Could a comet’s tail pointing towards the Sun possibly indicate alien technology, or is it just a strange quirk of dust physics? While interstellar comet 3I/ATLAS speeds by our planet at a blistering 130,000 miles per hour, it is clearly not the speed or, for that matter, the origin of this visitor that is so fascinating, but its weird “anti-tail.”
In normal comets, a trail of dust and gas stretches away from the heating influence of the solar radiation pressure and the solar wind, creating a glowing trail called an anti-tail that always points away from the Sun. However, pictures taken by the NASA Hubble Space Telescope starting July 21 and as recent as December 13 by the Teerasak Thaluang telescope in Thailand confirm the existence of a long, thin object extending hundreds of thousands of miles directly towards the Sun despite being no optical phenomenon but a stable anti-tail phenomena that has been present for several months when the comet passed perihelion and began its journey away from the Sun.
Harvard astronomer Avi Loeb dubs it a “real physical jet” and has identified it as the fourteenth anomaly exhibited by 3I/ATLAS. It is a list of anomalies that includes both non-gravity related acceleration, a subtle and measurable boost in velocity that cannot be accounted for by the strength of the solar gravity, a nickel to cyanide ratio that’s anomalous in the gas ejection, highly negative polarization of scattered light, and a trajectory that’s precisely tuned to encounter assorted planets. Avi Loeb has speculated that the antitail may result from a “swarm of objects” that follow an artificial object, known colloquially as a “mothership,” that launches probes. Avi Loeb feels that science neglects the potential for these probes at the expense of abundant evidence, suggesting that the interstellar medium has had enough time to allow advance civilizations to disseminate their artifacts across the galaxy.
Mainstream comet research scientists, however, have a more realistic explanation with a basis in dust mechanics. David Jewitt of UCLA observes the following with respect to the comet 3I/ATLAS: The sunlight side of the comet is strongly heated as the comet comes close to the Sun, and this causes large dust grains to be ejected towards the Sun. These dust grains with a size of about 100 microns are large enough for the solar pressure to be gradual for them to be redirected towards the usual antisolar tail. This has actually been seen in other comets as well, for instance, comet C/2014 UN271 Bernardinelli–Bernstein.
Modeling the anti-tail region of 3I/ATLAS in detail, such as the Haser-type analysis of radial outflow in the Hubble images, confirms the dust-grain model. The coma is modeled here as a flow of sublimating ice particles of constant velocity, whose lifetimes depend on the angle of solar illumination. In the direction toward the Sun, particles have longer lifetimes before complete sublimation takes place, thus extending the “snow line” and creating a brighter feature. The observed snow line in the Solar direction, projected for size, is about 30,000 km, much larger than in the perpendicular directions.
The non-gravitational acceleration measured close to perihelion has also been examined in detail. NASA’s Jet Propulsion Laboratory’s navigation engineer Davide Farnocchia measured radial and transverse accelerations on the order of just a few hundred millionths of Earth’s gravity. If it were caused by gas jets from the sublimation process, this amount of acceleration would mean that 3I/ATLAS could potentially shed as much as 10-13% of its mass in several months, although preliminary observations indicate that it has held on to its nucleus. This particular non-gravitational force could be attributed to gas jets of volatile compounds like CO2 molecules, which dominate the outer coma, but other theories, including propulsion systems, cannot be completely eliminated at this point, writes Loeb.
Even instrumentation around our solar system has been enlisted to investigate this comet. Hubble imaging, James Webb modulation, and even Mars Reconnaissance Orbiter’s High Resolution Imaging Science Camera assistance in determining positions and compositions. Partial comet images with dual tails, one composed of gas and one composed of dust, taken by ESA’s JUICE space probe, orbiting tens of millions of kilometers away from the comet, are a wonder. Gemini North observations demonstrate changeovers in comas from reddish to greenish in color due to heightened diatomic carbon emissions in post-perihelion stages.
An understanding of the anti-tail is also informed by years of study of cometary dust, beginning with the Giotto comet encounters and the Rosetta comet encounter of 67P/CG. Comet encounters showed the dependence of coma shape on the size and velocity of the dust particles ejected, which in the case of larger particles can have velocities of less than 1 m/s. The anti-tail of 3I/ATLAS appears to reflect the continuous ejection of larger particles from the sunlight side of the comet that is seen in simulations of dust ejection velocities.
Whether the 3I/ATLAS comet is a natural survivor from the Milky Way galaxy’s thick disk or an engineered body, the opportunity it presents to test the physics of comets in an interstellar environment is tantalizing. When it passes by at its closest point at 167 million miles away from our planet, it will be closely observed for any changes in its mysterious stream of materials headed towards the sun, so it will never be seen again as it traverses into interstellar space.
