“No one knows where the comet came from. It’s like glimpsing a rifle bullet for a thousandth of a second,” said David Jewitt, the University of California, Los Angeles astronomer in charge of Hubble’s newest observations. That ephemeral target is 3I/ATLAS, just the third confirmed interstellar object ever seen in our solar system, and now the target of the most precise image ever taken of such a visitor.
On July 21, as the comet whizzed by at an incredible 134,000 miles per hour relative to the Sun, NASA’s Hubble Space Telescope took a lock on it at 320 miles above Earth. The image that followed showed ateardrop-shaped cocoon of dust streaming behind an unseen icy core material shed as solar warming began to agitate the surface of the comet. Hubble’s Wide Field Camera 3, built for high-resolution imaging in ultraviolet to near-infrared wavelengths, was able to tease apart the faint dust plume even as the object moved so fast and was 277 million miles away.
The nucleus itself is obscured, even to Hubble’s optics, but the new data limit its size to between 1,000 feet and 3.5 miles in diameter. That width is smaller than previous ground-based measures, courtesy of Hubble’s capability to distinguish the coma’s scattered light from the surrounding sky. The rate of dust loss, as determined from the brightness of the plume, is the same as that of comets first observed at 300 million miles from the Sun, indicating that 3I/ATLAS is acting precisely like long-period comets which are indigenous to our own system.
Its origin, though, is beyond a doubt extraterrestrial. Comet’s hyperbolic path and high velocity approximately 60 kilometers per second suggest it was thrown out of another star system and has been wandering in the Milky Way for billions of years. Astrophyisicists Aster Taylor and Darryl Seligman put its age at between 3 and 11 billion years, based on the gravitational “slingshot” accelerations it would have gained through numerous stellar encounters. Some models point to a possible origin in the galaxy’s thick disk, a population of older, high-velocity stars above and below the Milky Way’s plane.
The technical challenge of imaging such a fast, distant object is formidable. Hubble’s tracking system had to match the comet’s apparent motion over 75 arcseconds per hour while maintaining sub-arcsecond stability to avoid smearing the image. Multi-band photometry from the Palomar 200-inch telescope and other observatories in g, r, and i filters determined its colors to have a spectral slope of approximately 1.3% per 100 nanometers, much bluer than interstellar comet 2I/Borisov. Its blueness may be indicative of variations in surface composition or the lack of certain gas emissions within the coma.
The rates of dust production were estimated based on the A(0°)fρ factor, resulting in measurements close to 287 centimeters, characteristic of active solar system comets. It can be inferred from the sunward extent of the coma that micrometer- to millimeter-sized dust grains are expelled at velocities ranging from 0.01 to 1 meter per second, and the rate of total mass loss is between 0.1 and 1.0 kilograms per second. These figures closely track Borisov’s inbound activity but are vastly larger than the near-zero dust emission of ‘Oumuamua, the initial interstellar object discovered in 2017.
Companionship between the three recognized interstellar guests emphasizes their variation: ‘Oumuamua was shaped like a cigar and quiet, Borisov was chemically ordinary but not very wet, and 3I/ATLAS seems larger, more rapid, and possibly older. Its activity should increase as it runs toward perihelion on October 30, 2025, at 1.35 astronomical units from the Sun, although Earth will be on the opposite side of the Sun, making direct observation difficult.
To close those gaps, astronomers are coordinating observations among platforms. The James Webb Space Telescope will search for volatiles like water, carbon dioxide, and ammonia in August and December, while the Neil Gehrels Swift Observatory and TESS will track its light curve for rotation signals. Ground-based telescopes such as the Lowell Discovery Telescope will take advantage of low-sun angles to monitor the comet close to perihelion, and even NASA’s MAVEN orbiter at Mars might see its coma from 19 million miles.
3I/ATLAS will exit the solar system by year’s end, never to return. Its fleeting passage offers a rare laboratory for testing theories of planetesimal formation and survival over cosmic timescales. As Jewitt noted, “This latest interstellar tourist is one of a previously undetected population of objects bursting onto the scene… We’ve crossed a threshold.” For astronomers, the challenge now is to catch the next one before it slips back into the dark.
