Subaru Telescope measures Comet Encke's 7-by-3-kilometer nucleus

That result agreed with about 327 kelvin predicted by an empirical model, meaning a calculation built from previous observations. The agreement was close.

A nucleus's surface temperature helps show how it absorbs sunlight and radiates heat into space. Those thermal properties can help researchers understand the comet's activity, including the release of gas and dust that surrounds it.

The measured heat was consistent with the earlier prediction.

Why can a comet's shape alter its path?

Comet Encke is named for Johann Encke, a 19th-century German astronomer who studied its orbit and showed that it returns to the Sun's vicinity about every 3.3 years. Encke also found that its orbital period was gradually shortening. The cause remained unexplained.

In the mid-20th century, American astronomer Fred Whipple proposed that gas and dust escaping as a comet approaches the Sun create a reaction force that changes its orbit. His “dirty snowball” model described nuclei as mixtures of ice and dust. The model remains a cornerstone of comet research.

Comet nuclei preserve material from the birth of the solar system 4.6 billion years ago, making them valuable for studying the solar system's early history. Encke became one of the cases that helped scientists recognize the “non-gravitational effect,” in which gas jetting acts like a rocket thruster and pushes on a nucleus.

The size and direction of that effect depend on the nucleus's shape and spin, and on where and in which direction gas escapes. The shape measured in this study cannot be identified as the direct cause of Encke's shortening orbital period. It does, however, provide basic information for understanding the comet's activity and long-term orbital evolution.

The shape is a clue to the changing orbit, not the explanation itself.

Can ground-based telescopes see comet nuclei?

In 2021, the same team detected another comet's nucleus in the mid-infrared. Together with the Encke result, such observations are gradually revealing cometary nuclei that remain difficult to study directly from the ground.

The mid-infrared is well suited to capturing a nucleus's heat but difficult to observe from Earth. Water vapor in the atmosphere absorbs much of this radiation, while the telescope and atmosphere themselves emit strong thermal radiation that can interfere with measurements.

The summit area of Maunakea, at an altitude of about 4,200 meters, is dry and contains little water vapor. Combined with Subaru's large 8.2-meter aperture, those conditions allowed COMICS to perform high-resolution imaging and spectroscopy, which measures radiation at individual wavelengths.

COMICS ended its observations on Subaru in 2020. At present, no instrument on Maunakea can routinely conduct mid-infrared observations, although a plan to restart COMICS is underway under Ootsubo, the leader of the Encke study.

Ootsubo said: “Our study of Comet Encke showed that mid-infrared observations can directly capture thermal emission from the nucleus itself.”

He added: “By restarting COMICS, we hope to study the shapes, temperatures and surface materials of many more cometary nuclei in detail and get closer to revealing their true faces.”

Maunakea's dry air made a difficult kind of comet science possible, and the team now wants the instrument back.

What has the study settled?

The findings were published as “Photometric and Spectroscopic Observations of Comet 2P/Encke's Nucleus in the Mid-infrared Wavelength Region” by Takafumi Ootsubo and colleagues in The Astronomical Journal in 2026. The DOI is 10.3847/1538-3881/aea614.

Together, the measured shape and temperature give researchers a reference point for modeling Encke's activity and orbital evolution. They do not identify the specific cause of its changing orbital period.

The study supplies Encke's measured size and 2003 temperature; the orbital explanation remains open.