’s own note makes the intent crystal clear:

While it poses no threat, comet 3I/ATLAS presents a great opportunity … to perform an observing exercise due to its prolonged observability from Earth and high interest to the scientific community. ^[11]

In practical terms, starting today:

Professional observatories and skilled amateurs worldwide will submit precise measurements of the comet’s position.
Those measurements will be used to stress‑test orbit‑calculation software and coordination procedures, mirroring what would happen if a truly hazardous object were discovered. ^[12]

So if you see headlines about the UN, planetary defences and 3I/ATLAS, they’re talking about a planned drill, not a secret crisis.

Mars helped nail down its orbit: ESA’s ExoMars data

Also feeding into today’s story is a fresh analysis highlighted by Universe Today: ESA has used its ExoMars Trace Gas Orbiter (TGO) around Mars to dramatically sharpen the comet’s predicted path. ^[13]

Here’s what the new work shows:

When 3I/ATLAS passed about 29 million km from Mars in early October, ESA’s ExoMars TGO and Mars Express spacecraft captured images from a different vantage point than Earth. ^[14]
By combining those spacecraft observations with ground‑based data, ESA’s Near‑Earth Object Coordination Centre improved the comet’s ephemeris (predicted positions) by about a factor of 10 in accuracy. ^[15]
This is the first time astronomers have used a spacecraft orbiting another planet to triangulate the orbit of an interstellar object, making 3I/ATLAS a test bed for future planetary‑defence techniques. ^[16]

The sharpened trajectory confirms:

3I/ATLAS is currently racing away from the Sun at up to ~250,000 km/h (~155,000 mph). ^[17]
It will pass Earth at a safe 270 million km or so on 19 December 2025, before heading outward for its eventual encounter with Jupiter in March 2026. ^[18]

For planetary‑defence planners, this is a dress rehearsal for the kind of multi‑platform tracking (Earth + deep‑space spacecraft) they’d like to use during a real threat scenario.

Is the close encounter with Jupiter dangerous?

Short answer for Earth: no. But it is scientifically fascinating.

A recent preprint highlighted by IFLScience modelled 3I/ATLAS’ past and future motion, running hundreds of simulated “clones” of the comet to see how its trajectory behaves. ^[19]

That work suggests:

3I/ATLAS likely entered the Solar System from the direction of the Sagittarius constellation and will depart toward Gemini. ^[20]
On 16 March 2026, it will pass close to Jupiter’s Hill radius — the region of space where Jupiter’s gravity dominates. This may significantly tweak its outbound path, depending on the exact details of its motion and outgassing. ^[21]

Crucially, all credible orbital solutions still show:

No impact risk to Earth, now or after the Jupiter encounter.
The “course‑altering encounter” is about how the comet escapes into interstellar space, not about steering it toward our planet. ^[22]

New images: a spectacular ion tail and rising activity

Another thread in today’s coverage comes from fresh ground‑based imagery. A new analysis on Techno‑Science, based on the Virtual Telescope Project in Italy, showcases a composite image taken on 10 November 2025 that reveals 3I/ATLAS in striking detail: ^[23]

The comet shows a bright nucleus and a compact coma (the fuzzy “head”).
A sharply defined ion tail stretches about 0.7° across the sky — longer than the apparent diameter of the full Moon.
A faint anti‑tail, an optical effect where dust appears to extend in the “wrong” direction due to perspective, is also visible. ^[24]

Physically, that ion tail is built as:

Solar ultraviolet light strips electrons from gas molecules leaving the comet, creating ions.
The solar wind then sweeps those charged particles straight away from the Sun, so the ion tail always points roughly anti‑sunward, regardless of the comet’s direction of motion. ^[25]

Spacecraft and large telescopes have also been watching. Observations from Hubble, JWST, Mars orbiters and solar missions show that 3I/ATLAS:

Is rich in carbon dioxide and carbon monoxide, with unusually strong gas emissions compared to many Solar‑System comets. ^[26]
Has changed color from reddish to greenish to bluish over the past few months, likely reflecting evolving gas and dust chemistry as it warmed near the Sun. ^[27]

Taken together, the images released over the last week by NASA (including Mars‑based HiRISE and MAVEN views) and independent observatories give us the closest look ever at an interstellar comet in action. ^[28]

Where is 3I/ATLAS today, and can you see it?

