Dark Comet 1998 SH2: The Asteroid That Was a Comet

What happens when an asteroid we’ve tracked for twenty-seven years simply refuses to show up where the math says it should be?

Until August 2025 the catalogs were unanimous: object (875163) 1998 SH2 was an asteroid found in September 1998 and tracked for twenty-seven years without ever being seen to do anything cometary. It is now also comet P/1998 SH2, a dark comet caught by its motion rather than its glow.

The dark comet problem

A dark comet is an object that behaves like a comet without looking like one. Astronomers normally identify comets by their comas and tails, the glowing envelopes of gas and dust that appear when solar heat vaporizes their ices. That escaping gas also acts like a tiny thruster, gently nudging the comet off a purely gravitational path. Dark comets show the nudge but not the glow.

In recent years, fourteen near-Earth objects have been flagged as morphologically inactive yet subject to non-gravitational accelerations too strange to dismiss. Researchers split them into two families. Inner dark comets are small, tens of meters or less, on orbits close to Earth’s. Outer dark comets are hundreds of meters across on orbits resembling those of Jupiter-family comets. For the outer group, the measured accelerations are far too large for radiation forces to explain.

Object (875163) 1998 SH2 sat quietly in the asteroid catalogs for all of that time.

Its credentials were suspicious, though. The Tisserand parameter with respect to Jupiter is 2.9, squarely in Jupiter-family comet territory. Infrared data from the NEOWISE mission gave a diameter of 380 ± 57 meters and a very dark albedo of 0.058, compatible with comet nuclei. But the images showed no coma and no tail, and nobody had measured an anomaly, so it stayed an asteroid on paper. We’ve seen before how a body’s rotation and surface can hide dramatic physics, and our study of the comet that reversed its own spin shows how outgassing torques quietly reshape small worlds.

What did a missed radar echo reveal?

A 70-meter antenna pointed at empty sky is very hard to ignore.

On 30 August 2025, 1998 SH2 passed within 0.02 astronomical units of Earth, about eight times the distance to the Moon. NASA’s Goldstone DSS-14 antenna was scheduled to ping it with radar on 26 August. The pointing was based on an orbit fitted to 148 optical measurements spanning 1998 to 2016, with a formal uncertainty of just ±24 arcseconds. Observations ran for 40 minutes. Nothing came back. The system itself was healthy, since other asteroids observed the same day returned clean echoes.

Five days later, on 31 August, the Wykrota Observatory of the Centro de Estudos Astronômicos de Minas Gerais in Serra da Piedade, Brazil, recovered the object. It sat 153 arcseconds away from the gravity-only prediction, a 19-sigma discrepancy. That single measurement explained the radar failure and demanded a missing force.

Adding a transverse non-gravitational acceleration to the orbit model fixed everything at once. Farnocchia, Hainaut, Seligman and colleagues report a fitted value of A₂ = (−1.4 ± 0.1) × 10⁻¹¹ m/s², which reconciled the entire 1998–2025 data arc, and a second Goldstone attempt on 2 September found the echo within two minutes.

Set that against the Yarkovsky effect (the recoil from uneven thermal emission), whose strongest plausible acceleration on a body this size is about 1.3 × 10⁻¹² m/s². The measured value is ten times larger. Sunlight can’t do this. Escaping gas can, at a water production rate of roughly 1.2 × 10²⁴ molecules per second.

The tail that only big mirrors could find

They saw a comet, faint but unmistakable, exactly where the dynamics said one had to be.

Routine survey images told a different story at first. The ATLAS survey (the same system that discovered interstellar comet 3I/ATLAS) collected 53 images in September 2025, and in every single one 1998 SH2 looked like an ordinary point of light. Activity was real, but hiding below survey sensitivity. Only large apertures and careful stacking could dig it out.

Table 1 — What each telescope saw of 1998 SH2 in September 2025 (Farnocchia et al., Nature Astronomy, 2026)

The coma extends at least 10 arcseconds from the nucleus, and its contribution to the total brightness grew from 0.06 to about 0.24 magnitudes between 13 and 30 September, hinting that activity was increasing. Dust-dynamics modeling dates the release of the tail material to a continuous window between late August and 7 September 2025. The morphology firmly rules out a one-off event like a small impact, and rules out rotational shedding too. Sublimation, slow and steady.

