# Keck and Gemini Characterization of Hayabusa2# Rendezvous Target 1998 KY26

Bryce T. Bolin, Christoffer Fremling, Matthew Belyakov, Jin Beniyama, Marco Delbo, Robert Jedicke, Ian Wong, Laura-May Abron, Keith S. Noll, Andrew W. Stephens

► To cite this version:

Bryce T. Bolin, Christoffer Fremling, Matthew Belyakov, Jin Beniyama, Marco Delbo, et al. Keck and Gemini Characterization of Hayabusa2# Rendezvous Target 1998 KY26. The Astronomical Journal, 2025, 169, 10.3847/1538-3881/adccbe. insu-05099858

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# Keck and Gemini Characterization of Hayabusa2# Rendezvous Target 1998 KY26

Bryce T. Bolin¹, Christoffer Fremling²,³, Matthew Belyakov⁴, Jin Beniyama⁵,⁶, Marco Delbo⁵,⁷, Robert Jedicke⁸, Ian Wong⁹, Laura-May Abron¹⁰, Keith S. Noll¹¹, and Andrew W. Stephens¹²

¹Eureka Scientific, Oakland, CA 94602, USA; bbolin@eurekasci.com

²Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA

³Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA

⁴Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

⁵Université Côte d'Azur, CNRSgrange, Observatoire de la Côte d'Azur, F 06304, Nice, France

⁶Institute of Astronomy, Graduate School of Science, The University of Tokyo, Mitaka, Tokyo, 181-0015, Japan

⁷University of Leicester, School of Physics and Astronomy, Leicester, LE1 7RH, UK

⁸Institute for Astronomy, University of Hawai'i at Mānoa, Honolulu, HI 96822, USA

⁹Space Telescope Science Institute, Baltimore, MD 21218, USA

¹⁰Griffith Observatory, Los Angeles, CA 90027, USA

¹¹Goddard Space Flight Center, Greenbelt, MD 20771, USA

¹²Gemini Observatory/NSF NOIRLab, Hilo, HI 96720, USA

Received 2025 February 4; revised 2025 April 8; accepted 2025 April 12; published 2025 May 12

## Abstract

Near-Earth object (NEO) 1998 KY26 is a target of the Hayabusa2# spacecraft, which it will rendezvous with in 2031 July. The asteroid has been noted to rotate rapidly and has a large out-of-plane nongravitational acceleration. We present observations consisting of deep-g- and R-band imaging obtained with the Keck I/Low Resolution Imaging Spectrometer (LRIS) and visible spectroscopy from Gemini North/Gemini Multi-Object Spectrograph (GMOS) taken of 1998 KY26 on 2024 June 8–9 when the asteroid was ~0.037 au from the Earth. The asteroid does not show evidence of a dust coma and has a surface brightness profile similar to nearby background stars in the deep images. The spectrum of 1998 KY26 from the combined LRIS and GMOS observations most closely resembles Xe-type asteroids, possessing a spectral slope of 6.71% ± 0.43% 100 nm⁻¹, and color indices g − r = 0.63 ± 0.03, r − i = 0.15 ± 0.03, i − z = 0.05 ± 0.04, and implies a diameter of ~10 m. From our deep image stacks, we compute a 3σ upper limit on the dust production of 1998 KY26 of <10⁻⁵ kg s⁻¹, <10⁻² kg s⁻¹, and <10⁻¹ kg s⁻¹ assuming µm, mm, and cm size dust particles. In addition, we compare the orbit of 1998 KY26 and other known asteroids with large nongravitational parameters to NEO population models and find that the majority, including 1998 KY26, likely originated from the inner Main Belt, while the second most numerous group originates from the outer main belt, followed by a third group possibly originating from the Jupiter Family Comet population. Given its inner Main Belt origin, its Xe-type spectrum, and rapid rotation, we hypothesize that the nongravitational acceleration of 1998 KY26 may be caused by the shedding of large dust grains from its surface due to its rotation rather than H₂O vapor outgassing.

*Unified Astronomy Thesaurus concepts: Near-Earth objects (1092); Asteroid surfaces (2209)*

## 1. Introduction

With a semimajor axis of *a*, of 1.23 au, eccentricity, *e*, of 0.2, inclination, *i*, of 1.48, and absolute magnitude, *H* of 25.7¹³ the Apollo near-Earth object (NEO) 1998 KY26 was first noted for being one of the smallest asteroids studied by ground-based radar (S. J. Ostro et al. 1999). The X-band (3.5 cm wavelength) radar detection of 1998 KY26 by the Goldstone Solar System Radar facility provided a diameter estimate between 20 and 40 m, making it stand out as a potential target for testing the detection of the thermal recoil Yarkovsky Effect (D. Vokrouhlický et al. 2000). The asteroid was also noted for having a 10.7 minutes rotation period, well below the ~2.2 hr critical spin period limit for surface material near the equator to remain gravitationally bound to the asteroid in the absence of cohesive forces (P. Pravec & A. W. Harris 2000).

