The Hubble Space Telescope, in collaboration with ground-based observatories and other space telescopes, has documented two distinct cometary phenomena: the fragmentation of Comet C/2025 K1 (ATLAS) and the reversal of rotation for Comet 41P/Tuttle-Giacobini-Kresák. These observations provided astronomers with insights into the physical processes affecting comets as they interact with the Sun, from their structural integrity to their rotational dynamics.
Comet C/2025 K1 (ATLAS) Fragmentation
Observation and Timeline
Comet C/2025 K1 (ATLAS), discovered in May 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS), fragmented into at least four distinct pieces during late 2025. Some observations suggested the presence of potentially five fragments. Each fragment subsequently developed its own coma, a cloud of gas and dust surrounding its nucleus.
The Hubble Space Telescope captured detailed images of the fragmentation between November 8 and November 10, 2025, taking three 20-second exposures daily. John Noonan, a research professor and co-investigator at Auburn University, stated that the observation was serendipitous, occurring after the team's original comet target became unviewable. Noonan noted the fragmentation upon reviewing the images.
Ground-based telescopes also documented the event. The Gemini North telescope in Hawaii observed glowing fragments on November 11 and December 6, 2025. Gianluca Masi of the Virtual Telescope Project in Italy imaged what appeared to be three or four fragments in early November. Separately, astronomers at the Asiago Observatory in Italy observed two fragments, approximately 1,200 miles (2,000 km) apart, on November 11.
Hubble's capabilities provided clear resolution of individual fragments, which appeared as bright blobs or hazy smudges through ground-based instruments.
Characteristics and Mechanism
Prior to its fragmentation, Comet C/2025 K1 was estimated to be approximately 5 miles (8 kilometers) across. It is classified as a long-period comet, believed to originate from the distant Oort Cloud.
The comet reached its closest approach to the Sun, or perihelion, on October 8, 2025. This occurred within Mercury's orbit, approximately one-third the Earth-Sun distance, or 31 million miles (50 million kilometers). Researchers estimate the comet began to disintegrate approximately eight days prior to Hubble's observation, likely in late October or early November, after enduring intense heating and stress during perihelion.
The fragmentation is attributed to strong solar gravity and the pressure of solar wind on the comet, which is a loosely held collection of ice and dust. Sublimation-driven gas jets, combined with gravitational forces and internal structural weaknesses, are also factors in such disintegration events. During the observation period, one of the smaller fragments was also observed to break apart.
Scientific Significance
This event marked one of the earliest observations of a comet's breakup, occurring days after the process began, rather than weeks or months later. This proximity to the initial fragmentation provided data for understanding the physical processes occurring at a comet's surface.
Comets are considered remnants from the early solar system, preserving primordial materials. Observing the interior of a comet immediately after fragmentation offers an opportunity to study unprocessed, ancient material, which assists scientists in distinguishing a comet's primitive properties from changes induced by solar heating and cosmic radiation. Initial ground-based analysis indicates Comet K1 is chemically anomalous, showing a depletion of carbon compared to other comets.
An observation revealed a delay between the comet's initial breakup and subsequent bright outbursts detected by Earth-based instruments. Scientists have proposed theories for this, including the time required for a layer of dry dust to form over newly exposed ice and then be ejected by gas, or for heat to accumulate beneath the surface to build pressure before expelling a dust shell. Cometary brightness is primarily attributed to sunlight reflecting off dust grains.
The findings related to Comet C/2025 K1's fragmentation were published in the journal Icarus on February 6. The comet's fragmented remains are currently located approximately 250 million miles from Earth in the constellation Pisces and are projected to exit the solar system permanently.
Comet 41P/Tuttle-Giacobini-Kresák Spin Reversal
Observation and Reversal Details
Astronomers, utilizing data from the Hubble Space Telescope, NASA's Neil Gehrels Swift Observatory, and the Lowell Discovery Telescope, observed Comet 41P/Tuttle-Giacobini-Kresák undergoing a reversal in its spin direction. This phenomenon marks the first time such a rotational reversal has been documented for a comet.
Comet 41P, a Jupiter-family comet with an approximate 5.4-year orbit, passed its perihelion in 2017. Observations from March 2017 by the Lowell Discovery Telescope showed the comet spinning. By May 2017, data from the Swift Observatory indicated the comet's rotation had slowed significantly, spinning approximately three times slower with a period of 46 to 60 hours. Subsequent Hubble observations in December 2017 revealed that the comet's spin had accelerated again, to a period of about 14 hours.
David Jewitt, a planetary scientist at the University of California at Los Angeles who analyzed the combined data, inferred that the comet had slowed, nearly stopped, and then reversed its direction of rotation before accelerating in the new direction.
Mechanism and Implications
The observed changes in rotation are attributed to outgassing jets from the comet's surface. As Comet 41P approached the Sun, heating caused its volatile ices to sublimate into gas, producing jets that acted as small thrusters. Jewitt explained that unevenly distributed jets can significantly alter a comet's rotation, especially for smaller comets like 41P. The comet's nucleus, approximately 0.6 miles (1 kilometer) in diameter, is relatively small, making it susceptible to the torque generated by these gas jets.
This observation provided an opportunity to study evolutionary processes within comets over a human timescale. Modeling suggests that continued rotational changes could lead to structural instability for Comet 41P, potentially resulting in its fragmentation or disintegration if centrifugal forces overcome its weak gravitational binding.
Observations from 2001 indicate a substantial decline in Comet 41P's overall activity by 2017, with gas production significantly reduced. This suggests a rapid surface evolution, possibly due to the depletion of near-surface volatile materials or the formation of insulating dust layers.
Comet Characteristics and Data Access
Comet 41P/Tuttle-Giacobini-Kresák originated from the Kuiper Belt and its orbit has been gravitationally influenced by Jupiter. It has likely been in its current orbit for approximately 1,500 years.
The discovery of the spin reversal was made by Jewitt during his review of archival data from the Mikulski Archive for Space Telescopes, a public repository for astronomical mission data. The findings were detailed in The Astronomical Journal.