Last summer, astronomers were observing the near approach of a space object believed to be an asteroid that had been tracked since its discovery in 1998. However, in a surprising development, the object failed to appear where it was predicted to be after decades of observations.
Now, a team of researchers reports in a new study that this mysterious orbital body is no asteroid at all, but instead may be an elusive class of space objects that scientists are still working to fully understand.
The research, published in the journal Nature Astronomy, identifies the newly suspected reasons for this unexpected “dark” twist in the developing story of 1998 SH2, a celestial body once thought to be a common space rock that is now upending past thinking on how to protect Earth from potential impact risks.
1998 SH2’s August Surprise
Since its discovery in 1998, the object designated 1998 SH2 has been monitored by astronomers continuously, providing close to three decades of observations that inform us about its orbit, leaving what researchers thought would be little room for surprises.
That all changed last August as the anticipated return of 1998 SH2 approached. Based on radar observations, the object appeared to be absent from its expected orbital path—a mystery that initially baffled astronomers.
Eventually, once the near-Earth object was spotted again by Brazil’s Southern Observatory for Near Earth Asteroids Research, it had moved significantly beyond its expected path. But what could account for such an unusual display?
When AN Asteroid Goes “Dark”
Now, according to Jet Propulsion Lab and CalTech researcher Davide Farnocchia and colleagues, 1998 SH2 was likely steered off course as a result of jets of gas escaping from beneath the object’s surface—a phenomenon known as weak cometary outgassing—meaning that the suspected asteroid is actually probably a type of comet.
However, unlike most comets, this one showed none of the typical characteristics astronomers associate with them, including a glowing tail or coma. This helped to explain the confusion over its initial classification, and additional observations of layered composite imagery did finally reveal these features, albeit faintly.
This confirmation that the object was indeed releasing material meant that 1998 SH2 was a rare variety of space objects known as dark comets, enigmatic bodies whose subtle behavior falls somewhere between comets and asteroids.
Planetary Defense Implications
One of the more concerning aspects of the discovery involves 1998 SH2’s unexpected orbital shift, which presents the question of whether other suspected asteroids might turn out to be dark comets. If so, could any of these shifty space objects pose a risk to Earth?
Fortunately, Farnocchia and his colleagues say there is currently no indication that 1998 SH2 poses an impact threat to Earth in the foreseeable future. Still, the team’s findings do have important implications for planetary defense, since their findings underscore the possibility that additional near-Earth objects currently classified as asteroids may turn out to be hidden comets.
Given the unpredictable orbital shifts and changes in acceleration these objects display, challenges associated with identifying and tracking them could complicate long-term impact forecasts, and influence how future deflection missions are designed.
Fortunately, the team’s study also highlights beneficial findings, which include the possibility that a hidden population of dark comets could potentially resolve questions about how water was delivered throughout the early solar system, eventually making its way to a young planet Earth billions of years ago.
Additionally, such possibilities could help to explain past observations of enigmatic objects like ‘Oumuamua, the first confirmed interstellar object, whose detection in 2017 and unexplained acceleration during subsequent observations have fueled scientific debate over its origins for many years.
Farnocchia and his colleagues’ new paper, “Non-gravitational acceleration indicative of cometary activity of near-Earth object,” appeared in Nature Astronomy.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.