Plate Nº 76 · recorded November 11, 2026
Space & AstronomyReported finding
Asteroid 1995 BJ4 Grew a Tail — and It May Be the Largest Main-Belt Comet
A machine-learning survey flagged asteroid 1995 BJ4 with 99.9% confidence; its 32-arcsecond tail could make it the largest main-belt comet ever found — if ice, not a collision, feeds it.
By Marcus Bennett3 min read660 words
In brief
- Asteroid 6993 (1995 BJ4) is about 7.5 miles (12 km) across, larger than 99% of asteroids.
- A machine-learning model in the SNAPS pipeline flagged the activity on August 5, 2026, with 99.9% confidence.
- The tail was visible in Zwicky Transient Facility images dating back to June 25, 2026, extending at least 32 arcseconds.
- The study appeared on arXiv on October 22, 2026, and is not yet peer-reviewed.
- The asteroid reaches perihelion, about 2.85 times Earth's distance from the sun, in January 2027.
A machine-learning model flagged asteroid 6993 (1995 BJ4) as active on August 5, 2026, with 99.9% confidence — and the 7.5-mile (12 km) rock now carries a tail stretching at least 32 arcseconds across the sky. If that tail comes from vaporizing ice, 1995 BJ4 becomes the largest-known main-belt comet, an object larger than 99% of all asteroids.
Researchers at Northern Arizona University and other institutions published their study on October 22, 2026, on arXiv, a preprint server. The paper has not yet passed peer review, so its conclusions should be read as preliminary.
What did astronomers actually see?
The discovery came through the Solar System Notification Alert Processing System (SNAPS), an automated pipeline at Northern Arizona University. The team trained a machine-learning model to recognize the fuzzy look of active asteroids — rocks that shed dust and gas instead of sitting inert. On August 5, the model flagged an image of 1995 BJ4 with 99.9% confidence.
The researchers then searched earlier images from the Zwicky Transient Facility, a survey telescope in California. The tail had already been visible on June 25, 2026. It appears narrow and straight, extending at least 32 arcseconds — a measure of apparent size on the sky, where one arcsecond is 1/3600 of a degree — beyond the asteroid's main body.
The object orbits in the main asteroid belt between Mars and Jupiter, near the belt's outer edge. Astronomers have confirmed only about 20 main-belt comets to date, so each new candidate matters.
Takeaway: an automated survey caught a 12-km asteroid spouting a tail it had been wearing since at least June 25, 2026.
What is making the asteroid active?
The team tested three explanations.
- Rotation. An asteroid spinning fast enough can fling material off its surface. But 1995 BJ4 rotates once every 3.9 hours — too slow to match the observations. The team rejected this option.
- Impact. A collision with another space rock could have blasted dust off the surface. That dust would spread out and fade over time. The asteroid looked slightly brighter in June and July than in August and September, which fits the picture of dispersing collision debris.
- Sublimation. Sunlight could be warming buried ice, turning it directly from solid to gas — the process that builds the comas and tails of ordinary comets. The study has not ruled this out.
Both surviving explanations remain live, and the researchers state plainly that continued observations will decide the question.
Takeaway: rotation failed the test; a collision and vaporizing ice are still standing, and brightness measurements slightly favor the collision — for now.
Why the stakes are bigger than one asteroid?
If sublimation wins, 1995 BJ4 sets two records. It becomes the largest-known main-belt comet at 12 km across, and it orbits farther from the sun than any known main-belt comet. Its perihelion — the closest point in its orbit to the sun — sits about 2.85 times Earth's distance from our star. Finding ice that far out, in a body that big, would tell scientists something about how water was distributed in the early solar system.
The object also resists classification. Its colors and reflectivity do not fit neatly into existing asteroid categories, leaving real uncertainty about what its surface contains.
Takeaway: confirming ice would hand this object two records and a fresh clue about where water condensed billions of years ago.
What happens in January 2027?
The decisive test arrives next month, astronomically speaking. 1995 BJ4 reaches perihelion in January 2027, its closest approach to the sun. If sunlight is vaporizing ice, activity should increase around that date. If the tail keeps fading instead, the impact explanation gains ground.
The team also wants follow-up data on the asteroid's composition to pin down what its odd surface is made of.
January 2027 will tell us whether this asteroid is a wounded rock or a comet wearing an asteroid's name.
via arxiv.org (Original)
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News editor covering regulation, policy and their impact on general science news at SciBeat.
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