Recently, 1998 KY26 was selected as a rendezvous target of the Hayabusa2# extended spacecraft mission, which will reach the asteroid in 2031 (M. Hirabayashi et al. 2021). The rendezvous is expected to last ~1 yr and will include close proximity operations to determine the bulk properties of the asteroid's surface as well as characterize its regolith (S. Kikuchi et al. 2023). The spacecraft is also expected to characterize the dust environment surrounding the asteroid as well as touch down on the asteroid and conduct kinetic experiments with projectile firing (T. Saiki et al. 2020).

In addition to being observed by radar and a spacecraft mission rendezvous target, 1998 KY26 has been identified to possess an orbit with large nongravitational accelerations, exceeding those provided by thermal recoil forces, and in the out-of-plane direction (D. Farnocchia et al. 2023; D. Z. Seligman et al. 2023; D. Jewitt 2025). In some cases, authors refer to these objects as “dark comets,” although there is no direct evidence yet for their activity being driven by the sublimation of cometary volatiles such as H₂O. 1998 KY26 has radial, transverse, and out-of-plane acceleration components of 1.60 ± 0.88 × 10⁻¹⁰ au day⁻², -1.38 ± 0.57 × 10⁻<fnref n="13" /> au day⁻², and 2.70 ± 0.65 × 10⁻¹¹ au day⁻², with the out-of-plane noted for being particularly large and significant (D. Z. Seligman et al. 2024). The nongravitational acceleration of

<figure><img src="image_3.png" /></figure>

<footnote n="13">¹³ Taken from JPL Small-Body Database: https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=1998ky26, accessed on 2024 January 13.</footnote>
1998 KY₂₆ is interpreted by D. Z. Seligman et al. (2024) as possibly being caused by the outgassing of volatiles such as H₂O. The total of all three nongravitational acceleration components equals to ~1.62 × 10⁻¹⁰ au day⁻², implying that 1998 KY₂₆ has a mass-loss rate of ~3 × 10⁻⁵ kg when assuming that the ejected material consists of H₂O molecules escaping from the asteroid's surface at 350 m s⁻¹. We note that the uncertainties are large on the estimated nongravitational acceleration components and thus the total acceleration could be much lower than this.

This work discusses the physical characterization of 1998 KY₂₆ using imaging taken with the Keck/Low Resolution Imaging Spectrometer (LRIS) and spectroscopy taken with the Gemini North/Gemini Multi-Object Spectrograph (GMOS). We use the combination of visible imaging and spectroscopy to study the physical properties of 1998 KY₂₆, an approach previous demonstrated by B. T. Bolin et al. (2021, 2022, 2024, 2025). We also compare the orbital elements of 1998 KY₂₆ and other asteroids showing strong nongravitational acceleration with NEO population models (M. Granvik et al. 2018; A. Morbidelli et al. 2020; D. Nesvorný et al. 2023) to better understand their origins and predicted physical properties.

## 2. Observations

We used the LRIS instrument on the 10.0 m Keck I telescope to observe 1998 KY₂₆ on 2024 June 8 14:29 under program 2024A_N019 (PI: B.T. Bolin) when the asteroid was at R.A. = 23 31 00.2, decl. = -00 00 18, 0.037 au (14.3 lunar distances) from Earth, a heliocentric distance of 1.012 au, and had a phase angle of 93°.3. LRIS possesses a blue camera consisting of two 2K × 4K Marconi CCDs with a pixel scale of 0.135 pixel⁻¹ and a red camera consisting of two Lawrence Berkley National Laboratory 2k × 4k fully depleted, high-resistivity CCD detectors with the same pixel scale (J. B. Oke et al. 1995; C. Rockosi et al. 2010). Both cameras were used with the 2 × 2 binning with two readout amplifiers. The 560 nm dichroic was used in combination with an SDSS-equivalent g filter ($\lambda_{eff}$ = 467.2 nm, FWHM = 126.3 nm; J. E. Gunn et al. 1998) in the blue camera and Cousins R ($\lambda_{eff}$ = 649.2 nm, FWHM = 167.1 nm, A. W. J. Cousins 1976) in the red camera, similar to the observational set up of B. T. Bolin et al. (2020, 2023a).

A set of 30 s exposures was taken in both the blue and red cameras using the g and R filters in the field containing 1998 KY₂₆ using the sideral tracking rate to provide background stars for photometric calibration and surface brightness profile comparison without the background stars becoming trailed. A series of four 180 s simultaneous exposures in the two cameras immediately followed, tracking the telescope nonsidereally using 1998 KY₂₆'s on-sky motion rate of 8.87 minutes⁻¹. The asteroid was observed at an airmass of ~1.25 with Keck/LRIS. Sky conditions were clear and background stars in the images containing the 1998 KY₂₆ field had an average FWHM of ~1.08.

The LRIS images were reduced using the LPipe reduction software (D. A. Perley 2019). The images were stacked to increase the signal to noise of the detections (e.g., P. J. Whidden et al. 2019), mosaic of the g and R-band image stacks is shown in panels (A) and (B) of Figure 1. The g and R-band photometry measurements were completed using background solar analog stars from the Pan-STARRS1 catalog

(K. C. Chambers et al. 2016) and color transformations described in J. L. Tonry et al. (2012) to convert the Pan-STARRS1 catalog magnitudes to their corresponding g and R filter equivalents. The LRIS g- and R-band photometry of 1998 KY₂₆ and standard stars were measured using a 1.62 radius circular aperture (6 pixels) and then subtracting from it the median pixel value within a 2.97–4.32 (11–16 pixels) radius sky background annulus.