As of 27 November 2025:

3I/ATLAS is located in the constellation Virgo.
It sits about 292 million km from Earth — roughly 1.95 AU.
Its observed brightness is around magnitude 10, based on recent comet observers’ reports. ^[29]

What that means for skywatchers:

A magnitude‑10 object is far too faint for the naked eye and challenging even for standard binoculars.
Under dark skies, a small to medium amateur telescope (e.g., 15–20 cm / 6–8 inches aperture) can potentially pick up the fuzzy glow. ^[30]
For mid‑northern latitudes (like the UK), 3I/ATLAS currently rises in the early hours, culminates in the morning sky, and sets around early afternoon local time; it’s best attempted just before dawn, when Virgo is reasonably high and the sky still dark. ^[31]

Because visibility depends heavily on your location and local conditions, observers are encouraged to use a live sky tool or updated ephemeris (for example, TheSkyLive or similar apps) to get precise coordinates and rise/set times for their exact site. ^[32]

Why planetary‑defence experts are excited about this harmless comet

If 3I/ATLAS is no danger to Earth, why are planetary‑defence teams so invested?

The answer is that it is almost the perfect training target:

It’s faint and fuzzy.
Comets have comae and tails that can distort position measurements. The IAWN campaign explicitly aims to test and refine methods for accurate astrometry of extended objects, which is crucial if a real threat turns out to be a comet rather than an asteroid. ^[33]
It’s interstellar and fast.
Because 3I/ATLAS isn’t bound to the Sun, any trajectory errors grow quickly. That makes it an excellent stress test for orbit‑prediction software and international data‑sharing pipelines. ^[34]
It’s scientifically rich.
Its exotic chemistry and probable great age give planetary scientists a chance to study material from another star’s planetary system — and to practise coordinating observations from dozens of facilities, from Mars orbiters to JWST and JUICE. ^[35]

ESA’s Comet Interceptor mission (planned for later this decade) and NASA/ESA next‑generation survey telescopes will rely on exactly this sort of rapid‑response coordination to chase future interstellar visitors. 3I/ATLAS is, in effect, the prototype rehearsal. ^[36]

Fact‑check: Is 3I/ATLAS an alien spacecraft?

Speculation about interstellar objects is inevitable, especially after the debates around ʻOumuamua. Some researchers, most prominently Avi Loeb, have argued that certain anomalies — like unusual outgassing or non‑gravitational accelerations — could hint at artificial origins, and 3I/ATLAS has inspired similar discussion in op‑eds and preprints. ^[37]

However, the mainstream scientific view as of today is clear:

NASA officials state that all available data are consistent with a natural comet, and no “technosignatures” (signals of technology) have been observed. ^[38]
Spectroscopic and imaging studies show typical cometary gases, dust, and jets, albeit in unusual proportions — especially CO₂ and CO — which can be explained by its interstellar origin and long exposure to cosmic rays. ^[39]
The non‑gravitational forces acting on the comet appear consistent with outgassing and radiation pressure, the same physics seen in many active comets. ^[40]

In other words, 3I/ATLAS is mysterious, but not magical. It’s a rare opportunity to study a piece of another star system — not evidence that aliens are paying us a visit.

The bottom line for 27 November 2025

As of today:

A UN‑endorsed observing campaign officially begins, turning 3I/ATLAS into a world‑wide planetary‑defence exercise. ^[41]
ESA’s Mars‑based observations have tightened the comet’s orbit by an order of magnitude, confirming a safe flyby of Earth in December. ^[42]
New images from professional and amateur observatories reveal dramatic ion and dust tails, as the comet brightens and evolves on its outbound leg. ^[43]
For skywatchers with telescopes, 3I/ATLAS is now a challenging but reachable target in Virgo, around magnitude 10. ^[44]

It’s a rare moment when planetary defence, deep‑space science and backyard astronomy all converge on the same object — a tiny, ancient iceberg from another star, briefly illuminated in our skies before it vanishes back into the dark between worlds.

How Mars Spacecraft Enhanced Planetary Defense: Tracking Comet 3I/ATLAS