For nearly three decades, 1998 SH2 looked like a fairly ordinary near-Earth asteroid. It was discovered in 1998, catalogued, and followed through a 4½-year orbit around the Sun. Then, during a close but safe pass by Earth in August 2025, its motion refused to match the path astronomers had calculated.
The explanation, announced by NASA on July 16, is a quiet but satisfying one: 1998 SH2 is a comet. Its activity is so faint that earlier observations did not reveal the familiar coma or tail. Yet gas escaping from its surface has been giving the object a small, persistent push, enough to alter its orbit in a way gravity alone could not explain.
The clue was in the orbit
NASA’s Deep Space Network had been preparing to observe 1998 SH2 with planetary radar after its 2025 approach, when it came within about 2 million miles of Earth. The object did not appear where its predicted orbit said it should. That does not mean it was lost or threatening. It means the prediction was missing a subtle force.
Researchers at NASA’s Center for Near-Earth Object Studies combined older observations with new, precise measurements of its position. The mismatch was consistent with non-gravitational acceleration: a tiny thrust created when sunlight warms ice mixed into the object’s rocky surface and gas escapes into space.
That is normal comet behavior. The surprise was that 1998 SH2 had been hiding it so well. A bright comet advertises itself with a visible coma and tail. This object produced far less material, and the signs were beyond the reach of many earlier observations.
A faint tail settles the question
The team asked astronomers at several major observatories to take a closer look, including the Canada-France-Hawaii Telescope, the Danish 1.5-meter telescope at La Silla and the European Southern Observatory’s Very Large Telescope. Their images showed a weak but clear tail.
That evidence confirmed the interpretation. The object will now carry an additional comet designation, P/1998 SH2. The old provisional asteroid name is still part of its observational history, which is why both names will continue to appear in discussions of the discovery.
It is a useful reminder that the asteroid-comet boundary is not always visually obvious. Both are leftovers from the early solar system, but comets generally contain more volatile ice. If that material is buried beneath a dark, rocky crust or only escapes in small amounts, a comet can look asteroidal until a favorable observing geometry or a carefully measured orbit reveals what it is doing.
Why planetary-defense teams care
The result is not a warning about P/1998 SH2. NASA says the 2025 pass was safe. The value of the discovery lies in method. When a comet outgasses, its trajectory can shift more noticeably than that of a purely rocky asteroid. Tracking those shifts helps astronomers understand how an object’s orbit changes and improves the quality of future calculations.
The work also bears on a smaller, stranger population called dark comets. These objects show unexplained deviations in their motion but no obvious coma or tail. NASA notes that only about a dozen have been identified since the first discovery in 2016. Some may prove to be ordinary, weakly active comets when telescopes catch them at the right time.
That is one reason near-Earth surveys do not end when an object gets a name. Orbits are refined for years, sometimes decades. New instruments, a close approach or a better image can turn an old catalogue entry into a new scientific story.
For 1998 SH2, the decisive evidence was neither a dramatic flyby nor a spectacular tail. It was the small gap between where the object should have been and where it actually was.
Why an object can change categories
A designation is a description of the evidence available at the time, not a permanent verdict. In this case, decades of observations established a reliable orbit, but they did not reveal a bright, active coma. Better astrometry exposed the small push from escaping gas, and deep images supplied the visible tail that settled the question.
That sequence is exactly why catalogues are revised. Scientists keep older designations in the record because they explain how an object was first understood. The additional comet designation tells future observers that its path can be affected by activity that may be faint, intermittent or strongly dependent on where it is in its orbit.
The discovery also illustrates the value of routine follow-up. An object may pass safely by Earth many times without attracting public attention, while its measurements quietly improve the map of the solar system. When one detail stops fitting, researchers have a reason to look again.