The GMOS instrument on the 8.1 m Gemini N telescope was used to observe 1998 KY₂₆ the following night, on 2024 June 9 13:32, under program GN-2024A-DD-108 (PI: B.T. Bolin) when the asteroid was at R.A. = 23 45 02.0, decl. = +00 37 14, 0.039 au (15.2 lunar distances) from the Earth, a heliocentric distance of 1.011 au, and had a phase angle of 82°.1. A similar approach was taken to observe 1998 KY₂₆ with GMOS as previously described by B. T. Bolin et al. (2023b). The GMOS detector consists of three 2048 x 4176 Hamamatsu chips with an effective pixel scale of 0.0807 pixel⁻¹. The detectors were rebinned in both the spatial and spectral directions by a factor of two. The R400 grating was used with the 1.5 slit, the GG455_G0305 order-blocking filter, and a central wavelength of 700 nm to provide spectral coverage between 450 and 1030 nm at an average effective spectral resolution of 0.15 nm (I. M. Hook et al. 2004). A nearby solar analog star, HD 215428, was observed for telluric and slope correction using the same GMOS instrument settings.

A seeing FWHM of ~0.4 was measured from nearby background stars, and the observations were taken at the local parallactic angle for both the observations of 1998 KY₂₆ and the solar analog star. The telescope was tracked at the asteroid's sky motion rate of 9.89 minute⁻¹, and 6 x 600 s exposures were taken, and the asteroid was observed at airmass of ~1.5. In two of the six exposures, 1998 KY₂₆'s spectral trace was contaminated by background stars crossing its path; these exposures were excluded from the analysis, leaving an effective total integration time of 2400 s. The DRAGONS Gemini reduction software (K. Labrie et al. 2023) and a custom pipeline were used to reduce the 1998 KY₂₆ and solar analog spectra.

## 3. Results

We measured brightnesses of g = 21.57 ± 0.03, R = 20.72 ± 0.03 mag from the 720 s LRIS images. In addition, we stacked the g- and R-band images into a composite g + R stack, as seen in Panel (C) of Figure 1, to search for evidence of an extended coma from dust production in 1998 KY₂₆. From the g + R target and comparison star composite stacks, we measured the azimuthally averaged surface brightness profile of 1998 KY₂₆ and nearby comparison stars, as shown in Panel (D) of Figure 1. The surface brightness profile of 1998 KY₂₆ does not show any evidence of an extended coma when compared to the surface brightness profile of nearby stars.

We computed the GMOS reflectance spectrum of 1998 KY₂₆ by dividing the asteroid spectrum by the solar analog spectrum and normalizing to unity at 550 nm, as shown in red in Figure 2. We rebinned the spectrum by a factor of 70 to increase the signal-to-noise ratio. We do not show the data shortward of 550 nm and longward of 930 nm due to the sharp decrease in the signal-to-noise ratio of the spectrum in these wavelength ranges.
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Deep g- and R-band imaging of 1998 KY<sub>26</sub> taken with Keck/LRIS on 2024 June 8. Panel (A): a median combination stack of 4 x 180 s g filter LRIS images of 1998 KY<sub>26</sub>. An arrow indicating a width of 15" is shown for scale. The cardinal, solar, and orbital motion directions are indicated. Panel (B): a median combination stack of 4 x 180 s R filter images of 1998 KY<sub>26</sub>. Panel (C): a combined stack of all g + R filter images of 1998 KY<sub>26</sub>. The pixel scale is 0.27. Streaks and points are due to incompletely removed background star trails in the image stacks. Panel (D): the azimuthally averaged surface brightness profile of the combined 4 × 180 g + R filter stack plotted as yellow data points with error bars. The error bars are smaller than the marker for the first five data points. A nearby comparison star's azimuthally averaged surface brightness profile is overplotted as a piecewise between pixel orange line.</figcaption></figure>

We incorporate the LRIS g- and R-band photometry into the combined GMOS + LRIS reflectance spectrum of 1998 KY<sub>26</sub> by dividing the equivalent band fluxes by the corresponding flux of a solar analog and normalizing them to a wavelength of 550 nm. The normalized reflectivity of 1998 KY<sub>26</sub> in the g and R bands are plotted as green and orange data points in Figure 2, respectively. We use the combined GMOS + LRIS spectrum to compute the spectral slope between an equivalent SDSS g-band wavelength (λ<sub>eff</sub> = 467.3 nm) and an equivalent SDSS r-band wavelength (λ<sub>eff</sub> = 614.1 nm) to obtain a spectral slope value of 10.48% ± 0.35%/100 nm, which flattens out to an overall spectral slope of 6.71% ± 0.43%/100 nm when extending this range redward to the equivalent i-band wavelength (λ<sub>eff</sub> = 745.9 nm). Additionally,

we compute color indices between the SDSS g, r, i, and z (λ<sub>eff</sub> = 892.3 nm) bands: g − r = 0.63 ± 0.03, r − i = 0.15 ± 0.03, i − z = 0.05 ± 0.04. These colors are comparable to the equivalent g − r = 0.60 ± 0.06 and r − i = 0.21 ± 0.05 color measurements derived from previously published photometric observations of 1998 KY<sub>26</sub> taken by S. J. Ostro et al. (1999).

