Non-gravitational acceleration indicative of cometary activity of near-Earth object

Asteroids and comets are classically differentiated based on their orbits, composition, and observed cometary tails and activity levels¹. However, in recent years, a continuum of hybrid objects have become more apparent², eroding the traditional boundaries between these two classes. For example, some asteroids have been found to display comet-like activity and are referred to as active asteroids^3,4. Conversely, there are distinct populations of objects such as ‘Manx comets’⁵ and Damocloids^6,7,8 that are on comet-like orbits but do not display cometary activity—at least during extant serendipitous surveys and targeted observations. This continuum of activity spanning small-body populations is not unique to our Solar System. Unlike 2I/Borisov⁹ and 3I/ATLAS¹⁰, the first discovered interstellar object, 1I/‘Oumuamua, showed no evidence of a cometary dust tail in deep imaging^11,12,13. However, 1I/‘Oumuamua was clearly affected by non-gravitational accelerations, suggesting the possibility of undetected cometary outgassing¹⁴.

In recent years, 14 near-Earth objects have been identified as inactive yet subject to non-gravitational perturbations¹⁵, seemingly inconsistent with radiation forces that perturb the motion of asteroids^16,17,18. The appropriate classification of these enigmatic objects as asteroids or comets remains unclear; they are informally referred to as ‘dark comets’. These objects have been divided into two distinct populations: inner dark comets, which are tens of metres in size or smaller and have orbits close to that of the Earth, and outer dark comets, which are hundreds of metres in size or larger and have orbits similar to those of Jupiter family comets. For the inner dark comets, non-gravitational perturbations exhibit a term in the direction perpendicular to the orbit plane, whereas radiation forces are mostly in the orbital plane^19,20. Recent refinements to radiation models that take into account asymmetries in the shape of the asteroid may explain the direction of the observed non-gravitational forces on inner dark comets²¹. For the outer dark comets, the magnitude of the detected non-gravitational perturbations is simply too large to be caused by radiation effects and may instead result from outgassing, with cometary activity below the sensitivity threshold of telescopic observations. For one of these objects, (139359) 2001 ME₁, this interpretation was confirmed by the recent detection of activity in images from the Solar and Heliospheric Observatory taken in 2018, when the object was near perihelion²².

Near-Earth object (875163) 1998 SH₂ is currently classified as an asteroid as no cometary activity has been detected. However, its orbit is in line with that of other outer dark comets¹⁸ (Fig. 1) and its Tisserand parameter with respect to Jupiter is 2.9, which is typical of Jupiter family comets²³. Moreover, based on infrared measurements from the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE)²⁴, 1998 SH₂ has a diameter of 380 ± 57 m and a low albedo of 0.058 ± 0.024, which is compatible with what is observed for comets²⁵ and also C-, D- and P-type asteroids²⁶.

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a, The distribution of eccentricity and semimajor axis for dark comets (empty circles), 1998 SH₂ (filled circle) and Jupiter family comets (crosses). Outer and inner dark comets are clearly delineated by orbits and brightness. The size of the circles scales with the absolute magnitude H. The solid curves delimit the area corresponding to the definition of a Jupiter family comet where the Tisserand parameter with respect to Jupiter, T_j, is between 2 and 3. b, Orbits of the outer dark comets (red), inner dark comets (light grey) and 1998 SH₂ (black) compared with those of the Earth (blue) and Jupiter (cardinal). The object 1998 SH₂ is clearly in family with the outer dark comets on a Jupiter family comet-like orbit.

1998 SH₂ made a close approach to Earth at a distance of 0.02 au on 30 August 2025. Monostatic radar observations²⁷ were scheduled for Goldstone’s DSS-14 70-metre antenna for 26 August and 2 September 2025. The pointing for the 26 August track had a formal 3σ uncertainty of ±24″, based on an orbit estimated from 148 optical astrometric measurements from 17 September 1998 to 27 October 2016. 1998 SH₂ completed two full orbits around the Sun in the timespan between the last measurement and the radar track. Unexpectedly, 1998 SH₂ was not detected during the 26 August track within DSS-14’s 70″ primary beam width, indicating a systematic effect that had not previously been accounted for.

