New research led by scientists at NASA’s Jet Propulsion Laboratory in Southern California has revealed the identity of a mysterious near-Earth object by precisely tracking its motion through space and using powerful observatories that image faint celestial objects.
This object has a dual personality: previous images had not shown obvious comet-like activity, which suggested it might be an asteroid, but more recently its motions have proven to be erratic like a comet. Scientists detailed their findings in a study published in the journal Nature Astronomy.
The puzzle began on August 28, 2025, when the object, tentatively known as Asteroid 1998 SH2, passed safely within 2 million miles (3 million kilometers) of our planet during its four-and-a-half-year orbit around the Sun. Researchers observing 1998 SH2 with NASA’s Deep Space Network (DSN) planetary radar system calculated its position using data from previous orbits and took into account the effects of the Sun and the planets’ gravity on its path. But when 1998 SH2 didn’t appear where they expected, they realized that something unexpected was affecting the object’s motion.
By using optical astrometry to precisely measure the object’s position in the sky, researchers were able to identify the cause.
“When we measured non-gravitational perturbations affecting the motion of 1998 SH2 and recognized that they were not consistent with the object being an asteroid, we suspected that the object might be an active comet,” said study leader David Farnocchia, a navigation engineer at NASA’s Center for Near-Earth Object Studies at JPL.
Although 1998 SH2’s orbit around the Sun was well tracked from 1998 to 2016, the object had completed two solar orbits without additional observations by telescopes until the 2025 DSN efforts. Analyzing all observations collected since the object was discovered in 1998, researchers determined disturbances in the motion of 1998 SH2 and hypothesized that the object may produce a small thrust by expelling gas into space, causing it to deviate from its predicted path.
This venting occurs as a result of the Sun heating the ice mixed with rocky material, causing the ice to turn into gas. As with regular comets, this activity creates a trademark bright tail and coma – the gas and dust surrounding the comet’s nucleus. But when an object produces very small amounts of gas and dust, its tail and coma cannot be detected by most observatories.
1998 SH2’s close approach to Earth in August 2025 provided the paper’s authors with the perfect opportunity to gather observational evidence of visible cometary activity. They reached astronomers at the Canada-France-Hawaii Telescope, a 3.6-metre (12-foot) optical/infrared telescope near the summit of Mauna Kea, Hawaii, and the 1.5-metre (5-foot) Danish Telescope at the European Southern Observatory in La Silla, Chile. Astronomers with the powerful European Southern Observatory’s 8.2-meter (27-foot) Very Large Telescope also tracked the object on the Chilean mountain Cerro Paranal.
“The images we collected from these observatories showed a weak but clear tail, confirming that 1998 SH2 is indeed a comet,” said Oliver Hainaut, astronomer at the European Southern Observatory and co-author of the study. “That’s how science works – you make a hypothesis, and you set out to test it. This data is exactly what was needed to confirm our hypothesis that 1998 SH2 was a comet.”
As a result of the investigation, 1998 SH2 will receive an additional comet provisional designation, P/1998 SH2.
The research also sheds light on another, even more unusual, class of objects called dark comets. Like 1998 SH2, dark comets display significant irregularities, or disturbances, in their trajectories, but lack other visible evidence of comet activity – no coma, tail, or visible exhaust. These mysterious objects fall into two distinct populations: larger objects with orbits similar to Jupiter-family comets (short-period comets with highly elliptical, or eccentric orbits), and smaller objects that orbit closer to the Sun. Since the discovery of the first black comet in 2016, about a dozen more comets have been identified.
The paper’s authors suggest that many large dark comets, with orbits similar to those of 1998 SH2, could become regular comets if astronomers get the right opportunity to observe them with powerful telescopes capable of visualizing incredibly faint objects. And by analyzing the motions of all near-Earth objects using precise astrometry data, researchers may uncover more comets that were previously designated as asteroids if they exhibit comet-like non-gravitational perturbations.
“This work shows the importance of continuing to monitor near-Earth objects,” Farnocchia said. “Due to outgassing, the motion of comets is more perturbed than that of asteroids. Detecting these perturbations could be an important diagnostic tool for planetary defense that will help understand which objects may be comets rather than asteroids, how their orbits evolve, and how this affects their Earth impact risks.”
NASA’s upcoming Near-Earth Object (NEO) Surveyor will collect data that can be used to support this effort. The first space survey telescope to be built for planetary protection, this next-generation mission will look for some of the hardest objects to find near Earth, such as dark asteroids and comets that don’t reflect much visible light.
NASA’s Center for Near Earth Object Studies, the Goldstone Solar System Radar Group and NEO Surveyor are all managed by JPL and supported by the agency’s Planetary Defense Coordination Office in Washington. Caltech in Pasadena manages JPL for NASA. DSN receives programmatic oversight from the SCAN (Space Communications and Navigation) Program Office, also at NASA Headquarters.
More information about planetary radar, NASA’s Near Earth Object Study Center, and near-Earth objects can be found at:
https://www.jpl.nasa.gov/asteroid-watch
news media contact
Ian J. o’neill
Jet Propulsion Laboratory, Pasadena, California.
818-354-2649
ian.j.oneill@jpl.nasa.gov
Karen Fox / Molly Wasser
NASA Headquarters, Washington
202-358-1600
karen.c.fox@nasa.gov / molly.l.wasser@nasa.gov
2026-046