We compare the spectrum of 1998 KY<sub>26</sub> with the mean Bus-DeMeo asteroid spectra (S. J. Bus & R. P. Binzel 2002; F. E. DeMeo et al. 2009), finding that it resembles Xe-type spectra by visual inspection. We compute the χ<sup>2</sup> statistic for the spectrum of 1998 KY<sub>26</sub> compared to the spectrum of 19 different asteroid types—S-complex asteroid types (S, Sq, Sv, Q), C-complex types (B, C, Cg, Cgh), X-complex
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Visible spectra of 1998 KY26 taken with Gemini N/GMOS on 2024 June 9. The Gemini N/GMOS spectrum of 1998 KY26 is plotted in red, covering 550–920 nm and normalized to unity at 550 nm (indicated by the light blue cross). The normalized reflectivity from the Keck/LRIS g and R filter (converted to r, K. Jordi et al. 2006) observations are included in green and orange, respectively. The gaps in the spectrum at 640, 780, and 850 nm are due to incomplete removal of telluric features. The data are rebinned by a factor of 70 using an error-weighted mean. The spectral ranges of K-, C-, Xe-, and D-type asteroids from F. E. DeMeo et al. (2009) are overplotted in purple, gold, aqua, and orange.</figcaption></figure>

types (X, Xc, Xe, Xk, Xn), and other assorted types (A, D, K, L, O, R, V)—from the Bus-DeMeo asteroid spectral catalog and determine the closest match to be Xe-types, with a reduced $\chi^2$ metric of ~1, followed by K-types with a reduced $\chi^2$ of ~3. By comparison, the reduced $\chi^2$ between the spectrum of 1998 KY26 and C-types is ~22, and with D-types is ~14. We estimate the size of 1998 KY26 with $D = 1329 (10^{-H/5}/\sqrt{p_v})$ from H. N. Russell (1916), where D is the diameter of an asteroid, H, is its absolute magnitude, and $p_v$ is its visual albedo. We assume 1998 KY26 has $p_v \sim 0.5$, the same as for since Xe-type asteroids have been shown to have albedos ~0.5 (M. Delbo et al. 2019), which combined with its H of ~25.7 results in a D estimate of ~10 m.

Using the composite spectrum of 1998 KY26 and the combined g + R-band LRIS image stack, we estimate upper limits on the Johnson V-band equivalent ($\lambda_{eff} = 550.0$ nm, H. L. Johnson & W. W. Morgan 1953) surface brightness of faint coma centered on the target. We find a V-band equivalent 3$\sigma$ upper limit on the surface brightness in a 1.62–2.16 annulus of $<1.01 \times 10^{-7}$ Jy arcsec$^{-2}$, corresponding to a total flux of $<6.50 \times 10^{-7}$ Jy. We estimate the absolute magnitude H of a single grain of dust using $H = 5 \log_{10}(1329/D\sqrt{p_v})$. We assume that dust from 1998 KY26 has $p_v \sim 0.5$, the same as for since Xe-type asteroids. For $\mu$m, mm, and cm-scale dust, we obtain H values of ~60, ~46, and ~41.

The V-band brightness of a single grain of dust is found using the phase function from E. Bowell et al. (1988):

$$V = H + 5 \log_{10}(r_h \Delta) + 2.5 \log_{10} \times [(1 - G) \Phi_1(\alpha) + G \Phi_2(\alpha)] \quad (1)$$

where for the 2024 June 8 observations of 1998 KY26, $r_h$ is its heliocentric distance of 1.012 au, $\Delta$ is its geocentric distance of 0.037 au and $\alpha$ is its phase angle of 93.3. G is the phase coefficient, for which we use the value of 0.2, the average value for X-type asteroids (P. Vereš et al. 2015). $\Phi_1(\alpha)$ and $\Phi_2(\alpha)$ are

the basis functions normalized at $\alpha = 0^\circ$ described in E. Bowell et al. (1988).

We obtain $V \simeq 57, 42$, and 37 mag, or a flux of $\sim 3.8 \times 10^{-20}$ Jy, $\sim 3.8 \times 10^{-14}$ Jy, and $\sim 3.8 \times 10^{-12}$ Jy for $\mu$m, mm, and cm-scale dust particles, respectively. Dividing the 3$\sigma$ upper limit on the total flux inside the 1.62–2.16 annulus of $<6.50 \times 10^{-7}$ Jy described above, we obtain 3$\sigma$ upper limits on the dust particle number of $<10^{13}, <10^7$, and $<10^5$ for $\mu$m, mm, and cm-scale dust, or a total dust mass of $<10^{-2}$ kg, $<10$ kg, and $<100$ kg assuming that the dust has the same grain density of Aubrite meteorites: $\sim 3.15 \text{ g cm}^{-1}$ (R. J. Macke et al. 2011). Aubrites are linked to Xe-types (M. Ćuk et al. 2014; M. P. Lucas et al. 2019), suggesting that this is a reasonable estimate for the density of dust produced by 1998 KY26. Dividing the total dust mass 3$\sigma$ upper limits by the integration time of 720 s, we obtain 3$\sigma$ upper limits on the dust production of $<10^{-5}$ kg s$^{-1}$, $<10^{-2}$ kg s$^{-1}$, and $<10^{-1}$ kg s$^{-1}$ for $\mu$m, mm, and cm-scale dust particles.

