This Japanese spacecraft is due to land on an asteroid, but it could be in for a surprise

A Japanese spacecraft tasked with landing on an asteroid near Earth will likely have a harder time than initially thought.

That's according to astronomers who've been studying the asteroid with some of the most powerful telescopes on Earth.

They used multiple telescopes in Chile, and one on the Spanish island of La Palma, to study the size and rotation speed of asteroid 1998 KY26, which is the target of the Hayabusa2 mission.

Hayabusa2 is set to land on the asteroid, collect a sample of it, and return it to Earth for study.

Smaller, faster, trickier?

Observations of asteroid 1998 KY26 show it's almost three times smaller and spinning much faster than previously thought.

The asteroid is the target for the Japan Aerospace Exploration Agency's (JAXA) Hayabusa2 mission, which successfully gathered samples from another asteroid, Ryugu, on 22 February 2019.

Mission scientsts then extended the mission, and Hayabusa2 is now set to gather a sample from asteroid 1998 KY26 in 2031.

"We found that the reality of the object is completely different from what it was previously described as," says astronomer Toni Santana-Ros from the University of Alicante, Spain, who led a study of 1998 KY26.

The observations were combined with radar data to reveal the asteroid is just 11 metres wide and is spinning twice as fast as initially thought.

It spins once every 5 minutes, whereas previous data had estimated the asteroid was about 30 metres in diameter and span once every 10 minutes.

"The smaller size and faster rotation now measured will make Hayabusa2’s visit even more interesting, but also even more challenging,” says co-author Olivier Hainaut, an astronomer at ESO in Germany.

Hayabusa2's landing is described as 'kissing' the asteroid, as it will briefly make contact, gather a sample then manoeuvre away from the spinning space rock.

Knowing it's smaller and spinning twice as fast makes things more difficult.

“The amazing story here is that we found that the size of the asteroid is comparable to the size of the spacecraft that is going to visit it! And we were able to characterise such a small object using our telescopes, which means that we can do it for other objects in the future,” says Santana-Ros.

"Our methods could have an impact on the plans for future near-Earth asteroid exploration or even asteroid mining."

Why kiss an asteroid?

Asteroids are primordial, unspoiled remnants left over from the formation of our Solar System.

By learning more about them, scientists can in turn learn more about the conditions under which our Solar System formed around a young Sun, 4.5 billion years ago.

Gathering spacerocks that fall to Earth – meteorites – is a key piece of the puzzle, but any rocks that have fallen to Earth are contaminated during their journey through our atmosphere, and during the period of time they've rested on Earth before being discovered.

Sample-return missions like Hayabusa2 or NASA's OSIRIS-REx enable pristine samples to be gathered and returned to Earth, where they're studied under laboratory conditions.

An extended odyssey

1998 KY26 is to be the final target asteroid for Hayabusa2.

Hayabusa2 explored the 900-metre-wide asteroid 162173 Ryugu in 2018, sending asteroid samples back to Earth in 2020.

Ryugu samples were found to be rich in organic molecules.

With fuel left over, the spacecraft was then sent on a mission to 1998 KY26, which it should encounter in 2031.

This will be the first time a spacecraft has encountered a tiny asteroid.

These new, ground-based observations of 1998 KY26 were carried out to support preparation for the encounter.

As well as gathering information on the asteroid's size and spin, the team also found the asteroid has a bright surface and is likely made of a solid chunk of rock that could have come from a piece of a planet or another asteroid.

But, say the team, they can't rule out that the asteroid is made of rubble piles stuck together.

"We have never seen a ten-metre-size asteroid in situ, so we don't really know what to expect and how it will look," says Santana-Ros.

The telescopes used in the study were the Very Large Telescope, the Gemini South Telescope, the Southern Astrophysical Research Telescope, the Víctor M. Blanco Telescope and the Gran Telescopio Canarias.