For nearly three decades, astronomers tracking a small, dark rock zip across our stellar neighborhood thought they knew exactly what it was. Discovered in 1998 and designated as 1998 SH2, the object was cataloged as a standard near-Earth asteroid. It lacked the glowing, majestic tail usually associated with icy visitors from the outer solar system, quietly completing an orbit around the Sun every 4.5 years.
But in a discovery that reshapes our understanding of planetary tracking, a team led by scientists at NASA’s Jet Propulsion Laboratory (JPL) unmasked 1998 SH2’s true identity: it is actually an active comet.
This revelation, published in Nature Astronomy, marks a historical first in observation methods and offers a critical new asset for how we intend to defend the Earth from catastrophic space impacts in the future.
The Radar Intercept That Went Missing
The breakthrough occurred in late August 2025 during what should have been a routine celestial flyby. Object 1998 SH2 was scheduled to pass safely within roughly 2 million miles (3 million kilometers) of Earth.
Engineers prepared the massive 70-meter antenna at NASA’s Goldstone Deep Space Network complex in California, locking onto the coordinates where mathematical predictions insisted the asteroid should be.
The radar beam fired. The echo never returned.
The object was completely missing from its expected path. It turned up 153 arcseconds off-course—a massive discrepancy in precision astrometry.
"After we measured the nongravitational perturbations affecting the motion of 1998 SH2 and recognized they weren't compatible with the object being an asteroid, we suspected the object could be an active comet,"
explained Davide Farnocchia, a navigation engineer with NASA's Center for Near-Earth Object Studies (CNEOS) and the study's lead author.
How a Comet Hides in Plain Sight
Asteroids are rocky, metallic bodies whose movements are overwhelmingly governed by the gravitational pull of the Sun and the planets. Comets, conversely, are rich in volatile ices. As they near the heat of the Sun, these ices turn directly into gas—a process called outgassing.
When gas escapes, it behaves exactly like a collection of tiny, chaotic rocket thrusters, introducing nongravitational forces that subtly push the object off its calculated orbit.
| Feature | Asteroids | Comets (Like P/1998 SH2) |
| Composition | Rock, iron, silicate minerals | Ice, dust, frozen gases, organic compounds |
| Orbital Stability | Highly predictable; shaped entirely by gravity | Prone to sudden orbital drifting due to outgassing thruster effects |
| Appearance | Clean, sharp point of light | Faint or bright shroud (coma) and tail |
Because 1998 SH2 had such an exceptionally faint outgassing rate, standard survey telescopes never noticed a glowing cloud or tail. It looked like a dead rock.
To confirm their theory, the team aimed some of the most powerful instruments on the planet at the stray object, including the European Southern Observatory's Very Large Telescope in Chile. The high-sensitivity imaging caught what decades of scanning missed: a weak, unmistakable tail of escaping material. The object has since been officially reclassified with a dual identity as comet P/1998 SH2.
The Planetary Defense Shield
This cosmic unmasking is much more than a bookkeeping correction for astronomers; it highlights a critical vulnerability in planetary defense.
If an asteroid is on a collision course with Earth, scientists can project its path decades in advance with pinpoint precision. However, if that object is secretly a comet, its self-generated thruster effects make it an unpredictable threat. A hidden comet could drift into an Earth-crossing trajectory unexpectedly, drastically reducing the warning time we would have to deploy deflection missions.
Furthermore, the study indicates that 1998 SH2 belongs to an elusive class of "dark comets"—objects that drift like comets but remain completely dark, lacking any visible tails.
By mastering the art of detecting comets strictly by how much they wiggle off-course, NASA is developing an entirely new diagnostic radar shield. Knowing the precise mass, composition, and trajectory changes of these stealth objects ensures that if humanity ever needs to launch a kinetic impactor or interceptor to deflect a space rock, we will know exactly what we are hitting—and where it will actually be.