For nearly thirty years, 1998 SH2 was considered an asteroid with an orbit close to that of Earth. Scientists have just discovered an unusual behavior and trajectory changes.
The discovery began on August 28, 2025, when 1998 SH2 passed about 3 million kilometers from our planet. Researchers wanted to observe it with the planetary radar of NASA's Deep Space Network. But the object did not appear at the location predicted by calculations.
a and b: Views from the ATLAS telescope, taken on September 3 and 4, 2025. The slight elongation of the object is due to its motion during a 30 s ATLAS exposure.
c: Median stack of the eight exposures from September 3 and 4 at a scale of 2″ per pixel. Neither coma nor tail are visible here.
This difference could not be explained solely by the gravitational attraction of the Sun and planets. By analyzing the measured positions since its discovery, astronomers identified so-called non-gravitational perturbations. They indicated that an additional force was gradually modifying its trajectory.
The most likely explanation was then outgassing. When solar heat reaches ice present beneath the surface, it turns into gas. As it escapes, this gas acts like a tiny thruster: it exerts a weak push, but sufficient to shift the object's predicted position.
The researchers then used several telescopes to verify this hypothesis. Observations made with the Canada-France-Hawaii Telescope, the Danish telescope at the European Southern Observatory, and the Very Large Telescope revealed a faint tail around 1998 SH2.
This structure, difficult to detect, indicates that the object exhibits cometary activity. It therefore receives a new designation: P/1998 SH2. Its initial appearance had classified it among asteroids because no obvious coma or tail had been observed before.
This case shows that the boundary between asteroids and comets is sometimes less clear than it seems. Some objects may contain ice while producing too little gas and dust to be directly identified by telescopes.
The discovery also has implications for planetary defense. Outgassing can alter the trajectory of a comet, which is not the case for an asteroid. It is therefore essential to identify these perturbations to improve the tracking of near-Earth objects and the assessment of their future trajectory.