Can a spacecraft land on a teeny tiny asteroid? Japan's Hayabusa2 will certainly try

The Hayabusa2 mission is facing a new challenge after observations showed the asteroid it is going to rendezvous with and touchdown on in 2031 is much smaller than previously thought; it's also spinning much faster.

"We found that the reality of the object is completely different from what it was previously described as," Toni Santana-Ros said in a statement. Santana-Ros is an astronomer at the University of Alicante and affiliated with the University of Barcelona, and led the observations of the asteroid with the European Southern Observatory's Very Large Telescope (VLT) in Chile.

Astronomers had previously used brightness measurements to calculate the asteroid, designated KY26, should be about 98 feet (30 meters) across and spin on its axis once every 10 minutes.

However, a new set of observations from telescopes around the world, including the VLT and combined with radar data, has shown how astronomers had overestimated KY26's properties. Instead of being 98 feet wide, KY26 is really a mere 36 feet (11 meters) across, which is so small it could actually fit inside the dome housing one of the VLT's eight-meter telescopes. Meanwhile, its rotation is so fast it completes one revolution every five minutes.

"The smaller size and faster rotation now measured will make Hayabusa2's visit even more interesting, but also even more challenging," Olivier Hainaut, who is an astronomer at the European Southern Observatory, said in the statement.

Having rendezvoused with the 2,953-foot-wide (900-meter-wide) asteroid Ryugu in 2020 and returned a sample to Earth, the Japanese Aerospace Exploration 'Hayabusa2 mission is now on an extended mission taking it to KY26 in the year 2031, with a plan to orbit the space rock and touchdown in an attempt to learn more about the structure and composition of small asteroids.

"We have never seen a 10-meter-size [33-foot] asteroid in situ, so we don't really know what to expect and how it will look," said Santana-Ros.

The observations strongly suggest that the asteroid is made of chunks of solid rock, rather than being a loose, dusty, rubble pile — although Santana-Ros is at pains to point out that a rubble pile cannot be completely ruled out yet. Hayabusa2 will be able to determine KY26's exact structure and composition when it arrives.

Hayabusa2's mission is designed to learn more about small asteroids, which are the type that most frequently impact Earth. However, the VLT's observations prove that we can learn much about them without even leaving terra firma.

"We now know we can characterize even the smallest hazardous asteroids that could impact Earth, such as the one that hit near Chelyabinsk in Russia in 2013, which was barely larger than KY26," said Hainaut.

Santana-Ros sees even more possibilities beyond planetary defense. "Our methods could have an impact on the plans for future near-Earth asteroid exploration or even asteroid mining," he said.

The observations of KY26 were reported on Sept. 18 in the journal Nature Communications.