On 31 August 2025, the Southern Observatory for Near Earth Asteroids Research Wykrota-Centro de Estudos Astronômicos de Minas Gerais observatory in Serra da Piedade, Brazil, recovered 1998 SH₂, providing the first tracking data since 2016. 1998 SH₂ was found 153″ from the gravity-only prediction, corresponding to a 19σ offset, which explains the failed radar detection. Adding a transverse non-gravitational acceleration A₂(1 au/r_H)², where r_H is the heliocentric distance²⁸, led to an estimate A₂ = (−1.4 ± 0.1) × 10⁻¹¹ m s⁻², which corrected the offset and enabled a fit to the entire data arc (more details in ‘Orbital fit’ in Methods). The detected non-gravitational perturbations were then confirmed by more than 200 additional optical observations and the delay measurement from the successful radar track on 2 September 2025 (see Extended Data Table 1 and Extended Data Fig. 1 for details). The optical observations included 40 high-precision measurements with 0.1″ astrometric uncertainties that we collected with the 1.54-metre Danish Telescope (Dk154) in La Silla, Chile.

The transverse acceleration A₂(1 au/r_H)² is generally used to model the so-called Yarkovsky effect²⁰, which is due to radiative recoil of anisotropic thermal emission. As such, transverse non-gravitational acceleration via Yarkovsky measurements has been estimated on ~500 near-Earth asteroids²⁸. A₂ captures the dependence of the non-gravitational acceleration on the physical properties of the asteroid, for example, it is inversely proportional to the asteroid’s diameter. The largest possible A₂ compatible with the Yarkovsky effect for an object of the size of 1998 SH₂ is ~1.3 × 10⁻¹² m s⁻², which is 10 times smaller than the detected acceleration. We therefore conclude that this acceleration must be caused by another mechanism such as cometary outgassing. A nuclear radius of 170 m and this acceleration yield an estimate¹⁶ of the production rate (if driven by ougassing of H₂O) of Q(H₂O) ≈ 1.2 × 10²⁴ molecules per second.

To investigate the possibility that 1998 SH₂ could be a comet, we looked for observational evidence of cometary activity. The Asteroid Terrestrial-impact Last Alert System (ATLAS)²⁹ survey obtained 53 images of 1998 SH₂ from 3 September 2025 to early October. However, no individual exposure revealed cometary activity. Figure 2 shows two individual images and a stack of 8 × 30 s observations collected by the ATLAS 0.5 m telescope on Haleakala through the ATLAS o-band filter from 3 and 4 September, with a seeing of 5″ and when the brightness of 1998 SH₂ was V ≈ 15. 1998 SH₂ looks stellar like an asteroid would.

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a,b, Individual cutout o-band exposures from the ATLAS-Chile telescope taken on 3 (a) and 4 (b) September 2025. The cutouts are 9.3′ on a side (1.86″ per pixel), with 5′ indicated. The asteroid is marked using crosshairs. The slight elongation of the asteroid is due to its motion over a 30-s ATLAS exposure. c, A median stack of the eight exposures from 3 and 4 September at a pixel scale of 2″ per pixel. No coma nor tail are apparent.

On 17 September 2025, we obtained 3 × 120 s gri exposures of 1998 SH₂ using the 3.6 m Canada-France-Hawai‘i Telescope (CFHT) on Maunakea, Hawai‘i, tracking non-sidereally. Although the object’s full-width at half-maximum (FWHM; 0.65″) was comparable to that of field stars (0.67″), the object displayed a low-surface-brightness tail extending west over approximately 20″. The tail was also visible in a stack of 11 × 180 s images also obtained with CFHT on 18 September (with variable seeing from 1.01″ to 1.54″), but was not detected on 23 September (6 × 180 s images, ~1.0″ seeing) or 30 September 30 (6 × 180 s images, ~0.6″ seeing), when 1998 SH₂ was noticeably fainter as expected from the larger heliocentric and geocentric distances and illustrated by the increasing V magnitude derived from the ephemeris (see Table 1, which assumes a constant absolute magnitude). The 17 September 2025 images are shown in Fig. 3b.

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The linear greyscale covers the range 0 to 5σ of the sky noise. The arrows indicate the orientation and the projected anti-solar (−Sun) and negative heliocentric velocity (−V). The bars give the scale in arcseconds and kilometres. a, 13 September 2025, 3,600 s, R filter, Dk154. b, 17 September 2025, 360 s, gri filter, CFHT. c, 30 September 2025, 5,940 s, clear filter, VLT. Refer to Table 1 for the details. Dec, declination; RA, right ascension; ADU, analog-to-digital units.

