Asteroid 1998 SH2 Revealed as a Comet After 28 Years of Observation

Astronomers have identified that a near-Earth object observed for 28 years under the assumption it was an asteroid is, in fact, a comet masquerading as an asteroid.

On August 28, 2025, NASA's radar system was directed toward a region in space where the object known as "asteroid 1998 SH2" was expected to appear. However, the object was completely absent from its predicted location, leaving scientists puzzled.

What initially seemed like a routine observation turned into a scientific mystery, prompting the team to recalculate the object's trajectory with high precision. They discovered that a non-gravitational force was influencing its motion, causing deviations from the expected orbit.

Scientists had been tracking asteroid 1998 SH2 from 1998 until 2016, during which it completed two orbits around the Sun. Although earlier observations did not reveal comet-like features such as a tail or a coma, more precise measurements of its sky position indicated irregular movement, raising doubts about its true nature.

No further observations were attempted until 2025, when NASA's Deep Space Network, consisting of large radio antennas used for tracking space objects, was employed.

During its 4.5-year orbit, the object safely passed Earth at a distance of 3 million kilometers (2 million miles). Using data from previous orbits and accounting for the gravitational effects of the Sun and planets, scientists predicted its location and aimed NASA's antennas accordingly. The object's failure to appear led the team to conclude that an additional force beyond gravity was affecting its trajectory.

David Farnocchia, an engineer at NASA's Jet Propulsion Laboratory and lead author of the study, explained: "After measuring the non-gravitational perturbations affecting 1998 SH2's motion and finding them inconsistent with an asteroid, we suspected the object might be an active comet." Further examination of earlier observations revealed that the object produces a very small thrust by releasing gas into space, a gas emission responsible for its orbital deviation, a phenomenon common in comets.

When a comet approaches the Sun, the star's heat converts ice within it into gas, forming the characteristic tail and coma composed of gas and dust. In the case of 1998 SH2, the amounts of gas and dust emitted were significantly lower than typical comets, rendering its tail and coma invisible in most prior observations.

To confirm its identity, scientists decided to obtain clearer observations during its close approach to Earth in August 2025. They utilized three powerful telescopes: the Canada-France-Hawaii Telescope in Hawaii, the Danish telescope in Chile, and the Very Large Telescope of the European Southern Observatory, one of the largest telescopes worldwide.

Olivier Hainaut, an astronomer at the European Southern Observatory and co-author of the study, stated: "The images collected from these observatories revealed a faint but distinct tail, confirming that 1998 SH2 is indeed a comet."

Consequently, the object will receive an additional temporary cometary designation alongside its previous asteroid label, becoming P/1998 SH2.