1998 KY26
Near-Earth asteroid
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Through Comet and Science (journal)
1998 KY26 is believed be to most likely a solid chunk of rock, resembling one of the boulders seen on large rubble pile asteroids like Itokawa and Ryugu. Telescope observations have shown it to be an E-type asteroid, whose surface is bright white and rich in enstatite. It shares a similar composition and orbital inclination as 44 Nysa, which suggests that they formed together from a catastrophic collision event. Due to 1998 KY26's small size, its rotation and orbit are gradually accelerated by sunlight-related forces, such as the Yarkovsky and YORP effects. Studies from 2025 have shown no evidence of cometary activity and anomalous acceleration in 1998 KY26, which disproves the 2023 speculation that it might be a dark comet.
Observational history
Discovery
1998 KY26 was discovered by Tom Gehrels on the night of 28 May 1998, during routine observations for the University of Arizona's Spacewatch project. The Spacewatch project had the goal of discovering near-Earth asteroids and conducted its observations using a 0.9-meter (35 in) telescope at Kitt Peak, Arizona. In the discovery images, 1998 KY26 appeared as a small dot moving among background stars with an apparent magnitude of 18.9. During this time, 1998 KY26 was approaching Earth from an initial distance of 0.03 astronomical units (4.5 million km or 2.8 million mi).
Follow-up observations of 1998 KY26 began on 31 May 1998, starting with Spacewatch. Several other observatories around the world, including Kleť and Ondřejov Observatory in the Czech Republic, followed suit. With multiple days of observations, the Minor Planet Center (MPC) announced the discovery of 1998 KY26 on 1 June 1998, one week before the asteroid was set to make its closest approach to Earth.
First characterization
By the time 1998 KY26's discovery was announced, the asteroid was still approaching Earth and brightening in the sky. The asteroid was predicted to brighten up to apparent magnitude 16 and pass within 2.1 lunar distances (810,000 km; 500,000 mi) of Earth, providing astronomers the opportunity to study it in detail. From 2 to 8 June, astronomers monitored the brightness of 1998 KY26, revealing that it rotates rapidly with a period of several minutes. On 3 June, astronomers made the first measurement of 1998 KY26's color. During 1998 KY26's closest approach between 6 and 8 June, a team of astronomers led by Steven J. Ostro observed the asteroid with the Goldstone Solar System Radar in California, confirming earlier measurements of its rapid rotation and providing the first direct measurement of its size and shape. In July 1999, Ostro's team published their results in the journal Science and in a Jet Propulsion Laboratory press release, proclaiming 1998 KY26 as the smallest and fastest-rotating asteroid studied in detail at the time.
Hayabusa2 preparation
In 2020, the Japan Aerospace Exploration Agency (JAXA) announced its plan to visit 1998 KY26 in 2031 with its operational Hayabusa2 space probe. However, at the time the announcement was made, 1998 KY26 still had a large orbital uncertainty. The asteroid had not been observed since 2002 because it was too dim and far away from Earth. Astronomers had to wait for 1998 KY26's close approach to Earth every three and a half years before they could observe it. By monitoring 1998 KY26's position over several years, its orbital uncertainty can be reduced.
In December 2020, 1998 KY26 was predicted to pass within 0.47 AU (70 million km; 44 million mi) of Earth. During that time, the asteroid was predicted to brighten up to apparent magnitude 25, making it very dim and detectable to only the largest optical telescopes. Between 10 and 12 December, European and Japanese teams of astronomers observed 1998 KY26 with the Very Large Telescope and Subaru Telescope, respectively. The MPC reported the recovery of 1998 KY26 on 15 December 2020.