## 4. Discussion and Conclusions

Measurement of the nongravitational parameters of 1998 KY26 shows that it has a total acceleration of $\sim 1.62 \times 10^{-10}$ au day$^{-2}$, which given a 350 m s$^{-1}$ outflow speed implies a total mass loss of $\sim 10^{-5}$ kg s$^{-1}$ (D. Z. Seligman et al. 2024). This rate is in line with our 3$\sigma$ upper limit for the production of $\mu$m-sized dust particles. However, the mass-loss scenario described by D. Z. Seligman et al. (2024) assumes that the mass loss could be in the form of H$_2$O vapor molecules, which would escape the surface of 1998 KY26 at the speed of sound in H$_2$O vapor, i.e., $\sim 350 \text{ m s}^{-1}$ at 1 au from the Sun. We also note the true total acceleration of 1998 KY26 could be much lower due to the large uncertainties in the estimate of all the nongravitational acceleration components.

The spectral type and source location of 1998 KY26 seems to diminish the possibility that it contains volatiles. While some
asteroids in the Xe-type group show evidence of hydrated
spectral features (A. S. Rivkin et al. 1995), it seems less likely
that small Xe-types such as 1998 KY₂₆ would possess H₂O
vapor given that the majority of Xe-types show little to no
evidence of hydration features (S. Fornasier et al. 2011).
Additionally, meteoritic evidence suggests that Jupiter formed
quickly, reaching 20 Earth masses within the first Myr after the
formation of solids in the solar system (T. S. Kruijer et al.
2017), effectively locking the early ice line from reaching
within 3 au from the Sun (A. Morbidelli et al. 2016). This is
outside the range of where 1998 KY₂₆ is likely to have
originated within the Main Belt. Additionally, the lack of
hydration among asteroids in the inner Main Belt is supported
by the observation that no active asteroids whose activity has
evidence of being driven by the sublimation of volatiles are
found in the inner Main Belt (D. Jewitt & H. H. Hsieh 2022).

Instead, the mass loss of 1998 KY₂₆, as suggested by its
nongravitational acceleration, may be due to a different
mechanism than the outgassing of H₂O. The short rotation
period of 1998 KY₂₆ of ~10 minutes, well below the stability
limit of ~2 hr, suggests that material near its surface may be
unstable in the absence of cohesive forces (P. Pravec &
A. W. Harris 2000). While small asteroids have been observed
with rotation periods below 2 hr (B. D. Warner et al. 2009),
small asteroids on the scale of ~100 m have been observed to
sporadically eject dust and take on an active appearance as a
result of surface mass shedding due to their rapid spins
(J. N. Purdum et al. 2021; D. Jewitt & H. H. Hsieh 2022).

Surface material ejected from a rotationally unstable
asteroid leaves its surface at speeds comparable to the
asteroid's gravitational escape speed (e.g., D. Jewitt et al.
2014). This is also true for the fragments of asteroids that
have undergone collisional disruption (e.g., B. T. Bolin et al.
2018a, 2018b; F. Moreno et al. 2017). Using the upper limit
on the diameter of 17 m estimated from a nondetection of
1998 KY₂₆ in ground-based mid-infrared observations
(J. Beniyama et al. 2025), and assuming a density of
2600 kg m⁻³ (i.e., a typical density for Xe-type asteroids;
B. Carry 2012), its escape speed is ~0.01 m s⁻¹. Rescaling
the mass loss of 3 × 10⁻⁵ kg s⁻¹, assumed for H₂O molecules
escaping at 350 m s⁻¹ D. Z. Seligman et al. (2024), to an
ejection speed of ~0.01 m s⁻¹ results in a mass-loss rate of
~0.1 kg s⁻¹, comparable to our 3σ upper limits on the
production of mm and cm-sized dust derived from our deep
imaging observations. Additionally, manuscript by Santana-
Ros et al.<fnref n="14" /> currently in review at time of writing suggests that
that 1998 KY₂₆ may have an even smaller diameter of ~10 m,
similar to our size estimate from Section 3. The ~10 m size
estimate from our measurements and the measurements of
Santana-Ros would imply a mass-loss rate of ~0.1 kg s⁻¹
similar to our 3σ upper limit on the production of mm-size
dust. Additionally, Santana-Ros find a rotation period of ~5
minutes for 1998 KY₂₆ further strengthening our argument that
it is rotating faster than the critical spin limit for surface
material to remain gravitational bound to the asteroid.
Furthermore, the NEO Bennu was observed by the OSIRIS-
REx spacecraft to be undergoing mass loss in the form of the
ejection of mm- to cm-sized particles from its surface, similar
to the size of the particles compatible with our mass-loss upper
limit estimate for 1998 KY₂₆ (C. W. Hergenrother et al. 2020).