Encouraged by the CFHT detection, we obtained a deep sequence of 100 × 60 s images through a clear filter on 30 September 2025, using the 8 m European Southern Observatory (ESO) Very Large Telescope (VLT) Unit 1 (UT1) on Paranal, Chile (seeing 0.60″). The object appeared slightly broader than field stars (0.71″) and exhibited a long (>20″), narrow tail extending to the southwest (Fig. 3c). We subsequently reviewed images that we had previously taken with the Dk154 at the ESO La Silla Observatory, Chile, on 13 and 14 September 2025. A stacked series of 48 × 75 s R-filter images (1.64″ seeing) also shows the tail (Fig. 3a). The telescopes and instruments used for these observations, and the data processing steps are described in ‘Optical observations’ in Methods.

Inspection of the CFHT, VLT and Dk154 images of 1998 SH₂ reveals a small but clearly resolved faint coma extending at least 10″ from the object. We reported the detection of cometary activity to the Minor Planet Center. As a result, 1998 SH₂ is now a dual-status object and has received an additional comet provisional designation, P/1998 SH₂, as well as a comet number. The coma’s contribution to the total brightness is modest, increasing from 0.06 ± 0.01 to 0.24 ± 0.01 magnitudes over the 3 epochs (13–30 September 2025; see ‘Cometary activity’ in Methods for details on these measurements). While part of this increase could be due to a rotational variability of the nucleus, this suggests a possible increase in activity during this period. Moreover, the images show a faint, narrow tail extending at least 20″ from the nucleus. Using a dust dynamics model, we converted the tail’s position angle to a dust release date (illustrated in Extended Data Fig. 2). Data from all three epochs consistently point to the tail dust being released between 1 and 7 September 2025. The images are not sensitive to dust released before or after these dates and therefore, this analysis cannot determine when the emission process started or ended.

A more detailed analysis of the Dk154 image from 13 September, which offered the best spatial resolution due to the object’s closest approach to Earth, indicates that the tail is composed mostly of large grains with radius a ≈ 400 μm released from the nucleus between 29 August and 7 September 2025 (see ‘Analysis of the tail’ in Methods and Extended Data Fig. 3). The gas production rate we obtained is fully capable of lifting even larger grains provided that the nucleus is not considerably denser than ~1,300 kg m⁻³. Laboratory simulations suggest that large particles are ejected when ice sublimation occurs below the surface³⁰.

Collectively, the images demonstrate that dust release took place continuously over the late August to late September 2025 period, and do not imply that the activity was confined to that range. Rather, this suggests a sublimation-caused cometary activity process and firmly precludes instantaneous processes (such as a small impact) or repetitive processes (such as rotation-induced shedding), which would produce distinct morphological signatures. Furthermore, the object’s surface brightness, shown in Extended Data Fig. 4, decreases with distance r from the nucleus as r⁻ⁿ, with a steep exponent n decreasing from 4.3 to 3.6 over the course of the observations. For an object releasing dust at a constant rate, the expected value is n = 1, increasing to n = 1.5 if solar radiation pressure materially affects dust evolution^31,32. A steeper slope (n > 1.5) can be caused by the destruction of grains over time or by an increase in the activity level.

The object passed perihelion on 21 July 2025, weeks before the observations. Therefore, the increase of activity between the beginning and the end of September took place several weeks after perihelion. Comet outgassing is often asymmetric around perihelion. A complex interplay between the nucleus shape, rotation state and thermal processing governs the onset and evolution of cometary activity. In general, outgassing is highly variable and does not follow a universal pattern ³³. This asymmetry can also be driven by low thermal conductivity of the body, causing a lag between the incoming radiation and the heat reaching subsurface volatile ices. For instance, in the case of comet 67P/Churyumov–Gerasimenko, a lag of several months has been estimated³⁴ at a depth of 0.5 m. The post-perihelion delay of 1998 SH₂’s activity therefore support the hypothesis that the sublimation took place below the surface.

