Peeling away the dust: a rare direct look at comet Encke’s bare nucleus

Comets are time capsules from the dawn of the solar system, but their solid hearts — the nucleus — are almost always wrapped in a thick veil of dust and gas that hides them from view. For one famous comet, 2P/Encke, astronomers have pulled off a rare feat: looking straight at the bare nucleus from the ground, in thermal infrared light, and measuring its shape, size and warmth.

The hidden heart of a comet

A comet’s nucleus is a chunk of ice and dust, a leftover planetesimal from about 4.6 billion years ago, and its shape, size and interior record both how it formed and how it has eroded over countless trips around the Sun. The trouble is that within about three times the Earth-Sun distance, the nucleus is buried under an optically thick cloud of dust called the coma. Seeing it directly usually requires a spacecraft flyby, as with Giotto at Halley or Rosetta at comet 67P, or radar during a close pass. And in mid-infrared light, the glowing dust grains normally drown out the nucleus entirely. Usually.

Why Encke is special

Comet 2P/Encke is a short-period comet, circling the Sun every 3.3 years on an orbit that lies entirely inside Jupiter’s, and it is an old, evolved body that serves as a key evolutionary benchmark. Crucially, it has very low dust activity, so its coma is thin enough that the nucleus’s own thermal glow can be detected directly. A team led by Takafumi Ootsubo used the mid-infrared camera and spectrometer on the 8.2-meter Subaru Telescope to observe it, from data taken in 2003 and now fully analyzed.

A bare nucleus, measured

The comet appeared as a sharp point of light, like a star, rather than a fuzzy blob, confirming the astronomers were seeing the nucleus itself and not a shroud of dust. As the elongated nucleus rotated, once every roughly 11 hours, its brightness rose and fell, betraying its shape: an elongated body with a long-to-short axis ratio of about 2.3 to 1, roughly 3.6 by 1.6 kilometers, a potato-shaped splinter a few kilometers across. Its surface glowed at about 328 kelvin, around 55 degrees Celsius, just what one would expect for a body warmed by the Sun at that distance.

A whiff of the surface

The infrared spectrum was a smooth continuum, with none of the silicate dust feature near 10 microns that a dusty coma would betray, again confirming that the nucleus dominates. But it did show a subtle dip in emissivity near 9.5 microns, a faint fingerprint of the surface material, and the very same feature seen in another low-activity comet, 460P/PanSTARRS. It is a hint that these bare comet surfaces may share a common trait.

What’s measured, and what’s modeled

The thermal brightness, the point-source appearance, the temperature and the smooth spectrum are direct measurements. The shape and size, however, come from a model: an idealized smooth, uniform, elongated ellipsoid rotating about a fixed axis whose orientation was taken from earlier studies. So the 2.3-to-1 ratio and the 3.6-by-1.6-kilometer size carry real uncertainties and rest on those assumptions, and the data come from a single night. Even so, the result is a rare prize: a direct, ground-based portrait of the naked nucleus of a historic comet, an ancient, elongated chunk of the early solar system caught with its dust down.

© 2026 Homer Dávila Gutiérrez, FRAS — SKYCR.ORG. All rights reserved. Publication: Ootsubo, T., Kawakita, H., Watanabe, J., et al., «Photometric and Spectroscopic Observations of Comet 2P/Encke’s Nucleus in the Mid-infrared Wavelength Region», The Astronomical Journal, 172, 299 (2026). DOI: https://doi.org/10.3847/1538-3881/aea614