Hayabusa2’s flyby of asteroid Torifune

On Sunday, July 5, the Japan Aerospace Exploration Agency (JAXA)'s Hayabusa2 probe performed a close flyby of the asteroid Torifune. 

What is asteroid Torifune?

Torifune is a near-Earth asteroid roughly 450 meters (1,475 feet) in diameter. Telescopic observations from Earth revealed an elongated shape, which Hayabusa2 has now imaged at high resolution to show two distinct lobes. 

The asteroid got its name from a contest led by JAXA, in which a committee of Hayabusa2 team members and a group of schoolchildren reviewed names submitted to the campaign by the public. The winner, "Torifune," is an abbreviation of Ame-no-torifune, the name of a both Japanese god and his ship, which are said to be able to travel safely and steadily at high speed.

Why did Hayabusa2 perform this flyby?

This flyby was part of Hayabusa2’s extended mission. The spacecraft has already completed its primary mission, which was to collect samples of the near-Earth asteroid Ryugu and return them to Earth in 2020.  

Hayabusa2 aimed to fly past Torifune as close as possible without colliding, and it came as close as 800 meters (0.5 miles) from the asteroid’s center. Because of Torifune’s small size and low reflectivity, Hayabusa2 wasn’t able to plot its exact trajectory past the asteroid until a few days before the flyby, requiring last-minute navigational adjustments. JAXA saw the flyby as an opportunity to test the kind of orbital maneuvering that would be necessary for a planned collision with an asteroid, also known as the kinetic impactor method of asteroid deflection. This added to Hayabusa2’s value to planetary defense efforts, along with the mission’s study of its asteroid targets themselves. 

What’s next for Hayabusa2?

The mission’s final objective is to conduct a close flyby of the asteroid 1998 KY26.

At just 11 meters (36 feet) in diameter, 1998 KY26 would be the smallest asteroid visited up-close by a spacecraft. Hayabusa2 is expected to reach this final target in 2031, when it will enter into orbit around the asteroid and attempt to land on its surface. This will be challenging due to KY26’s small size and extremely rapid rotation. 

This final objective will also serve humanity’s efforts to defend our planet from impacts. 1998 KY26 is about half the size of the asteroid that exploded above Chelyabinsk, Russia in 2013. Asteroids of this size collide with Earth once every 10 to 100 years or so, on average, and can cause significant damage, but are too small to be easily spotted by ground-based telescopes. Missions like Hayabusa2 can yield important discoveries about these potentially dangerous little space rocks.