Grain sizes are the odd part. Most particles run around 400 micrometers, narrowly confined to the 300–600 μm range, more like fine sand than the micron-scale dust typical of comets.

Laboratory work suggests grains this size get launched when ice sublimates beneath the surface and pressure builds until it vents. The timing supports that picture. 1998 SH2 passed perihelion on 21 July 2025, yet its activity peaked weeks later, a thermal lag that fits heat slowly soaking down to buried ice, much as observed on comet 67P/Churyumov–Gerasimenko.

The Minor Planet Center has now granted the object dual status. It is both asteroid (875163) and comet P/1998 SH2. The 2 September radar track adds one more superlative: at under 600 meters, and most likely 380, it’s the smallest comet ever observed by radar, with a circular polarization ratio of 0.09 and a radar albedo near 0.04, both closer to comet nuclei than to typical rocky near-Earth asteroids.

Planetary defense has a classification problem

You can’t deflect what you’ve misclassified.

As of late 2025, there are 2,009 known near-Earth asteroids with Jupiter Tisserand parameters between 2 and 3, the comet-like range. Of these, 285 are formally “potentially hazardous asteroids”, meaning bodies at least 140 meters wide whose orbits come within 0.05 au of Earth’s. If a meaningful fraction of them are dormant or weakly active comets, three things change.

• Their trajectories get less predictable, since outgassing forces exceed radiation forces and can shift long-term impact probabilities.
• Deflection design changes: a kinetic impactor like NASA’s DART mission behaves differently against a porous, volatile-rich comet nucleus than against a coherent rocky asteroid, and impact-effect models diverge too.
• The impactor census tilts toward comets.

To be clear, 1998 SH2 itself poses zero impact risk for the foreseeable future. The concern is the objects we haven’t tested yet.

There’s a deeper, older question hiding here as well. If volatile-rich bodies masquerading as asteroids are common near Earth today, a similar population probably existed when our planet was forming, and such objects may have helped deliver Earth’s water. The result even feeds back into the most famous orbital mystery of the modern era: 1I/’Oumuamua accelerated anomalously with no visible coma, and 1998 SH2 now proves that decades of invisible outgassing are entirely possible. We examined how careful physics tames sensational claims about interstellar visitors in our assessment of whether 3I/ATLAS is really an alien visitor, and how a passing spacecraft can catch these travelers in the act in the JUICE observations of comet 3I/ATLAS.

• The August 2025 miss was 19 standard deviations wide, about 153 arcseconds, and it killed a scheduled radar observation.
• The extra push on its orbit is roughly ten times too strong to come from sunlight. Only cometary outgassing fits.
• Deep images from telescopes in Chile and Hawai‘i found a faint coma and a narrow tail over 20 arcseconds long. The object now carries a dual designation: asteroid (875163) and comet P/1998 SH2.
• It is the first object ever confirmed as a comet from its anomalous motion alone, and at roughly 380 meters across, the smallest comet ever detected by radar.
• Hundreds of hazardous asteroids may be comets.

For twenty-seven years, 1998 SH2 kept a secret in plain sight, and prying it loose took a missed radar echo, a Brazilian observatory’s sharp recovery, and some of the largest telescopes on Earth. We find that humbling. The sky we’ve cataloged is not always the sky that exists, and the difference between “asteroid” and “comet” turned out to be a question of how hard we were willing to look.

FreeAstroScience.com brought you this one, and we’d rather you left it suspicious of catalog entries than impressed by telescopes. Keep asking what an object has been measured doing. Never let your Mind sleep.

— Gerd Dani, President of FreeAstroScience

Primary source: Farnocchia, D., Hainaut, O. R., Seligman, D. Z., et al. (2026). “Non-gravitational acceleration indicative of cometary activity of near-Earth object.” Nature Astronomy, Springer Nature. Published 10 July 2026. https://doi.org/10.1038/s41550-026-02913-7