1998 SH₂ is the first object for which cometary activity was initially predicted based on non-gravitational perturbations to its motion and then confirmed by targeted observational data. The astrometric observation arc from 1998 to 2025 enables the detection of non-gravitational perturbations to the object’s motion, and favourable observing conditions following the 30 August 2025 close approach and access to large-aperture telescopes enabled the detection of cometary activity. This result suggests that also the other outer dark comets, which have similar size and orbital properties to that of 1998 SH₂, could be confirmed as active. While these objects have sufficient arc lengths to detect non-gravitational perturbations, the lack of sufficiently sensitive observations during favourable observing opportunities has prevented the observational confirmation of their cometary activity.

More broadly, as of late 2025 there are 2009 known near-Earth asteroids with a Jupiter Tisserand parameter between 2 and 3, the range corresponding to Jupiter family comets. Theoretical predictions suggest that a fraction of near-Earth objects with a Tisserand parameter in that range may be nearly extinct comets³⁵. If a substantial fraction of these are truly comets and contain volatiles, there could be important ramifications for the delivery of water to the Earth by an analagous population of volatile-bearing near-Earth objects present when the Earth was forming^36,37,38,39. Targeted searches for activity as in this paper or as done for (3552) Don Quixote⁴⁰ or main-belt comets⁴¹, and the advent of Vera Rubin Observatory’s operations⁴² may reveal that more objects with these orbital properties are outgassing. In particular, 285 of these objects, including 1998 SH₂, are classified as potentially hazardous asteroids, meaning that their orbits come within 0.05 au of the Earth’s orbit, and their sizes are estimated to be at least 140 m. These are the most promising candidates for future detections of activity because their proximity to Earth allows for higher-resolution imaging and better constraints on non-gravitational perturbations through orbit fitting.

Small-body surveys observe the sky to find near-Earth objects and enable mitigation efforts in case an object were found to be on a collision course. While the probability of an Earth impact for 1998 SH₂ over the foreseeable future remains zero even after the detection of non-gravitational perturbations, this may not be the case for other potentially hazardous asteroids that turn out to be comets. Because non-gravitational perturbations due to outgassing are larger than radiation-based ones, the future trajectory of these objects may be more uncertain than currently assumed. Fully capturing the resulting uncertainties may lead to a different assessment of any possibility of future impact.

Our discovery of 1998 SH₂ displaying cometary activity has broader implications for planetary defence. The possibility that a number of potentially hazardous objects currently classified as asteroids could turn out to be comets could increase the relative Earth impact risk from comets with respect to asteroids. The different physical properties of comets, for example, composition, directly affect the design of any deflection mission, such as the recent Double Asteroid Redirection Test⁴³. Moreover, models of the actual impact effects⁴⁴ if an object were to hit Earth can lead to different assessments based on the nature of the object. Therefore, planetary defence researchers should consider comet properties more consistently when evaluating the risk of and planning for impact threats.

Finally, our findings are relevant to the theoretical interpretations for the presence of non-gravitational acceleration affecting the motion of interstellar object 1I/‘Oumuamua despite the lack of detected cometary activity^11,12,13. This puzzle led to a variety of hypotheses, including theories that invoke (1) radiation pressure^45,46,47 or (2) comet-like outgassing^48,49,50,51 for the driver of the non-gravitational acceleration.

Orbital fit

The orbital solution was estimated through a least-squares fit⁵² to the available optical and radar astrometry. We corrected optical astrometry to remove star catalogue biases⁵³ and weighted data according to a statistical analysis of past performance for the different observatories⁵⁴. Our own Dk154 and radar observations were weighted based on the individual measurement uncertainty. The parameters estimated from the fit are the cometary orbital elements and the A₂ parameter used to model non-gravitational perturbations as a transverse acceleration A₂g(r_H) where g provides the functional dependence on the heliocentric distance r_H (ref. ⁵⁵). In this paper, we set g(r_H) = (1 au/r_H)², which is the typical choice to model the Yarkovsky effect²⁸.

Optical observations

In early September 2025, 1998 SH₂ was imaged during routine observations by the ATLAS survey²⁹. Typical ATLAS survey exposures are sidereally tracked and 30-s long, using either the ATLAS o (~r + i) or c (~ g + r) filter, with 4 exposures taken over a 30-min interval at the same footprint on the sky. The ATLAS projected pixel scale is 1.86″ on sky.