Hayabusa2# is designed to characterize the dust environment
around 1998 KY₂₆, and will provide a more stringent constraint
on its dust production rate when it reaches the asteroid in 2031.

We compare the orbital elements of 1998 KY₂₆ (a = 1.23,
e = 0.20, i = 1.48, and H = 25.7) with the M. Granvik et al.
(2018), A. Morbidelli et al. (2020), and NEOMOD
(D. Nesvorný et al. 2023, 2024) NEO population models
to estimate 1998 KY₂₆'s source within the Main Belt and
Jupiter Family Comet populations. We find that the most
likely source for 1998 KY₂₆ is the ν₆ resonance located on
the inner edge of the inner Main Belt (A. Milani &
Z. Knežević 1994) with a probability of ~0.67 as seen in
Table 1. The following two most probable sources are the
Hungarias at 1.9 au and the 3:1 mean motion resonance at
2.5 au, with probabilities of ~0.17 and ~0.16, respectively.
Combining these three sources strongly implies that 1998 KY₂₆
originated from the inner Main Belt inside of 2.5 au from the
Sun. We note that Hungarias, a source compatible with the orbit
of 1998 KY₂₆, consists mainly of Xe-types (M. Ćuk et al. 2014;
M. P. Lucas et al. 2019), agreeing with the classification of 1998
KY₂₆ from our spectroscopic observations.

Table 1 shows a complete list of the NEO source probabilities
for all of the objects with large nongravitational accelerations listed
by D. Z. Seligman et al. (2024). Sources for some large
nongravitational objects have been described by A. G. Taylor
et al. (2024) and are broadly consistent with our results. We
find that the majority of objects more recently described by
D. Z. Seligman et al. (2024) likely originate from the inner Main
Belt: 1998 KY<sub>26</sub>, 2005 UY<sub>6</sub>, 2012 UR<sub>158</sub>, 2016 GW<sub>221</sub>, 2013
BA<sub>74</sub>, 2016 NJ<sub>33</sub>, 2013 XY<sub>20</sub>, 2005 VL<sub>1</sub>, 2010 RF<sub>12</sub>, 2010
VL<sub>65</sub>, and 2006 RH<sub>120</sub>. Following our previous discussion of the
lack of hydration among asteroids in the inner main belt, the
likely origin of these asteroids suggests that their nongravita-
tional acceleration is due to mechanical means, such as the
shedding of surface material, as opposed to volatile sublimation.
Additionally, the small (≤10 m) size of some of these objects,
such as minimoon 2006 RH<sub>120</sub> (T. Kwiatkowski et al. 2009),
may suggest that the nongravitational accelerations of some of
these objects may be due to solar radiation pressure, as has been
detected for other 10 m-scale asteroids (e.g., B. T. Bolin et al.
2025).

The second most populous source of large nongravitational
asteroids is located in the outer main belt where objects 1998
FR<sub>11</sub>, 2001 ME<sub>1</sub>, and 2003 RM most likely originate, in the
vicinity of the 5:2 resonance located at the inner edge of the outer
main belt at 2.82 au (N. Todorović 2017). In addition to being
more volatile-rich compared to the inner main belt (e.g.,
H. Campins et al. 2010; A. S. Rivkin & J. P. Emery 2010), the
outer main belt is also the location of several asteroid families
(H. H. Hsieh et al. 2018; Y. Xin et al. 2024). The escape of
asteroids from the outer main belt, where main belt comet families
are located, into the NEO population suggests that some of the
large nongravitational asteroids escaping from this region of the
main belt (M. Granvik et al. 2017) may be retaining volatiles such
as H<sub>2</sub>O, which is driving their nongravitational accelerations. Two
of these objects, 1998 FR<sub>11</sub> and 2001 ME<sub>1</sub>, also have significantly
large Jupiter family comet (JFC) source probabilities of ~30%,
making the JFCs the third most likely source of large
nongravitational objects. This suggests that 1998 FR<sub>11</sub> and 2001
ME<sub>1</sub> could be weakly active cometary remnants that originated
from the volatile-rich Kuiper Belt (D. Nesvorný et al. 2017). We
note, however, that the difference in the JFC source probabilities

