An object that astronomers believed was an asteroid for nearly three decades has been reclassified as a rare “dark comet” after unexpectedly deviating from its predicted orbit.
Known as 1998 SH2, the object puzzled researchers when it failed to appear where calculations predicted it would in 2025. Follow-up observations and a year of analysis revealed that the object was exhibiting subtle comet-like behavior, despite lacking the bright tail and glowing coma normally associated with comets.
The discovery, published in Nature Astronomy, is providing new insights into an unusual class of solar system objects and raising fresh questions about how astronomers identify and track potentially hazardous bodies near Earth.
What Happened to Asteroid 1998 SH2?
Astronomers had monitored 1998 SH2 since its discovery in 1998, using years of observations to calculate its orbit with increasing precision.
When the object was expected to return for observation in August 2025, researchers pointed their telescopes toward its predicted location.
It wasn’t there.
Initially, the missing object puzzled astronomers because its orbit had been considered well understood.
The mystery deepened until Brazil’s Southern Observatory for Near Earth Asteroids Research detected the object at a different position than expected.
That unexpected shift suggested something other than gravity had slightly altered its trajectory.
Why Did Scientists Reclassify It as a Dark Comet?
A research team led by Davide Farnocchia of NASA’s Jet Propulsion Laboratory and Caltech investigated why the object’s orbit had changed.
Their analysis found evidence of weak cometary outgassing—a process in which gas escapes from beneath an object’s surface.
This escaping gas acts like tiny thrusters, producing a small but measurable force that can gradually change an object’s orbit.
Asteroids generally do not experience this type of self-propelled acceleration.
Because 1998 SH2 displayed this behavior, researchers concluded that it is more accurately classified as a dark comet than an asteroid.
What Is a Dark Comet?
Dark comets are among the most mysterious objects in the solar system.
They behave like comets by releasing gas that changes their motion.
However, unlike ordinary comets, they show little or no visible evidence of that activity.
Typical comets usually develop:
- A bright glowing coma surrounding the nucleus.
- A long tail of gas and dust pointing away from the Sun.
Dark comets lack these obvious visual features, making them appear almost identical to asteroids in telescope images.
As a result, they can remain misclassified for years.
Why Didn’t Astronomers Notice Earlier?
For nearly three decades, 1998 SH2 appeared to behave much like an ordinary asteroid.
It showed no obvious tail or surrounding cloud of gas that would have immediately identified it as a comet.
Only after researchers reanalyzed high-resolution observations did they detect faint signs of cometary activity.
Those subtle clues, combined with the unexpected orbital change, revealed the object’s true nature.
The case demonstrates how some dark comets can remain hidden in plain sight.
How Does Outgassing Change an Object’s Orbit?
When volatile materials beneath a comet’s surface are heated by sunlight, they can transform directly from solid ice into gas.
As that gas escapes into space, it produces a gentle thrust in the opposite direction.
Although the force is extremely small, it acts continuously over long periods.
Over months or years, this process can slightly alter the object’s speed and trajectory.
That appears to be what happened with 1998 SH2, causing it to arrive at a different location than astronomers had predicted.
Why Does This Matter for Planetary Defense?
Planetary defense programs rely on accurate predictions of where near-Earth objects will be years or even decades into the future.
Objects that experience non-gravitational forces such as outgassing are more difficult to track because their paths can slowly change over time.
If more objects previously classified as asteroids are actually dark comets, astronomers may need to account for these subtle effects when calculating future trajectories.
The discovery could improve long-term monitoring of potentially hazardous objects.
Is This Related to ʻOumuamua?
Researchers note that the discovery adds to growing interest in dark comets as a distinct class of celestial objects.
One of the best-known examples linked to this category is ʻOumuamua, the first confirmed interstellar object observed passing through the solar system in 2017.
ʻOumuamua exhibited unexplained acceleration without displaying a visible comet tail, leading some researchers to classify it as a dark comet. That interpretation remains an active area of scientific research, and alternative explanations have also been proposed.
The reclassification of 1998 SH2 strengthens evidence that similar objects may be more common than previously thought.
Why This Discovery Matters
Astronomers have discovered more than a million asteroids and thousands of comets, but dark comets occupy a poorly understood middle ground between the two.
Finding another confirmed example helps scientists better understand how these objects evolve and how common they may be throughout the solar system.
It also serves as a reminder that even well-studied objects can still surprise researchers decades after their discovery.
The Bottom Line
The object known as 1998 SH2 spent nearly 30 years being tracked as an asteroid before revealing its true identity as a rare dark comet. By detecting weak gas emissions that subtly altered its orbit, scientists solved the mystery of why it failed to appear where expected in 2025.
The finding not only expands scientists’ understanding of dark comets but also highlights the importance of continually refining models used to track objects that pass through our cosmic neighborhood.
TL;DR
- Object 1998 SH2 was believed to be an asteroid since its discovery in 1998.
- It failed to appear at its predicted location during observations in 2025.
- Scientists later found evidence of weak gas emissions, indicating it is a rare dark comet.
- Unlike typical comets, it has almost no visible tail or coma.
- The finding could influence how astronomers track near-Earth objects in the future.
