Mid-infrared observations with the Subaru Telescope have revealed the shape and surface temperature of the nucleus of Comet 2P/Encke. By analyzing how the nucleus’s thermal emission changed as it rotated, the researchers found that the nucleus is elongated, with a long axis of about 7 kilometers and a short axis of about 3 kilometers. They also determined that the surface temperature at the time of observation was 328 ± 7 kelvin (about 55°C). These results provide new clues for understanding how comets’ activity and orbits change, and for exploring the early history of the Solar System.
Figure 1: Conceptual illustration of the nucleus of Comet Encke (Credit: Kyoto Sangyo University (created using Microsoft Copilot))
The Nucleus: A Key to Understanding Comets
Appearing seemingly out of nowhere, trailing a hazy "coma" and a long "tail" across the night sky, comets have been objects of interest culturally, artistically, and scientifically throughout human history.
Comet Encke (2P/Encke; Note 1) is named for the 19th-century German astronomer Johann Encke, who studied its orbit in detail and showed that it is a periodic comet that swings by the Sun about every 3.3 years. Encke also found that the comet’s orbital period was gradually getting shorter, but the cause remained unexplained. The clue to this puzzle turned out to lie in the "nucleus" at the comet’s center.
In the mid-20th century, the American astronomer Fred Whipple proposed that when gas and dust jet out of a comet as it approaches the Sun, the reaction force changes the comet’s orbit. He also put forward the "dirty snowball" model, in which the nucleus is made of ice mixed with dust. This model remains a cornerstone of our understanding of comets.
Comet nuclei preserve material from the birth of the Solar System 4.6 billion years ago, which makes them an important subject of study. However, as a comet approaches the Sun, gas and dust released from the nucleus extend around it and form a cloud-like structure called a coma, so direct observation of the small nucleus at the center is difficult from ground-based observatories.
Revealing the Nucleus’s Shape and Temperature
A research team from the University of Occupational and Environmental Health, Kyoto Sangyo University, and Photocross Co., Ltd. analyzed mid-infrared data (wavelength of about 10 micrometers) for Comet Encke, obtained with the Subaru Telescope in November 2003. The observations were made with the Cooled Mid-Infrared Camera and Spectrometer (COMICS).
Mid-infrared observations can detect thermal emission from the nucleus itself. Thanks to the Subaru Telescope’s high spatial resolution, Comet Encke appeared as a point source, indicating that emission from the coma made only a small contribution to the observed signal.
The analysis showed that the comet’s brightness changed during the observations. This is thought to be due to the rotation of the elongated nucleus, changing the apparent size of the surface as seen from Earth. From this brightness variation, the team estimated that the nucleus is an elongated ellipsoid, about 7 kilometers along its long axis and about 3 kilometers along its short axis.
Earlier visible-light and radar observations had also suggested that Encke’s nucleus may be elongated, while spacecraft observations have revealed cometary nuclei with complex, pronouncedly nonspherical, shapes. This study provides an independent estimate of Encke’s shape based on the thermal emission from the nucleus itself.
By further analyzing thermal emission observed at multiple wavelengths, the team found that the nucleus had a surface temperature of 328 ± 7 kelvin (about 55 °C) when the comet was about 1.1 astronomical units (au) from the Sun (Figure 2). This agrees well with the temperature of about 327 kelvin predicted by an empirical model based on previous observations. A nucleus’s surface temperature is indictive of how it absorbs sunlight and radiates heat into space, providing insight into its thermal properties and helping researchers understand its activity.
Figure 2: Schematic of Comet Encke’s orbit at the time of observation (November 2003). The comet was 1.09 au from the Sun and 0.27 au from Earth. One au (astronomical unit) is approximately the average distance between Earth and the Sun. (Credit: Kyoto Sangyo University)
How Cometary Activity Can Change an Orbit
Comet Encke holds a special place in the history of comet research. It was one of the cases that led scientists to the "non-gravitational effect," in which gas jetting from the nucleus acts like a rocket thruster, pushing on the nucleus and altering its orbit. The size and direction of this effect depend on the nucleus’s shape and spin, and on where and in which direction the gas is released. The shape revealed in this study cannot be identified as the direct cause of the change in Encke’s orbital period. Even so, the shape and size obtained here provide fundamental information for understanding the comet’s activity and the long-term orbital evolution.
Maunakea: A Window to See Cometary Nuclei
In 2021, the same team also succeeded in detecting another comet’s nucleus in the mid-infrared (Subaru Telescope science result, April 5, 2021). Together with the present result, ground-based observations are gradually revealing the nature of cometary nuclei.
The mid-infrared is well suited to capturing a nucleus’s thermal emission, but it is a difficult wavelength range for ground-based observation: mid-infrared radiation is easily absorbed by water vapor in Earth’s atmosphere, and the telescope and the atmosphere themselves emit strong thermal radiation. The summit area of Maunakea, at an altitude of about 4,200 meters, is dry with little water vapor, making it well suited to mid-infrared observations. Taking advantage of this excellent environment and the Subaru Telescope’s large 8.2-meter aperture, COMICS was a world-class, uniquely-capable instrument that could perform imaging and spectroscopy with high spatial resolution.
COMICS ended its observations on the Subaru Telescope in 2020. Currently, no instrument on Maunakea can routinely perform mid-infrared observations, and a plan to restart COMICS is under way, led by Dr. Takafumi Ootsubo of the University of Occupational and Environmental Health, who led this study.
"Our study of Comet Encke showed that mid-infrared observations can directly capture thermal emission from the nucleus itself," says Dr. Ootsubo. "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."
These results were published on November 9, 2026, in The Astronomical Journal (Ootsubo et al., "Photometric and Spectroscopic Observations of Comet 2P/Encke’s Nucleus in the Mid-infrared Wavelength Region").
This research was supported by JSPS KAKENHI (grant numbers JP23K25930, JP24H00271, and JP26K00771), the Koyama Space Science Institute of Kyoto Sangyo University, and the Koyama Astronomical Observatory.
(Note 1) The "P" in "2P/Encke" stands for "periodic comet," and the "2" indicates the order in which a comet’s periodicity was confirmed. Comet Encke was the second comet whose periodicity was established, after Halley’s Comet.
The Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan, National Institutes of Natural Sciences with the support of the MEXT Project to Promote Large Scientific Frontiers. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has cultural, historical, and natural significance in Hawai`i.
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