<footnote n="14">14 https://www.researchsquare.com/article/rs-5821856/v1</footnote>
<table><caption>**Table 1**
Summary of Orbital Elements, Albedo, Diameter, and NEO Source Probabilities for 1998 KY<sub>26</sub> and Other Asteroids with Large Nongravitational Accelerations</caption><thead><tr><th>Object<sup>1</sup></th><th>a<sup>2</sup><br>(au)</th><th>e<sup>3</sup></th><th>i<sup>4</sup><br>(deg)</th><th>T<sub>j</sub><sup>5</sup></th><th>H<sup>6</sup></th><th>p<sub>v</sub><sup>7</sup></th><th>D<sup>8</sup><br>(m)</th><th>P<sub>Hun</sub><sup>9</sup></th><th>P<sub>c6</sub><sup>10</sup></th><th>P<sub>Pho</sub><sup>11</sup></th><th>P<sub>3:1</sub><sup>12</sup></th><th>P<sub>5:2</sub><sup>13</sup></th><th>P<sub>2:1</sub><sup>14</sup></th><th>P<sub>JFC</sub><sup>15</sup></th></tr></thead><tbody><tr><td>1998 KY<sub>26</sub></td><td>1.23</td><td>0.20</td><td>1.48</td><td>5.19</td><td>25.74</td><td>0.23/0.12<sup>a</sup></td><td>30<sup>2</sup></td><td>0.25/0.08</td><td>0.68/0.66</td><td>0.0/0.0</td><td>0.07/0.24</td><td>0.0/0.01</td><td>0.0/0.01</td><td>0.0/0.0</td></tr><tr><td>1998 FR<sub>11</sub></td><td>2.81</td><td>0.71</td><td>6.67</td><td>2.89</td><td>16.42</td><td>0.1/0.02<sup>b</sup></td><td>4900<sup>1</sup></td><td>0.0/0.0</td><td>0.01/0.0</td><td>0.0/0.0</td><td>0.06/0.05</td><td>0.61/0.83</td><td>0.01/0.10</td><td>0.30/0.02</td></tr><tr><td>2001 ME<sub>1</sub></td><td>2.63</td><td>0.87</td><td>5.96</td><td>2.67</td><td>16.52</td><td>0.1/0.02<sup>b</sup></td><td>4600<sup>1</sup></td><td>0.0/0.0</td><td>0.05/0.02</td><td>0.0/0.0</td><td>0.18/0.25</td><td>0.45/0.50</td><td>0.04/0.14</td><td>0.28/0.10</td></tr><tr><td>2005 UY<sub>6</sub></td><td>2.25</td><td>0.87</td><td>12.21</td><td>2.94</td><td>18.14</td><td>0.20</td><td>700</td><td>0.01/0.01</td><td>0.59/0.23</td><td>0.01/0.01</td><td>0.25/0.62</td><td>0.13/0.10</td><td>0.0/0.02</td><td>0.0/0.0</td></tr><tr><td>2003 RM</td><td>2.92</td><td>0.60</td><td>10.85</td><td>2.96</td><td>19.64</td><td>0.14</td><td>400</td><td>0.0/0.0</td><td>0.0/0.0</td><td>0.0/0.0</td><td>0.03/0.03</td><td>0.70/0.40</td><td>0.12/0.52</td><td>0.14/0.05</td></tr><tr><td>2012 UR<sub>158</sub></td><td>2.23</td><td>0.85</td><td>3.19</td><td>3.00</td><td>20.67</td><td>0.21/0.02<sup>b</sup></td><td>630<sup>1</sup></td><td>0.01/0.02</td><td>0.73/0.38</td><td>0.0/0.0</td><td>0.22/0.51</td><td>0.03/0.07</td><td>0.0/0.02</td><td>0.01/0.0</td></tr><tr><td>2016 GW<sub>221</sub></td><td>0.83</td><td>0.27</td><td>3.76</td><td>7.05</td><td>24.76</td><td>0.23</td><td>31</td><td>0.22/0.07</td><td>0.72/0.67</td><td>0.0/0.0</td><td>0.05/0.25</td><td>0.0/0.01</td><td>0.0/0.0</td><td>0.0/0.0</td></tr><tr><td>2013 BA<sub>74</sub></td><td>1.75</td><td>0.44</td><td>5.22</td><td>4.00</td><td>25.40</td><td>0.22</td><td>24</td><td>0.12/0.05</td><td>0.80/0.75</td><td>0.0/0.0</td><td>0.08/0.19</td><td>0.0/0.01</td><td>0.0/0.0</td><td>0.0/0.0</td></tr><tr><td>2016 NJ<sub>33</sub></td><td>1.31</td><td>0.21</td><td>6.62</td><td>4.94</td><td>25.53</td><td>0.23</td><td>22</td><td>0.31/0.10</td><td>0.59/0.71</td><td>0.0/0.0</td><td>0.11/0.19</td><td>0.0/0.01</td><td>0.0/0.0</td><td>0.0/0.0</td></tr><tr><td>2013 XY<sub>20</sub></td><td>1.13</td><td>0.11</td><td>2.85</td><td>5.52</td><td>25.64</td><td>0.23</td><td>21</td><td>0.28/0.08</td><td>0.70/0.36</td><td>0.0/0.0</td><td>0.02/0.55</td><td>0.0/0.0</td><td>0.0/0.0</td><td>0.0/0.0</td></tr><tr><td>2005 VL<sub>1</sub></td><td>0.89</td><td>0.22</td><td>0.24</td><td>6.65</td><td>26.45</td><td>0.23</td><td>14</td><td>0.22/0.07</td><td>0.68/0.71</td><td>0.0/0.0</td><td>0.10/0.22</td><td>0.0/0.01</td><td>0.0/0.0</td><td>0.0/0.0</td></tr><tr><td>2010 RF<sub>12</sub></td><td>1.06</td><td>0.19</td><td>0.88</td><td>5.79</td><td>28.42</td><td>0.23</td><td>5.7</td><td>0.21/0.09</td><td>0.74/0.76</td><td>0.0/0.0</td><td>0.05/0.15</td><td>0.0/0.01</td><td>0.03/0.0</td><td>0.0/0.0</td></tr><tr><td>2010 VL<sub>65</sub></td><td>1.07</td><td>0.15</td><td>4.23</td><td>5.75</td><td>29.22</td><td>0.23</td><td>4</td><td>0.22/0.09</td><td>0.71/0.75</td><td>0.0/0.0</td><td>0.07/0.16</td><td>0.0/0.01</td><td>0.0/0.0</td><td>0.0/0.0</td></tr><tr><td>2006 RH<sub>120</sub></td><td>1.03</td><td>0.02</td><td>0.59</td><td>5.93</td><td>29.50</td><td>0.23</td><td>3.5</td><td>0.27/0.05</td><td>0.73/0.82</td><td>0.0/0.0</td><td>0.0/0.13</td><td>0.0/0.0</td><td>0.0/0.0</td><td>0.0/0.0</td></tr></tbody></table>