The Dk154 observations were obtained with the Danish Faint Object Spectrograph and Camera (DFOSC)⁵⁶, using either the Cousins (Bessell) R filter or no filter. The detector is an E2V231-42, with a pixel scale of 0.396″ on sky. The telescope was tracked at a half of the apparent rate of the observed target, resulting in same trailing of field stars and the target.

The CFHT observations were obtained with the MegaPrime instrument⁵⁷, using the ‘gri.MP9605’ filter. While MegaPrime is a mosaic of 40 charge-coupled devices (CCDs), the detector used was ‘ccd23’, a Marconi/EEV CCD with a 0.187″ pixel scale.

The VLT observations were obtained with the Focal Reducer/low dispersion Spectrograph 2 (FORS2)⁵⁸, using the clear filter, optimized for throughput. FORS2 is equipped with a 1 × 2 mosaic of CCDs. We used ‘chip 1’, an MIT/LL detector, with a pixel 0.126″ pixel read binned 2 × 2 resulting in a 0.252″ on sky. The observation circumstances are listed in Table 1.

Image processing

The data were processed using standard electronic bias subtraction and flat-fielding with averaged twilight or dome exposures. Images from the Dk154 and VLT were aligned with subpixel accuracy using the centroids of tens of background stars. Stars were identified using Source Extractor⁵⁹ in the SEP implementation⁶⁰ and cross-matched with Astroalign⁶¹, resulting in the accurate offset between frames. The star-aligned frames were then stacked using a sigma-clipping median rejection, creating a deep star background reference image. The object was rejected by the median due to its motion and is thus absent from this reference. In the case of CFHT, as only three frames were available, a master background stack could not be produced. The stars and background objects were therefore manually masked in the three frames.

The ATLAS exposures were reduced using its standard image reduction pipeline²⁹. Transient sources were detected by subtracting the ATLAS o-band template image, and the asteroid was detected by linking catalogue detections using the ATLAS Moving Object Processing System. Eight of the subtracted images from 3 and 4 September 2025 were registered to a pixel scale of 2″ per pixel and further stacked using a ‘weighted median’ (50% quantile of cumulative weights) to search for a signal of cometary activity.

The ephemeris of 1998 SH₂ was retrieved from Jet Propulsion Laboratory (JPL)’s Horizons⁶² for each frame’s epoch. Using the previous inter-frame offsets and the reference image’s astrometric calibration, the necessary offsets to align the images on 1998 SH₂ were computed. The star-aligned reference image was also subtracted from the individual frames. This removed the extended signal (for example, stellar point spread function (PSF) wings and galaxies), leaving only small residuals near the star cores due to minor seeing variations, subpixel misalignment and VLT diffraction pattern rotation. These residual frames were then shifted and stacked. The sigma-clipping median rejected background star residuals, cosmic rays and blemishes, resulting in deep stacks showing the object on a clean, empty background (Fig. 3).

Cometary activity

The surface-brightness profile of the object was computed using the final stacks from Fig. 3, and compared with the profile of the PSF, evaluated by averaging several well-exposed field stars in the corresponding master background stack.

The flux was integrated in a series of circular annuli centred on either the object or the star template. The error was taken as the standard deviation of the individual pixels within each annulus. For the PSF profile of trailed images, we only used pixels in angular regions perpendicular to the star’s trailing direction. For the object’s profile, we rejected pixels within 5° of the tail’s position angle to avoid contamination. The PSF profile was normalized so that its peak flux matched that of the object. The resulting profiles are shown in Extended Data Fig. 4.

1998 SH₂ shows a clear flux excess over the PSF profile, extending beyond 10″. Quantitatively, this excess represented 0.06 ± 0.01, 0.210 ± 0.008 and 0.241 ± 0.008 mag for the Dk154, CFHT and VLT observations, respectively. The error bars seem small in absolute terms, but one must keep in mind that this is a relative measurement to the stellar profile. The total duration of each observation is unlikely to average out the possible rotational variability of the object, which could be in the 0.2–0.4 range⁶³ for such a small object. An intrinsic variation of the nucleus of 0.2 mag would cause a change of the (unchanged) coma contribution of ±0.04 mag. The overall increase from 0.06 to 0.2 is therefore likely significant, but the variation from 0.21 to 0.24 could be caused by the nucleus rotation.