In June 2024, 1998 KY26 made a closer approach to Earth at a distance of 12 LD (4.6 million km; 2.9 million mi). The asteroid was predicted to brighten up to apparent magnitude 20, once again providing astronomers the chance to further refine its orbit and study its properties. During May to June and October to November, a team of astronomers led by Toni Santana-Ros observed 1998 KY26 using various large optical telescopes around the world. They found that the asteroid was three times smaller and rotating twice as fast as previously thought. They additionally reanalyzed radar data from 1998 and found that it matches their new results. The team published their findings in September 2025, through the journal Nature Communications and in press releases by their affiliated institutions.
1998 KY26 will make its next close approach to Earth in 2028, at a minimum distance of 0.35 AU (52 million km; 33 million mi). Although the asteroid will be farther away, it may be observable with the James Webb Space Telescope (JWST) in March 2028 or October 2028. The JWST may be able provide a more definitive measurement of 1998 KY26's size and thermal properties before Hayabusa2's arrival in 2031.
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Name
This asteroid is currently known by its minor planet provisional designation 1998 KY26, given by the MPC upon its discovery. The provisional designation encodes the year, half-month, and order of the asteroid's discovery. It has not been given a formal name and a minor planet number, because it has not been observed over enough oppositions. Once the MPC gives 1998 KY26 a number, the asteroid's discoverer can propose a name for it. According to naming guidelines by the International Astronomical Union's Working Group for Small Bodies Nomenclature, near-Earth asteroids with perihelion distances smaller than 1.15 AU should be given mythological names not related to creation or the underworld.
Orbit and classification
1998 KY26 orbits the Sun at an average distance (semi-major axis) of 1.23 astronomical units (AU), completing one orbit every 1.36 years. It follows a slightly elliptical and tilted orbit, with an orbital eccentricity of 0.20 and an inclination of 1.5° with respect to the ecliptic. Throughout its elliptical orbit, 1998 KY26's distance from the Sun ranges from 0.98 AU at perihelion to 1.47 AU at aphelion. It crosses the orbit of Earth with a minimum orbit intersection distance of 0.00256 AU (383,000 km; 238,000 mi), allowing for close approaches to the planet.
Since 1998 KY26 comes close to Earth, it is classified as a near-Earth asteroid. It is specifically classified as an Apollo asteroid, because it has a wide, Earth-crossing orbit with a semi-major axis above 1 AU and a perihelion below 1.017 AU. It makes close approaches to Earth every three and a half years, although the distances vary from 0.005 to 0.5 AU (0.75 to 74.80 million km; 0.46 to 46.48 million mi). 1998 KY26 poses no threat to Earth; if it were on a collision course with the planet, the asteroid would most likely explode in the upper atmosphere and harmlessly drop its fragments to the ground.
The shape and low inclination of 1998 KY26's orbit makes it an accessible target for intercepting with a space probe. For a spacecraft to leave low-Earth orbit and enter a rendezvous trajectory toward 1998 KY26, it would require a total velocity change (Δv) of 4.2 km/s, lower than that needed for a rendezvous with the Moon (6 km/s). A Hohmann transfer of a spacecraft onto a rendezvous with 1998 KY26 requires a minimum Δv of 3.34 km/s. For a direct Earth-to-1998 KY26 transfer involving two burn impulses, the required Δv is even smaller at 0.82 km/s.
Non-gravitational acceleration
Due to 1998 KY26's small size and proximity to the Sun, it exhibits noticeable non-gravitational acceleration from sunlight-related forces, such as the Yarkovsky effect. The Yarkovsky effect involves the asteroid absorbing sunlight and asymmetrically re-emitting it as thermal radiation, which causes the asteroid to recoil and deviate from its orbital path. Since 1998 KY26 is rotating backwards (retrograde) relative to its orbital direction, the Yarkovsky effect gradually shrinks its orbital semi-major axis. As of 2025, the Yarkovsky effect is believed to be largely responsible for 1998 KY26's observed non-gravitational acceleration. Based on data as of 2024, the Jet Propulsion Laboratory calculates 1998 KY26's non-gravitational acceleration to be (3.66±3.87)×10−11 AU/day2, (−2.20±0.49)×10−14 AU/day2, and (−1.69±1.12)×10−12 AU/day2 in the radial (A1), transverse (A2), and normal (A3) directions, respectively. Although 1998 KY26 experiences a stronger solar radiation pressure than the Yarkovsky effect, the radiation pressure is less efficient at perturbing the asteroid, because it mainly acts in the radial direction.
