Reclassification follows unexpected orbital behavior
Scientists monitoring a near-Earth object cataloged as 1998 SH2 discovered that its motion could not be explained by gravity alone and concluded the object is an active comet rather than a purely rocky asteroid. The finding was prompted when radar observers failed to locate the target at its predicted position on Aug. 28, 2025, roughly two million miles from Earth.
Teams first used NASA's Deep Space Network planetary radar to scan the area of sky where 1998 SH2 was expected, based on its prior orbital solutions covering a 4.5-year period. When the object did not appear at the predicted coordinates, astronomers turned to optical telescopes to pinpoint its actual location. Analysis of the new observations showed systematic, non-gravitational perturbations in the object's trajectory that were incompatible with a typical asteroid.
Evidence points to cometary activity
The key difference that led researchers to reclassify 1998 SH2 is the presence of forces beyond gravity acting on the body — a hallmark of cometary mass loss. Comets, composed of volatile ices and dust, can develop a tenuous atmosphere (coma) and a tail when solar heating causes sublimation. Those outgassing processes can subtly alter an object's orbit over time.
“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,” said Davide Farnocchia, a navigation engineer at NASA JPL’s Center for Near-Earth Object Studies.
Prior observations of 1998 SH2 dated to 2016, meaning the object had completed two additional orbits around the sun since then. The newly measured deviations accumulated over those passages, producing the mismatch between predicted and observed positions that revealed the object's true nature.
Implications for tracking and classification
The case of 1998 SH2 illustrates challenges in distinguishing small solar system bodies and the importance of combining radar and optical tracking. Reclassification has consequences for orbital prediction and for the models used by planetary defense programs to assess future positions and potential close approaches.
- Object designation: 1998 SH2 (previously cataloged as an asteroid)
- Key date: Aug. 28, 2025 — radar observation failed to find the object at predicted location
- Primary evidence: measured nongravitational perturbations incompatible with a rocky asteroid
- Last previous observations: 2016
Data summary
| Item | Detail |
|---|---|
| Last pre-2025 observations | 2016 |
| Instrument | NASA Deep Space Network planetary radar; optical telescopes |
| Distance at 2025 attempt | ~2 million miles from Earth |
| Finding | Active comet with weak coma, not an asteroid |
Researchers noted that some objects on elongated near-Earth orbits can display significant perturbations when they outgas, and that ongoing monitoring is essential to refine orbital predictions. The transition in classification from asteroid to active comet for 1998 SH2 underscores the dynamic nature of small bodies in the inner solar system and reinforces the need for multi-instrument observation campaigns.