At radii r beyond the seeing disk, the excess flux follows a linear trend in the log–log plot, corresponding to a surface-brightness profile that evolves as r⁻ⁿ. A linear regression performed over 2 ≤ r ≤ 4 half-width at half-maximum yielded exponents n = 4.3, 3.8 and 3.6 for Dk154, CFHT and VLT, respectively. These values are considerably steeper than the n = 1 expected for a steady-state, isotropic coma³¹ or the n = 1.5 maximum expected when including the effect of solar radiation pressure³². Such steep slopes can be caused by the sublimation of icy grains over time, or by an increase in activity. Given that the fraction of extended flux relative to the total flux was possibly increasing with time, we favour the interpretation of increasing activity, with the rate of increase becoming slower with time.

In summary, the object was active on 13, 17 and 30 September 2025, and its activity level was possibly increasing throughout this period.

Analysis of the tail

The 1998 SH₂ tail morphology was analysed using the Finson–Probstein method⁶⁴, which models the motion of dust grains under the effects of solar gravity and solar radiation pressure. This analysis generates families of synchrones, connecting particles emitted at the same time, and syndynes, connecting particles with the same β, the ratio of the radiation pressure to the solar gravity. On the images, the synchrones appear as radial lines whose position angle (PA) is related to time of dust emission. To visualize and analyse these, the images were transformed to polar coordinates, shown in Extended Data Fig. 2, in which the nucleus appears as the broad bright band at low radii, and the tail as an horizontal feature.

The Dk154 image indicates that the dust in the tail was released between 30 August and 7 September 2025. The shallower CFHT data suggest a release window between 28 August and 7 September. Owing to Earth’s position nearly in 1998 SH₂’s orbital plane during the VLT observations, the synchrones nearly collapse into a single line: at PA ≈ 65° for dust emitted after 9 September, and at PA ≈ 244° for pre-7 September emissions (matching the PA of the projected negative velocity vector). The entire observed tail lies along this position angle. In summary, the tail in each image corresponds to the same continuous activity event that took place between 30 August and 7 September 2025.

Although the Dk154 image had worse seeing, the closer proximity of 1998 SH₂ to Earth provided the highest-spatial-resolution look at the object. Critically, the tail reveals additional information via the Finson–Probstein syndynes. The β value is related to the grain radius a (m) and density ρ (kg m⁻³) via:

where Q ≈ 1 is the radiation pressure efficiency, which depends on the grain material⁶⁴. While ρ can vary widely—from 1,000 kg m⁻³ (a traditional cometary value) to 1,900 ± 1,100 kg m⁻³ from in situ measurements⁶⁵ on comet 67P, and up to 3,000 kg m⁻³ for S-type asteroids⁶⁶—we adopt ρ = 2,000 kg m⁻³ for this analysis. Extended Data Fig. 3 shows a subset of the Dk154 image from Extended Data Fig. 2, with β and the corresponding radius a labelling the plotted syndynes. The PAs of the synchrones are also marked. To characterize the tail’s shape, a Gaussian profile was fitted to the tail at various distances from the nucleus, and its central PA and FWHM are marked as green symbols.

The peak of the tail is confined within the 0.0005 ≤ β ≤ 0.0010 range, corresponding to emission times from 29 August until 7 September 2025. The tail is too faint and diffuse to measure its position before 29 August with this method, and it is too close to the nucleus and lost in its glare after 7 September, but there is no indication of an abrupt change. The measured range of β corresponds to very large grains, on the order of 400 μm. Cometary dust grains typically follow a power-law size distribution⁶⁷ with an index of about −4, and up to an upper limit a_max. The fact that no grains appear below the β = 0.0005 syndyne suggests an upper size limit of a_max ≈600 μm. As grain brightness scales as a², the observed grains follow a brightness power-law distribution with an index of about −2. This implies that grains smaller than ~400 μm should be more numerous and brighter, which is not the case. This discrepancy strongly suggests that the actual grain size distribution is narrowly limited to the 300–600 μm range.