The Yarkovsky effect was first predicted for 1998 KY26 in 2000, but it was not detected until 2023. In a 2023 study, a team led by Darryl Seligman noticed that 1998 KY26 displayed a large normal acceleration, which seemingly could not be explained by the Yarkovsky effect. In that study, Seligman's team proposed that 1998 KY26 might be a dark comet, whose normal acceleration might be caused by unseen outgassing. The dark comet hypothesis for 1998 KY26 was disproven in 2025; observations from 2024 showed no evidence of ejected dust from outgassing, and a 2025 reanalysis led by Davide Farnocchia showed that the asteroid's non-gravitational acceleration can be fully explained by the Yarkovsky effect when using the asteroid's updated properties from 2024.
Physical properties
Shape, rotation, and structure
1998 KY26 is a tiny asteroid with a diameter of 11 ± 2 meters (36 ± 7 ft), roughly twice the width of the Hayabusa2 spacecraft (6 m or 20 ft). The asteroid's diameter was determined from radar and optical telescope measurements of its brightness and albedo. Due to its small size, the asteroid's gravity is expected to be extremely weak, with an estimated escape velocity of about 1 cm/s. Measurements of 1998 KY26's Yarkovsky acceleration indicate that the asteroid has an area-to-mass ratio of (5.54±0.97)×10−5 m2/kg and a bulk density of 2.8+1.6
−1.0 g/cm3.
1998 KY26 rotates rapidly with a period of 5.35 minutes, with a measurement uncertainty of 6 milliseconds. As it rotates, its brightness fluctuates due to its elongated shape. Observations of 1998 KY26's brightness variations from 1998 to 2024 show that the asteroid has an asymmetric and moderately elongated shape, with slight flattening along its rotation axes. Modeling of the asteroid's shape suggests that it has a large concavity close to its southern hemisphere.
The asteroid's rotational north pole points toward the ecliptic coordinates (λ, β) = (36°, −44°), giving it an axial tilt of 136° with respect to the ecliptic. This makes the asteroid's rotation retrograde, meaning it rotates backwards relative to its orbital direction. Calculations predict that the asteroid's rotation is gradually speeding up due to the YORP effect. The YORP effect is estimated to cause an annual fractional change of 7×10−6 to 1998 KY26's rotation period, meaning the rotation period decreases by roughly 2 milliseconds per year. The YORP effect has not yet been detected in 1998 KY26, and it is not expected to produce a noticeable change to its rotation period by 2030.
Due to 1998 KY26's small size and rapid rotation, centrifugal forces exceed the surface gravity at its equator. For this reason, the asteroid's surface is expected to lack loose boulders, as they would be flung away. For material to stick onto 1998 KY26's surface, it must have a minimum cohesive strength between 4 and 20 pascals. While this makes it possible for 1998 KY26 to have a rubble pile structure, models suggest that tiny, decameter-sized rubble piles are unlikely to form. 1998 KY26 is instead more likely to be a solid chunk of rock, resembling one of the boulders seen on the surfaces of larger rubble pile asteroids like Itokawa and Ryugu. Any loose material on 1998 KY26's surface would most likely exist as fine grains covering the asteroid's poles, where centrifugal acceleration is minimal.
Surface composition
Spectroscopic observations from 2024 have shown that 1998 KY26 is an E-type asteroid, meaning it is a highly reflective asteroid mainly made of enstatite. In visible light, 1998 KY26 appears white. Its visible spectrum most closely resembles those of EL- and EH-type enstatite chondrite meteorites, although its high geometric albedo of 0.52 aligns more closely with aubrite (enstatite achondrite) meteorites.
