Many of you may remember the interstellar object ʻOumuamua (Hawaiian for 'messenger') that visited our solar system in 2017. The object was unlike anything we had seen before, as it was the first known interstellar object to pass through our solar system.
One of the mysteries that baffled astronomers was the fact that it showed non‑gravitational acceleration (meaning that its acceleration could not be explained by the gravitational forces acting on it alone).
Now, a team of researchers from the University of California, Berkeley, and Cornell University may have found a surprisingly simple explanation for this puzzling behavior.
An artist’s impression of ʻOumuamua - Image Credit: joshimerbin via Shutterstock / HDR tune by Universal-Sci
Upon its discovery, scientists determined that 'Oumuamua lacked a coma or tail typically associated with comets. In addition, its distance from the sun was too large to produce significant water ejection through solar heating. As a result, astronomers were left to ponder the object's composition and what forces propelled it through space.
Theories ranged from the outgassing of an H2 iceberg to 'Oumuamua being a large and delicate snowflake pushed by solar radiation pressure. Some wilder theories even considered it to be a lightsail crafted by extraterrestrial intelligence or a spaceship traveling under its own power.
A surprisingly simple explanation
Researchers Jeniffer Berner (UC Berkeley) and Darryl Seligman (Cornell University) proposed a much simpler explanation in their paper published in the peer-reviewed journal Nature.
Bergner: "A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming hydrogen as a result. Our thought was: If this was happening, could you actually trap it in the body so that when it entered the solar system and it was warmed up, it would outgas that hydrogen?" Could that quantitatively produce the force that you need to explain the non-gravitational acceleration?
To her surprise, she discovered that studies conducted in the 1970s, 80s, and 90s revealed that when ice is struck by high-energy particles similar to cosmic rays, it results in the production and entrapment of significant amounts of molecular hydrogen (H2) within the ice. In fact, cosmic rays can penetrate deep into ice, converting up to a quarter of the water into hydrogen gas even at depths of several meters.
Bergner continues: "For a comet several kilometers across, the outgassing would be from a really thin shell relative to the bulk of the object, so both compositionally and in terms of any acceleration, you wouldn't necessarily expect that to be a detectable effect, but because 'Oumuamua was so small, we think that it actually produced sufficient force to power this acceleration."
ʻOumuamua, displaying a slightly reddish hue, is believed to have measured approximately 115 by 111 by 19 meters in size. However, due to its distance and relatively small size, telescopes could not accurately measure its dimensions, meaning that astronomers had to estimate its size based on its brightness and the changes in brightness as it tumbled.
Another artist’s impression of Interstellar Interloper ‘Oumuamua - Image Credit: NASA, ESA, Joseph Olmsted (STScI), Frank Summers (STScI)
It's worth noting that all comets observed within our solar system, including short-period comets originating from the Kuiper belt and long-period comets from the more remote Oort cloud, have varied in size from approximately one kilometer to hundreds of kilometers in diameter.
More 'Dark' comets and future research
Bergner, Seligman, and colleagues have discovered six additional comets that lack an observable coma and show slight non-gravitational acceleration (like ʻOumuamua).
These findings suggest that dark comets may be more common than previously thought. As there is still much to learn from these extraordinary objects, one of them (1998 KY26, thought to be an asteroid but later identified as a dark comet) will be the next target for Japan's Hayabusa2 mission.
Seligman stated that he thinks his colleague is right about the entrapped hydrogen hypothesis. Seligman: "water is the most abundant component of comets in the solar system and likely in extrasolar systems, as well. And if you put a water-rich comet in the Oort cloud or eject it into the interstellar medium, you should get amorphous ice with pockets of H2."
The Rubin Observatory Legacy Survey of Space and Time (LSST) is expected to uncover numerous interstellar and dark comets, potentially shedding light on the prevalence of hydrogen outgassing in comets. According to Seligman's calculations, the Vera C. Rubin Observatory in Chile, where the LSST will be carried out starting in early 2025, should be able to detect between one and three interstellar comets similar to 'Oumuamua each year, as well as numerous others that exhibit a visible coma, such as Borisov.
All in all, our understanding of these mysterious objects is expected to grow in giant leaps in the upcoming years. As always, we will keep you posted. If you are interested in more details about Berner and Seligman's research, be sure to check out their paper published in the peer-reviewed science journal Nature, listed below.
Sources and further reading:
Acceleration of 1I/'Oumuamua from radiolytically produced H2 in H2O ice (Nature)
'Oumuamua Might Be a Giant Interstellar Hydrogen Iceberg (Wired)
Could Solar Radiation Pressure Explain 'Oumuamua's Peculiar Acceleration? (Cornell University/arxiv)
Dark Comets? Unexpectedly Large Nongravitational Accelerations on a Sample of Small Asteroids (Cornell University/arxiv)
Could 'Oumuamua be an alien lightsail? (EarthSky)
Scientists say mysterious 'Oumuamua object could be an alien spacecraft (NBC)
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