From the gas production rate computed above, $Q_{{\mathrm{H}}_2\mathrm{O}}\approx 1.2\times 10^{24}$ molecules per second, we estimate the largest dust grain that can be lifted from the nucleus. For that critical radius, the gas drag equals the weight of the grain. Assuming a grain density of 1,000 kg m⁻³ and a nucleus density of 500–1,000 kg m⁻³, the critical radius can be estimated⁶⁸ as a ≈ 1.6 mm, confirming that the large grains observed can easily be lifted by the gas. Alternatively, assuming that the largest grains observed, with a ≈ 600 μm, correspond to the critical radius, the density of the nucleus would be ~1,300 kg m⁻³. We can therefore use this value as an upper limit to the nucleus density.

While typical cometary dust is in the micrometre range, very large grains (up to centimetere scale) have been detected. Such large grains were observed remotely, for instance^69,70, near comet C/2001 A₂ or in situ near comet 67P. Laboratory simulations suggest that these large particles are ejected when ice sublimation occurs below the surface, leading to a buildup of pressure that explosively expels the material³⁰. A fast rotation could also contribute to the ejection of large particles, for example, similar to one of the hypotheses formulated for 133P/Elst–Pizarro⁷¹. Notwithstanding the origin of the grains, they were continuously released from 29 August until 7 September 2025, with no indication of an abrupt start or stop at either these dates.

Radar

Radar observations of 1998 SH₂ occurred at Goldstone (8,560 MHz, 3.5 cm) on 26 August and 2 September 2025, dates that straddled the closest approach within 0.02 au on 31 August, when the asteroid was too far south for Goldstone to track. The 26 August observations did not produce a detection but observations on 2 September were successful. The radar observations used standard data acquisition and reduction techniques^72,73. At the time of the observations, problems with one of the klystron amplifiers limited the transmitter power to 240 kW, or slightly more than one-half of the nominal value.

On 26 August, we estimated that signal-to-noise ratios (SNRs) would be strong enough to obtain an echo within a few minutes. Given the diameter, and the fact that nearly all near-Earth asteroids (NEAs) >0.15 km in diameter have rotation periods slower than 2.1 hours, we expected an echo bandwidth of less than about 20 Hz. After observing for about 20 minutes without detecting an echo, we checked different frequency resolutions in case the echo was much narrower or wider than expected. After 40 minutes, there was still no echo, so we abandoned 1998 SH₂ and observed a different asteroid. Earlier during the observing session on 26 August, we detected radar echoes from 1997 QK₁, and after we stopped the 1998 SH₂ observations, we also detected echoes from 2025 QX₄, so we knew that the radar system was functioning well and suspected that the pointing was off for 1998 SH₂, which was later confirmed after the observations on 26 August concluded.

We began on 2 September with continuous-wave observations and saw an echo within 2 minutes. The echo has a bandwidth of 7 Hz and is centred on the Doppler frequency predicted by the ephemeris (Extended Data Fig. 1). We then transmitted coded waveforms with time delay resolutions of 10 μs, 11 μs and 1 μs (distance resolutions of 1,500 m, 1,650 m and 150 m) to estimate the range. The entire sequence of Goldstone radar observations spanned about 67 min and is summarized in Extended Data Table 1.

On the basis of infrared data obtained by the NEOWISE mission, the diameter and albedo of 1998 SH₂ are estimated²⁴ as of 380 ± 60 m and 0.058 ± 0.024. The width of a radar echo is given by:

where B is the bandwidth or Doppler broadening of the echo, D is the diameter, δ is the subradar latitude, λ is the wavelength and P is the rotation period. If the rotation period is known, then equation (2) constrains the pole-on extent of the asteroid. For 1998 SH₂, a rotation period has not been reported but the bandwidth, diameter and equation (2) allow us to estimate the period. Given the bandwidth of 7 Hz and a diameter of 380 m, equation (2) places an upper bound on the rotation period of 5.4 h under the assumption that the diameter is correct and that 1998 SH₂ is not considerably elongated.

The echo in Extended Data Fig. 1 shows a dip at frequencies near the middle of the echo that is consistent with a concavity, but due to the relatively weak SNRs, the dip is also consistent with receiver noise. The narrow spikes also resemble echoes seen from satellites of binary systems, where the broad echo is from the primary and the narrow echo is from the secondary. To check, we processed the continuous-wave data at different frequency resolutions but did not find convincing evidence for a companion. We also checked the 1 μs ranging data by processing it at four different frequency resolutions and summing all the runs. Radar observations of binary NEAs observed previously at this delay resolution often show echoes from two separate objects. The summed images at 1.0 μs × 0.5 Hz resolution show a small number of pixels with SNRs ~3.5 in delay-Doppler locations expected for an object in orbit relative to the main echo, but the pixels are also consistent with noise (we expect ~30 noise pixels this strong), so the evidence for a satellite, although intriguing, is not convincing.

