Interstellar object 3I/ATLAS is expected to make its closest approach to Earth in a few days, at a distance of just 167 million miles—a huge gap, but only a stone’s throw away on a cosmic scale.
It’s an exciting time, and astronomers will have an unprecedented opportunity to focus ground and space telescopes on this unusual visitor. For months, they had been tracking the object, widely believed to be a comet, roaring through the solar system.
Since NASA’s Hubble Space Telescope first observed the object on July 21, scientists have noticed a strange protrusion on the object, with a second tail that counterintuitively points directly toward the sun, rather than away from it like the familiar tails characteristic of solar system comets.
As Harvard astronomer Avi Loeb puts it, this “anti-tail” could be the result of “increased mass loss on the side facing the sun.” futuristic This resulted in larger pieces being broken off earlier this year. These larger fragments are less susceptible to solar radiation pressure, causing them to move more slowly and accumulate on the side facing the sun.
As Loeb noted in a recent update on his blog, the observations still clearly show 3I/ATLAS’s anti-wake more than a month after its perihelion, its closest pass by the Sun. He wrote that the image taken on December 13 by the Teerasak Thaluang Telescope in Rayong, Thailand “shows a distinct anti-tail, which is unusual for comets, pointing in the direction of the sun.”
Judging from the “thousands of images” taken since Hubble’s July observations showing the anti-wake of 3I/ATLAS, Loeb believes this is “not a perspective effect” but “a real physical jet.”
“Its nature is a mystery because the gas and micron dust particles are expected to be pushed away from the Sun by solar radiation pressure and solar wind, creating the appearance of a tail — as is common in solar system comets,” Loeb wrote.
As he tends to do, Loeb thinks it’s still possible we could see an alien spacecraft rather than a natural comet. He thinks the anti-tail may be “a group of objects that lag behind 3I/ATLAS as it moves away from the sun’s non-gravitational acceleration,” as he details in a paper that has not yet been peer-reviewed.
However, others are not convinced by this possibility, arguing that despite 3I/ATLAS’s interstellar origins, there is nothing unusual about the object’s two tails.
“It’s ejecting dust particles toward the sun because the dayside of the core is the hot side,” said David Jewitt, a UCLA astronomy professor and comet expert. sky and telescope last month.
“All of this is consistent with comet nuclei of typical size or smaller sublimating in sunlight and blowing out dust particles,” he added. “There’s nothing really shocking there.”
In a September 29 blog post, Penn State astronomer Jason Wright also criticized Loeb’s unusual conclusion that 3I/ATLAS may be an alien spacecraft, noting that several previous observations had resulted in “similar sunward enhancement” caused by large ejected dust particles that “were not swept away by the solar wind on the sun-facing side of the comet.”
ESA scientists also suggested that the observed secondary tail may be a “dust tail” composed of tiny solid particles, which is a typical feature of comets in the solar system.
Even Loeb himself is open to all possibilities, writing two other papers suggesting that the anti-tail is “the result of sunlight scattered by ice fragments on the sun-facing side of 3I/ATLAS.”
“These tiny ice particles evaporate before being significantly pushed back by solar radiation pressure, so they never emerge as a traditional comet tail,” he wrote in his latest blog post.
Still, Loeb thinks we should be prepared for the unexpected.
“By identifying anomalies, we can learn new things,” he concluded. “By ignoring them, we remain ignorant.”
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