**Notes.** (1) Object name; (2) semimajor axis; (3) eccentricity; (4) inclination; (5) Tisserand's parameter, where $T_j = \frac{a_j}{a} + 2 \cos i \sqrt{\frac{a}{a_j}(1 - e^2)}$ and $a_j = 5.2$ au is the semimajor axis of Jupiter; (6) absolute magnitude; (7) visible geometric albedo inferred from the albedo model of A. Morbidelli et al. (2020) and other sources as noted; (8) object diameter, calculated from the absolute magnitudes in column 6 and the albedos in column 7, as well as diameter measurements from other sources where noted. Source probabilities inferred from M. Granvik et al. (2018) and NEOMOD (D. Nesvorný et al. 2023); (9) $\nu_6$ resonance source probability; (10) 3:1 resonance source probability; (11) 5:2 resonance source probability; (12) 7:3 resonance source probability; (13) 8:3 resonance source probability; (14) 9:4 resonance source probability; (15) 11:5 resonance source probability; (16) 2:1 resonance source probability; (17) weak inner resonances source probability; (18) Hungaria source probability; (19) Phocaea source probability; (20) Jupiter family comet source probability. The orbital elements of these asteroids were taken from JPL HORIZONS accessed on 2025 January 13. The value of $p_{3:1}$ determined from NEOMOD was calculated by combining the NEO source probability for the weak inner Main Belt resonances and the 3:1 mean motion resonance. The value of $p_{5:2}$ determined from NEOMOD was calculated by combining the probability for NEOs originating from the 7:3, 5:2, and 8:3 mean motion resonances. The value of $p_{2:1}$ determined from NEOMOD was calculated by combining the probability for NEOs originating from the 9:4, 11:5, and 2:1 mean motion resonances.

<sup>a</sup> Albedos for 1998 FR<sub>11</sub>, 2001 ME<sub>1</sub>, and 2012 UR<sub>158</sub> derived by combining the diameter measured in NEOWISE observations and their absolute magnitudes.

<sup>b</sup> Albedo for 1998 KY<sub>26</sub> derived by combining the diameter measured by radar observations (S. J. Ostro et al. 1999) and its absolute magnitude. <sup>1</sup>Diameters for 1998 FR<sub>11</sub>, 2001 ME<sub>1</sub>, and 2012 UR<sub>158</sub> measured from NEOWISE observations. <sup>2</sup> Diameter for 1998 KY<sub>26</sub> measured from radar observations (S. J. Ostro et al. 1999).
for some large nongravitational objects between the M. Granvik et al. (2018) and D. Nesvorný et al. (2024) may be due to differences in assumption in the rate at which JFCs become dormant versus disrupting once they reach perihelion passages below 2.5 au (D. Nesvorný, private communication).

## Acknowledgments

The authors wish to thank Alessandro Morbidelli for his help with computing the NEO source probabilities for the objects studied in this manuscript, and Josh Walawender for supporting the Keck/LRIS observations. The authors also wish to thank the Gemini North operations staff for help coordinating the observations described in this manuscript.

Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 nonprofit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

Based on observations obtained at the international Gemini Observatory, a program of NSF NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the U.S. National Science Foundation on behalf of the Gemini Observatory partnership: the U.S. National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea).

Keck and Gemini North Observatory is located on Maunakea, land of the Kānaka Maoli people, and a mountain of considerable cultural, natural, and ecological significance to the indigenous Hawaiian people. The authors wish to acknowledge the importance and reverence of Maunakea and express gratitude for the opportunity to conduct observations from the mountain.

Facilities: Keck:I (LRIS), Gemini:Gillett.

## ORCID iDs

Bryce T. Bolin https://orcid.org/0000-0002-4950-6323

Christoffer Fremling https://orcid.org/0000-0002-4223-103X

Matthew Belyakov https://orcid.org/0000-0003-4778-6170

Jin Beniyama https://orcid.org/0000-0003-4863-5577

Marco Delbo https://orcid.org/0000-0002-8963-2404

Robert Jedicke https://orcid.org/0000-0001-7830-028X

Ian Wong https://orcid.org/0000-0001-9665-8429

Laura-May Abron https://orcid.org/0009-0000-8709-5273

Keith S. Noll https://orcid.org/0000-0002-6013-9384

Andrew W. Stephens https://orcid.org/0000-0002-4434-2307

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