We searched for rotational variations in the bandwidths and spectral shapes by summing groups of 5 runs (spanning about 5 min each) processed at 0.5 Hz resolution. We did not see any variations that are statistically significant, so evidently the bandwidth did not change significantly over an interval of 1.06 h.

For 1998 SH₂, we estimate a circular polarization ratio, that is, the ratio of the echo power in the same circular (SC) polarization state to that in the opposite circular (OC) polarization state, of SC/OC = 0.09 ± 0.03, which is lower than the average of ~0.3 seen for hundreds of other NEAs observed with radar^74,75. This ratio is also lower than those observed for (433) Eros (0.28 ± 0.06), (25143) Itokawa (0.27 ± 0.04), (4179) Toutatis (0.29 ± 0.01), (101955) Bennu (0.18 ± 0.01) and (65803) Didymos (0.20 ± 0.02), which have been visited by spacecraft. The circular polarization ratio of 1998 SH₂ could indicate that the near-surface is less rugged at decimetre spatial scales than the surfaces of the asteroids imaged by missions. However, modelling results^76,77 indicate that surface texture and composition also strongly influence circular polarization ratios so roughness is not the only possibility. The low ratio of 1998 SH₂ is inconsistent with those seen for V-, E- and some X-class NEAs (SC/OC > 0.6). The ratio is consistent with values estimated for the other spectral types, particularly a small sample of M types, and also with the lower end of the distribution for optically dark BC types and bright SQ types. The circular polarization ratio has been measured for 8 comets^{78,79,80,81,82} and ranges between 0.105 and 0.59; 1998 SH₂’s value is lower than this range.

Radar echoes from some comets show a wide ‘skirt’ caused by centimetre- to decimetre-sized coma particles surrounding the nucleus. Coma echoes have been seen in radar echoes of numerous comets that were very active (for example, C/1996 B₂ (Hyakutake)), but are not always detected from some comets that show a coma at optical wavelengths. We searched but do not see a wide coma echo for 1998 SH₂, which is consistent with the low level of activity observed in the optical images. Thus, we conclude that cometary activity by 1998 SH₂ on 2 September was too low to detect with radar observations at Goldstone.

The 1-μs echo occupies 2 rows, and given that the radar could illuminate only ~1/2 of the surface if the object were a sphere, this establishes that the diameter of 1998 SH₂ is <600 m, a result that is consistent with the value of 380 m from NEOWISE²⁴. The diameter of 380 m also indicates that 1998 SH₂ is the smallest comet ever observed by radar. We used echo power spectra processed at 0.5-Hz resolution (Extended Data Fig. 1) to estimate a radar cross-section of 0.0048 km² ± 35%, where the uncertainty accounts for systematic pointing and calibration errors. If we adopt the diameter of 380 m, then we obtain a radar albedo of ~0.04, which is lower than most observed among NEAs but overlaps many estimated for comet nuclei⁸⁰. The radar albedo is a function of the near-surface bulk density^80,83,84 and a value of 0.04 suggests a surface with porosity that is not highly compacted. The implication is that the radar albedo more closely resembles those seen from comet nuclei than from NEAs.

Meteoroid stream

Given that 1998 SH₂ closely approaches Earth, the possibility of a meteoroid stream giving rise to a meteor shower does exist. Assuming any meteoroid ejection occurs at relatively low velocity, a meteoroid stream would be expected to move in parallel to 1998 SH₂. On 30 August 2025, when the orbit of 1998 SH₂ is 0.02 au from Earth, any potential shower would appear to originate from the geocentric radiant α_g ≈ 172.3°, δ_g ≈ −0.1°, with a geocentric speed v_g ≈ 17.2 km s⁻¹. However, as this date corresponds to the start of the current activity, any meteoroids that may have been observed must have been released during previous activity. Also, while ~400 μm meteoroids would produce optical meteors, this radiant is close to the helion direction, and thus, probably would be only visible by meteor radars. An in-depth simulation to better model any potential meteoroid stream is beyond the scope of our work here.