Interstellar Comet 3I/ATLAS Shows Water Loss But No Technosignatures

Scientists studying interstellar comet 3I/ATLAS reported that SOHO data showed heavy water loss after its October perihelion, while a Breakthrough Listen search with the Green Bank Telescope found no technosignatures. Together, the new results suggest the rare interstellar object behaved like a normal comet as it moved away from the Sun.

The comet gave researchers a brief view of material made around another star, because 3I/ATLAS is only the third confirmed interstellar object seen entering the solar system. The comet was already leaving, so the observation window was short and the object grew fainter and harder to track.

Radio search of interstellar comet 3I/ATLAS for technosignatures

In a preprint on arXiv, a Breakthrough Listen team led by Ben Jacobson-Bell at the University of California, Berkeley, said the 100-meter Robert C. Byrd Green Bank Telescope observed 3I/ATLAS on Dec. 18. The scan covered radio frequencies from 1 to 12 gigahertz but revealed no candidate technosignatures.

The group reported that all detected signals were traced to human-made radio interference, according to a ScienceAlert report, despite online discussion that the interstellar comet might be something other than a natural body. The authors wrote: "We report a nondetection of candidate signals down to the 100 mW level."

Speculation and structure claims around interstellar comet 3I/ATLAS

Some speculation had also centred on ideas from Harvard astrophysicist Avi Loeb, who argued that processed Hubble images showed three inner jets spaced at roughly equal angles, plus a strong sunward “anti-tail”. Loeb said that such a pattern would need an unlikely viewing angle if random, and argued the geometry deserved careful study.

Loeb also noted that gas dynamics could still explain the structure, so the suggestion did not rule out normal cometary physics. The attention to these images added to wider interest in whether an interstellar comet could display features that differ from those of typical solar-system comets.

Water loss and SOHO view of interstellar comet 3I/ATLAS

Another arXiv preprint, led by Michael Combi, used the Solar and Heliospheric Observatory’s SWAN instrument to examine the comet’s hydrogen coma. SWAN first detected this coma on Nov. 6, nine days after perihelion, when the Sun’s ultraviolet light broke apart water molecules and produced a glow in ultraviolet wavelengths.

By modelling that hydrogen emission, the team estimated a water production rate of 3.17 x 10@29 molecules per second when 3I/ATLAS was 1.40 astronomical units from the Sun. By Dec. 8, the rate appeared to have declined sharply to between 1 x 10@28 and 2 x 10@28 molecules per second.

NASA said 3I/ATLAS never threatened Earth, passing no closer than about 270 million km, or 1.8 astronomical units, while travelling at up to about 210,000 km per hour relative to the Sun. Remote sensing suggested a nucleus size somewhere between roughly 440 metres and 5.6 km, showing the difficulty of sizing a dusty, fast object.

Both types of observation carried limits. The radio search could have missed narrow signals outside the observed bands, transmissions that switched on and off, or non-radio technologies. SWAN’s ultraviolet hydrogen data also needed models that depended on changing solar ultraviolet output, and the work remained a preprint, still awaiting peer review.

The measurements so far pointed to a fast interstellar comet losing water as it departed the solar system, while its activity decreased with distance from the Sun. Taken together, SOHO observations and the Green Bank Telescope search supported the view that 3I/ATLAS behaved like an ordinary comet, with no confirmed signs of artificial origin.