- Oumuamua's mysterious acceleration is best explained by trapped hydrogen escaping from warming ice.
- Bergner and Seligman's 2023 Nature paper proposed the dark comet model for Oumuamua.
- 14 dark comets have since been found in our own solar system.
Rob Weryk was sorting through data from Haleakalā Observatory in Hawaii when a moving object caught his attention in October 2017. It was travelling too fast for any known solar system body, and it carried a Hawaiian name: ‘Oumuamua, meaning “first messenger from afar.” By the time astronomers grasped what they were seeing, it was already leaving.
It was the first confirmed object ever detected passing through our solar system from interstellar space.
Key figure
87 km/s
'Oumuamua's speed as it rounded the Sun – around 170 times faster than a commercial jet
An Object That Broke the Rules
What unsettled astronomers was not the speed. The suprising thing was the acceleration.
As ‘Oumuamua left the Sun, it sped up in ways that gravity and solar radiation could not explain. Rob Weryk, credited with the discovery, said its motion could not be accounted for by any standard orbit. The Spitzer Space Telescope spent 30 hours hunting for the infrared signature that would mark a comet.
It found nothing.
What is non-gravitational acceleration?
In comets, acceleration beyond what gravity predicts usually comes from sublimating ice releasing gas – acting like a thruster. ‘Oumuamua accelerated without producing any detectable gas or dust.
Harvard astronomer Avi Loeb proposed that only a light sail, a thin sheet of artificial material catching solar wind, could explain the data. His papers attracted wide attention. They attracted equally wide skepticism from colleagues, who saw the claim as premature and untethered from the available evidence.
The more patient explanation came from chemistry.
The Ice That Remembers Radiation
Jennifer Bergner, an astrochemist at the University of California, Berkeley, attended a seminar where Darryl Seligman, then a postdoctoral researcher at Cornell University, described ‘Oumuamua’s stubborn properties. She thought there was a simpler answer buried in decades-old laboratory experiments.
Water ice that forms at extreme cold – the temperatures of interstellar space – takes an amorphous, disordered form. Cosmic rays penetrate deep into this ice, splitting molecules and trapping hydrogen gas inside. When the ice warms near a star, it rearranges into a more stable crystal structure, and the trapped hydrogen escapes through cracks and channels.
Bergner and Seligman published their argument in Nature in March 2023. The key was size. In large comets, this outgassing effect is too small to matter. ‘Oumuamua, estimated at roughly 115 metres at its widest, was small enough that escaping hydrogen could produce the observed push. No dust would exit. No visible coma would form.
Just invisible gas, doing the work.
A New Population, and a Third Visitor
The dark comet hypothesis has since acquired its own extended family.
By late 2024, Seligman and colleagues had identified 14 objects in our solar system that show unexplained non-gravitational acceleration without any visible outgassing. One of them, 1998 KY26, is already scheduled for a visit by Japan’s Hayabusa2 spacecraft, which is expected to arrive in 2031.
Then, on July 1, 2025, astronomers confirmed 3I/ATLAS – the third known interstellar object.
More On Oumuamua
Interstellar Object 3I/ATLAS Has Nickel but No Iron
The interstellar comet 3I/ATLAS arrived with nickel but no iron, a chemical gap that challenges what we know about distant star systems.
→Unlike ‘Oumuamua, 3I/ATLAS looked and behaved like a conventional comet. It had a visible coma and tail, released water vapor, and proved unusually rich in carbon dioxide. The James Webb Space Telescope later detected methane in its coma – a hyper-volatile ice that sublimates at very low temperatures, pointing to a cold, distant origin. No mystery surrounded its acceleration, but the object confirmed that interstellar visitors are real, recurring, and diverse.
The Vera C. Rubin Observatory, which reached first light in 2025, imaged 3I/ATLAS during its commissioning phase. When its full ten-year sky survey begins, estimates suggest it could detect up to 50 interstellar objects per year.
The question that Weryk’s data raised in 2017 has shifted.
It is no longer whether interstellar visitors exist, but how many kinds there are.
Sources
- Primary Source: Everything We Know About 'Oumuamua (Astrum, YouTube)
- Additional Context:
- Acceleration of 1I/'Oumuamua from radiolytically produced H2 in H2O ice (Nature, 2023)
- NASA Researchers Discover More Dark Comets (NASA JPL, 2024)
- Third-ever confirmed interstellar object blazing through solar system (Phys.org, 2025)
Fact Check: Claim-by-Claim Verification Verified
The article accurately represents established scientific facts about 'Oumuamua, dark comets, and recent interstellar objects, with appropriate context on explanations and observations.
Commentary
- Article's "14 objects by late 2024" aligns with Dec 2024 PNAS paper doubling prior 7 to 14 total dark comets.
- H2 model for 'Oumuamua remains leading natural explanation but debated; no consensus against it in sources.
- Perihelion speed ~87 km/s is gravitational, not excess acceleration (article clarifies acceleration was non-gravitational outbound).
- Predictions for Rubin Observatory detection rates are estimates, not precise confirmed figures.
Sources used for verification
Academic/Peer-reviewed:
- Acceleration of 1I/'Oumuamua from radiolytically produced H2 in H2O ice - Nature
- Spitzer Observations of Interstellar Object 1I/'Oumuamua - arXiv
- Two Distinct Populations of Dark Comets Delineated by Orbits and Sizes - PNAS
- JWST detection of a carbon dioxide dominated gas coma surrounding interstellar object 3I/ATLAS - arXiv
Other reliable sources:
- NASA Researchers Discover More Dark Comets - JPL.nasa.gov
- Mark Your Calendars! NSF–DOE Vera C. Rubin Observatory Will Unveil First Look Images on 23 June 2025 - rubinobservatory.org
Fact-checked by Perplexity Sonar Pro on 2026-03-08
