Another success for Hayabusa 2 as it completes a flyby of asteroid Torifune

Hayabusa 2’s primary mission is now well in the past. JAXA’s asteroid-sampling spacecraft rendezvoused with asteroid Ryugu in June 2018. It studied the asteroid for 1.5 years and gathered a sample that was returned to Earth in December 2020.

After that successful endeavor, Hayabusa 2 was sent on its way to visit other targets, though another sample return is not possible. It’s on its way to visit a tiny asteroid called 1998 KY₂₆, a near-Earth object (NEO) only about 11 meters (36 feet) in diameter. But on its way, it also flew past another asteroid named Torifune (98943 Torifune).

Ground-based observations showed that Torifune is a near-Earth asteroid (NEA) measuring about 450 meters (1,476 feet) in diameter. It’s an S-type asteroid, meaning it’s a stony or siliceous type. These are high-density objects that make up about 17% of the asteroid population, making them the second most common type after carbonaceous C-type asteroids.

Hayabusa 2 began looking at Torifune in June with its Optical Navigation Camera—Telescopic (ONC-T). The ONC-T directly imaged Torifune on June 20 for navigation purposes.

Then on July 5, Hayabusa 2 came to within about 800 meters (2,625 feet) of the asteroid. It used ONC-T to capture images of Torifune that revealed details about its surface. While ground-based observations showed the asteroid was elongated, hinting at its contact binary nature, only these images confirmed it. In contact binaries, two separate asteroids orbited a common center of mass until they spiraled toward each other and joined into one. Contact binary asteroids aren’t expected to be rare.

Beginning about one hour before closest approach, Hayabusa 2 also observed Torifune with its other instruments: the NIRS3 (Near-Infrared Spectrometer), TIR (Thermal InfraRed Imager) and LIDAR (Light Detection and Ranging).

Hayabusa 2 was traveling very fast during this flyby. Its relative speed was 5 km/s (3.1 mi/s), making navigation and image capture challenging. It has less than half of its xenon propellant remaining, enough to power its ion thrusters to reach Torifune and 1998 KY₂₆, but not enough for any extra maneuvers.

Hayabusa 2’s next milestone will be in December 2027, when it swings past Earth. Then in June 2028, it will swing by Earth again. That will set it up for its rendezvous with 1998 KY₂₆ in July 2031.

The exact nature of 1998 KY₂₆ isn’t clear. Observations in optical and radar suggest that it’s a water-rich asteroid, and since it’s known to be a fast rotator, it’s almost certainly one single chunk of rock rather than a rubble-pile asteroid. It could also potentially be an X-type asteroid, which is a catchall term that encapsulates objects that look similar through a telescope but are composed of different materials.

Not all of Hayabusa 2’s data from its flyby of Torifune has yet reached Earth, so these are preliminary results. JAXA will release more in the near future.

Who’s behind this story?

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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