In the Bus–DeMeo and Tholen classification systems for asteroid spectral types, 1998 KY26 is classified as an X-type asteroid. This is because the visible spectra of E-type asteroids are often indistinguishable from those of metallic M-type and organic-rich P-type asteroids, so they are collectively categorized as X-type. 1998 KY26 exhibits a subtle absorption band at the visible wavelength of 0.49 μm, which might indicate the presence of troilite or other opaque minerals. This absorption band distinguishes the asteroid as an Xe-type in the Bus–DeMeo system. In the Solar System, E-type and Xe-type asteroids are relatively rare.
Early studies from 1998 previously suggested that 1998 KY26 might be carbonaceous and water-rich, but this was disproven by observations from 2024.
Origin
A 2026 study led by Eri Tatsumi found that 1998 KY26's surface composition resembles those of the X-type asteroids 44 Nysa and 135 Hertha. Although these two asteroids differ in composition, they share similar low-inclination orbits alongside many other asteroids in a region of the inner main asteroid belt, known as the Nysa–Polana complex. The compositions of Nysa and Hertha suggest that they originated from different layers of a now-destroyed parent body. Based on this idea, Tatsumi and colleagues proposed that 1998 KY26 and Nysa originated as fragments of this parent body's enstatite-rich crust, while Hertha originated from deeper layers.
Theoretical models indicate that within a few million years after the parent body's destruction, the semi-major axes of small fragments like 1998 KY26 significantly drift inward due to the Yarkovsky effect. These drifting asteroids eventually become captured in the ν6 secular resonance 2 AU away from the Sun, in which their apsidal precession rate matches that of Saturn. The ν6 secular resonance raises the eccentricity of these drifting asteroids, which ultimately directs them into Earth-crossing orbits. The low inclination of 1998 KY26's present-day orbit is consistent with those of asteroids in the Nysa–Polana complex, which supports the hypothesis that they share a common origin. Previous studies had suggested a possible origin from the highly-inclined Hungaria family, but Tatsumi and colleagues argued that it is implausible for 1998 KY26 to have undergone drastic changes to its inclination.
In 2026, Avi Loeb and colleagues proposed that 1998 KY26 might be the lost Soviet spacecraft Phobos 1. Their analysis, which as of June 2026 was not peer reviewed, argued that the spacecraft's available propulsion could account for the difference between the two trajectories and noted similarities in size, reflectivity and inferred shape. The proposed identification remains unconfirmed.
Exploration
In 2020, JAXA announced a mission extension for its Hayabusa2 spacecraft, which successfully retrieved samples of asteroid Ryugu in 2019. This extended mission, titled the Small Hazardous Asteroid Reconnaissance Probe (SHARP or ♯), tasked the spacecraft to visit two more near-Earth asteroids: 98943 Torifune in July 2026 and a rendezvous with 1998 KY26 in July 2031. These asteroids were chosen because they required a low amount of fuel (Δv < 1.6 km/s) to reach within 10 years. JAXA presented the initial plan in July 2020 with the choice of visiting either 1998 KY26 or 2001 AV43, but ultimately selected 1998 KY26 in September 2020.
When Hayabusa2 arrives, 1998 KY26 will become the smallest asteroid ever visited by a spacecraft. The exploration of 1998 KY26 will provide insights into the properties of decameter-sized asteroids, which will aid planetary defense efforts. During its rendezvous, Hayabusa2 will slowly approach 1998 KY26 and then enter orbit around the asteroid. The spacecraft will attempt to create an impact crater on 1998 KY26 by deploying its final remaining target marker and projectile. Hayabusa2 will then attempt to land on the surface of 1998 KY26. Due to the asteroid's rapid rotation, landing on its surface would be challenging; the safest place for Hayabusa2 to land would be the asteroid's poles, where centrifugal forces are minimal.