Archival data from Subaru Telescope reveals unexpected surface of distant comet
A chance discovery in archival images from the Subaru Telescope highlights how observations made in Hawaiʻi can continue producing new information years after the data is collected.
In publicly available data from Subaru’s Hyper Suprime-Cam, the surface of Comet 28P/Neujmin was found to behave unlike the dark asteroids it resembles.
The comet was more than 10 astronomical units from the sun, which is farther than Saturn’s orbit, and nearly dormant, with no visible cloud of gas and dust obscuring its solid nucleus.
A research team from the University of Occupational and Environmental Health, Kyoto Sangyo University, and the National Astronomical Observatory of Japan used the unusually clear view to make the first precise ground-based measurement of how a cometary nucleus reflects light near opposition, when the sun, comet, and Earth are almost perfectly aligned.
The results offer new clues about the tiny grains and open spaces that shape a comet’s surface.
Comets preserve ice and dust from the formation of the solar system about 4.6 billion years ago, making them natural time capsules. Their nuclei are difficult to study directly because sunlight normally warms their ice and creates a surrounding coma.
When a comet is far enough from the sun for its ice to remain frozen, its nucleus is visible but extremely faint.
Comet 28P/Neujmin appeared in Hyper Suprime-Cam survey images taken for a different scientific purpose. Subaru’s 8.2-meter primary mirror detected the faint nucleus, while HSC’s exceptionally wide field of view made the chance appearance possible.
The comet was also observed at a phase angle of just 0.33 degrees, providing a nearly face-on view under exceptionally favorable conditions.
Near opposition, shadows cast by surface particles nearly disappear, and scattered light can be redirected toward the observer, making an airless body appear brighter. Measuring this “opposition effect” helps researchers infer the size, spacing, and packing of surface particles.
The comet’s reddish color and low overall reflectance resemble dark asteroids thought to contain water-bearing material. Its behavior near opposition, however, was unexpectedly different. Comet 28P/Neujmin brightened much more strongly than those asteroid types and instead showed an effect comparable to brighter, more reflective asteroids.
The contrast suggests that a comet and an asteroid can have similar color and reflectance while possessing fundamentally different surface microstructures. Repeated passages near the sun may gradually reshape a comet’s surface as its ice turns directly into gas, leaving behind a distinctive arrangement of grains and voids.
The study demonstrates the continuing scientific value of Subaru Telescope observations made from Maunakea. By combining a large light-gathering mirror, a wide-field camera, and a publicly accessible archive, researchers can find solar system objects that were not the original targets of an observing program.
“The same archive likely contains many more solar system objects waiting to be found,” said lead author Dr. Takafumi Ootsubo of the University of Occupational and Environmental Health.
The team plans to apply the same approach to additional comets to investigate how their surfaces change over time and what those changes reveal about the solar system’s formation and evolution.
The study, “Opposition Effect of Comet 28P/Neujmin Observed with Subaru Hyper Suprime-Cam,” was published Aug. 25, 2026, in the Publications of the Astronomical Society of Japan.
