This artist’s concept depicts a near-Earth asteroid
with an elongated orbit. A few objects such as these can exhibit significant
perturbations in their motion around the Sun and, like the asteroid 1998 SH2,
could turn out to be regular comets with a weak tail and coma (the gas and dust
around a comet’s nucleus).
NASA/JPL-Caltech
New research led by scientists at NASA’s Jet Propulsion Laboratory in
Southern California has revealed the identity of a puzzling 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:
Past images hadn’t revealed obvious cometlike activity, suggesting it might be
an asteroid, but its motion recently proved to be irregular like that of a
comet. The scientists detailed their findings in a study published in the journal Nature Astronomy.
The puzzle began on Aug. 28, 2025,
when the object, provisionally known as the asteroid 1998 SH2, passed safely
within 2 million miles (3 million kilometers) of our planet during its 4½-year
orbit around the Sun. Researchers looking to observe 1998 SH2 with NASA’s Deep
Space Network (DSN) planetary radar system had calculated its position using
data from previous orbits and factored in the effects that the gravity of the
Sun and planets would have on its path. But when 1998 SH2 didn’t show up where
they expected, they realized that something unanticipated had been influencing
the object’s motion.
Object tracking
By using optical astrometry to
precisely measure the object’s position in the sky, the researchers were able
to identify the cause.
“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
with NASA’s Center for Near-Earth Object Studies at JPL and study lead.
Although 1998 SH2’s orbit around
the Sun had been well-tracked from 1998 to 2016, the object had completed two
solar orbits without additional observations by telescopes until the 2025 DSN
attempts. Analyzing all observations collected since the object’s discovery in
1998, researchers determined the perturbations to 1998 SH2’s motion and
hypothesized that the object may be generating a small thrust by venting gas
into space, causing it to deviate from its predicted path.
This venting results from the Sun
heating ice mixed with rocky material, turning the ice into a gas. With regular
comets, this activity forms a trademark bright tail and coma — the gas and dust
surrounding a comet’s nucleus. But when an object produces gas and dust in much
smaller quantities, its tail and coma may not be detectable to most
observatories.
Tail, coma emerge
The August 2025 close approach to
Earth of 1998 SH2 provided the perfect opportunity for the paper’s authors to
gather observational evidence of visible cometary activity. They reached out to
astronomers at the Canada-France-Hawaii Telescope, a 3.6-meter (12-foot)
optical/infrared telescope near the summit of Mauna Kea, Hawaii, and the
1.5-meter (5-foot) European Southern Observatory’s Danish Telescope in La
Silla, Chile, to observe. Astronomers at the powerful European Southern
Observatory’s 8.2-meter (27-foot) Very Large Telescope on the Chilean mountain
Cerro Paranal also tracked the object.
“The images we collected from these
observatories showed a weak but clear tail, thus confirming that 1998 SH2 is,
in fact, a comet,” said Olivier Hainaut, an astronomer with the European
Southern Observatory and coauthor of the study. “That’s how science works — you
form 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 an outcome of the investigation,
1998 SH2 will receive an additional comet provisional designation, P/1998 SH2.
Planetary defense implications
The research also sheds light on
another, even more unusual, class of objects called dark comets. Like 1998 SH2, dark comets exhibit significant
irregularities, or perturbations, in their trajectory but lack other visible
evidence of comet activity — there’s no coma, tail, or visible outgassing.
These enigmatic objects fall into two distinct populations: larger ones with
orbits similar to those of Jupiter-family comets (short period comets with highly elliptical, or eccentric, orbits), and smaller ones that
orbit closer to the Sun. Since the 2016 discovery of the first dark comet,
about a dozen more have been identified.
The paper’s authors suggest that
many of the larger dark comets, which have orbits like 1998 SH2’s, could turn
out to be regular comets if astronomers get the right opportunity to observe
them with powerful telescopes capable of imaging incredibly faint objects. And
by analyzing the motion of all near-Earth objects using precision astrometry
data, researchers may reveal more comets that were previously designated as
asteroids if they exhibit cometlike nongravitational perturbations.
“This work shows the importance of
continuously tracking near-Earth objects,” said Farnocchia. “Because of
outgassing, the motion of comets is more significantly perturbed than that of
asteroids. Detecting these perturbations can 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 that influences their
Earth impact risks.”
Hunting for near-Earth objects
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
defense, this next-generation mission will seek out some of the hardest-to-find
near-Earth objects, 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 all are
managed by JPL and supported by the agency’s Planetary Defense Coordination
Office in Washington. Caltech in Pasadena manages JPL for NASA. The DSN
receives programmatic oversight from the SCaN (Space Communications and
Navigation) program office, also at NASA headquarters.
More information about planetary radar, NASA’s Center for Near Earth Object Studies, and near-Earth objects can be found at: https://www.jpl.nasa.gov/asteroid-watch
Source: NASA Study Finds Near-Earth Asteroid Is Actually Comet - NASA
