Japan’s Hayabusa2 spacecraft is now just a few months away from its destination, a bizarre object named Ryugu that switches between spinning and tumbling through space.
New observations suggest asteroid 1998 KY26 is about 11 meters wide, or roughly the size of a school bus, and only takes a little over five minutes to complete a full rotation. It also bounces back more sunlight than previous models had guessed. The results, which were published Wednesday in Nature Communications by a team that included researchers from the European Southern Observatory, suggest navigation, imaging, and any close-up operations will be orders of magnitude more challenging.
A bus-sized target with a rapid five-minute day
Small as this is, gravity is almost absent and a spin that fast means the surface spins smoothly underneath any probe hovering above it. Depending on whether it is a monolithic solid or a “rubble pile,” with its grains held by gravity only, rapid rotation can come close to the rate at which material would be flung off by centrifugal effects. There’s not enough light-curve data and reflectivity to confirm either one, according to the research team headed up by Toni Santana-Ros.
High albedo is a double-edged sword. It reduces the complexities of thermal modeling; however, it may also saturate optical navigation sensors that are supposed to lock on high-contrast surface features. Habitable-zone surveys are further confounded by how much they can get bleached out from quick, high-res mapping and compositional scans for one.
Why the new numbers are important for operations
While the team behind Hayabusa2 has already demonstrated its prowess at close operations (sailing with precision around Ryugu), 1998 KY26 would require an even more delicate dance. Such low gravity, combined with the sensitive equipment, means that gentle thruster firings could rock the craft’s relative motion. Combine that with a whirling, nearly featureless surface and it is more difficult for guidance systems to track landmarks without blur or glare.
If impact experiments were accounted for, there would be less room to work. Smaller targets radiate energy in a different pattern, and rapid rotation can fling ejected material unpredictably. A more reflective surface can additionally “blind” stand-alone imagers, requiring adjustments or retargeting of the exposure time, approach viewing angle, or lighting scenarios.
The good news is that mission planners caught this packet early. The authors highlight the benefits of having these parameters in time for redesign, which allows JAXA to modify instrument sequences, tighten optical navigation thresholds, and develop proximity operations plans around a target that changes rapidly.
How astronomers measured KY26 during its close pass
The campaign to characterize the asteroid took advantage of a recent near pass by Earth, when it was temporarily bright enough that multiple facilities could make detailed observations. Using time-series photometry from multiple telescopes, including ESO facilities, the team deduced a rapid light curve from which they derived the rotation period. Thermal modeling and brightness comparisons limited its diameter and reflectivity, which indicated a smaller, shinier object than had previously been estimated.
“The scientific potential of characterizing a fast spinner like 2I/Borisov as it experiences solar heating on its journey through the Solar System is also complemented by technical challenges,” writes ESO astronomer Olivier Hainaut and colleagues. Those are rare targets to explore how microgravity, cohesion, and spin interact at the smallest asteroids — regimes in which even tiny differences in material strength or sunlight-driven torque (the YORP effect) can change the shape of an object over time.
Tiny asteroids, big stakes for science and safety
Very small asteroids were deliberately put at the top of JAXA’s Hayabusa2 list for its mission extension because none has been visited at close range. But these bodies do matter for science and safety. Objects a few tens of meters across enter Earth’s atmosphere much more often than the kilometer-scale behemoths, and—while they may not make it all the way to the ground—detonating in midair can have serious consequences. As such, the Chelyabinsk incident still serves as a stark reminder of just how much oomph even a small rock can pack.
Planetary defense advocates point out that more than 95% of the largest, civilization-threatening near-Earth objects have been discovered, but the population at tens-of-meters scale is being counted in a vastly under-calculated fashion. Missions like Hayabusa2 are helping to close that gap. They follow on decades of small-body exploration — from JAXA’s visits to Itokawa and Ryugu and NASA’s OSIRIS-REx, which returned samples from Bennu, to Psyche, now on its way there to investigate a metal-rich world.
What’s next for Hayabusa2 on its extended mission
Hayabusa2 has a mission to fly by the minuscule asteroid 2001 AV43, then home in on 1998 KY26. JAXA picked the duo out of more than 350 candidates within range and which would return valuable science considering leftover xenon propellant aboard Hayabusa2. Closer inspection of KY26 is likely to target its rotation, mass, surface texture, and composition — variables that are now defined by the new study’s tighter bounds.
Careful optimism from the veterans of small-body operations: “As the asteroid’s shape grew more irregular, its complexity grew higher,” says Dante Lauretta, OSIRIS-REx’s principal investigator, adding that at this point the overall concept of operations doesn’t need to be dramatically revised, but teams may need to adjust camera exposure settings, approach geometries, and guidance filters to account for a brighter target that is rotating more quickly.
Put simply, KY26 has gone from being a routine rendezvous opportunity into a tightrope act. That’s how discovery simply has to happen: at the boundary of what we can predict, where a spinning speck forces us to rethink ways of exploring the smallest worlds.