Asteroid 1998 KY26, the target of the extended mission of Hayabusa2. Image stack obtained with the Very Large Telescope of the European Southern Observatory. Credit: ESO / ESA NEOCC

On 6 December 2020, JAXA's Hayabusa2 spacecraft successfully completed its primary mission with the delivery of the sample container to Earth. Despite the great success of the project, the probe's mission might not be over yet. An extended mission is currently being considered: it will allow both a fly-by of a numbered asteroid, (98943) 2001 CC21, and more importantly a rendezvous with 1998 KY26, a small ~30 metre near-Earth asteroid first discovered in 1998 by the US-funded Spacewatch survey.

This target is particularly interesting, for a variety of reasons. It will, for example, be the first rendezvous with a tiny and fast-rotating asteroid. It is also an object that has been well observed with radar during its discovery apparition in 1998. This has allowed astronomers to derive an accurate tridimensional model of its shape.

However, despite the excellent characterisation effort back in 1998, the orbit of this asteroid was not known well enough to safely navigate the spacecraft. A recovery was needed, and the perfect opportunity was happening right now.

Our team, in the framework of our collaboration with ESO on near-Earth asteroids, was able to use ESO's VLT to target 1998 KY26 on 10 December 2020. At the time of the observation, the object was expected to reach magnitude 25.5, thus requiring very large telescopes. Recovering an object only observed 22 years earlier might appear challenging but, in this particular case, we could take advantages of the excellent radar detections obtained during the discovery apparition. This allowed the determination of a reasonably accurate ephemeris, with an uncertainty that could fit inside VLT's field of view. The object was successfully detected in the first set of frames, and then confirmed with an independent set of images collected two nights later. Around the same time, a Japanese team at the Subaru telescope also observed the target, providing an independent confirmation of the detection.

With the addition of our new observations, it is now possible to plan the mission in more detail. The object will become visible again in 2024, and observations collected during that apparition will be crucial to ensure the successful navigation of the spacecraft to the target, which should be reached in 2031.