Hayabusa2 achieves closest flyby in history — 400 meters from asteroid surface

It was revealed that when the spacecraft "Hayabusa2" explored the asteroid Torifune last month, it achieved the closest flyby crossing an asteroid in history. The Japan Aerospace Exploration Agency (JAXA) and partners announced the news. At approximately 400 meters from the surface, it broke the record set by China in 2012. In an asteroid flyby, precision laser range-finding via irradiation from a spacecraft was also successfully accomplished for the first time. It is said that the achievements expanded the possibilities of asteroid exploration and led to the acquisition of technologies useful for planetary defense and protecting humanity living on Earth from celestial collisions.

A flyby is an operational method where a spacecraft does not remain above a celestial body but conducts exploration for a very short time when passing nearby. To aim for the target point at closest approach, Hayabusa2 first repeated trajectory control by firing ion engines (electric propulsion engines) for acceleration while communicating with Earth. On the afternoon of the 5th last month, it transitioned to autonomous control by the spacecraft's computer. At 6:30 p.m. on the same day, it passed Torifune at its closest approach.

At closest approach, it was 100 million kilometers away from Earth, with a relative velocity of 5.3 kilometers per second. As a result of detailed analysis, it was found that the distance at this time was approximately 744 meters from the center of Torifune and approximately 400 meters from its surface. This surpassed the record of Chinese lunar orbiter "Chang'e 2" approaching within around 770 meters from the surface of the asteroid Toutatis in 2012, updating the shortest distance for an asteroid flyby.

Professor Hiroshi Takeuchi at JAXA Institute of Space and Astronautical Science explained at an online press conference on the 30th of last month that the course of the series of trajectory controls was extremely good. "Through onboard (autonomous) control, we reached this closest approach distance, which could not be done from the ground (via control through communication with Earth). It was controlled very beautifully," he stated. However, for about 10 minutes immediately before closest approach, data for "optical navigation," by which the spacecraft photographs celestial bodies to calculate relative positions, could not be incorporated, adversely affecting precision. Although it deviated by approximately 120 meters from the target point at closest approach in this way, it ended up exceeding the plan that aimed for approximately 800 meters from the center of Torifune, achieving approximately 744 meters.

Torifune is believed to have been formed by two asteroids colliding and combining, shaped like a snowman with a constriction. Through detailed analysis, it was found to measure approximately 840 meters in length and approximately 340 meters in width. It is said that the orbit of Torifune was also estimated with high precision through the flyby exploration.

Laser range-finding, achieved for the first time during an asteroid flyby, determines distance by measuring the time it takes for a laser pulse reflected from a target to return. Range measurements were successfully obtained about four and three seconds before closest approach to Torifune, at distances of roughly 20 km and 15 km, respectively.

Hayabusa2 remained above another asteroid Ryugu from June 2018 to November 2019, landing and collecting samples. Associate Professor Makoto Yoshikawa explained: "At that time, we descended to Ryugu at a speed of several dozen centimeters per second or slower. It was a laser altimeter built for that speed, and we had not envisaged using it at a high speed of 5.3 kilometers per second as in this case." JAXA and partners highlighted the value of the achievement, stating: "Like yabusame (horseback archery) controlling a horse while adjusting posture and shooting an arrow without missing the moment the target is captured, a laser was aimed at tiny Torifune from a spacecraft moving at high speed."

Through flyby exploration, observations via photography and infrared cameras were also successfully accomplished. Regarding the scientific results, Yoshikawa stated: "Fairly good observation data was obtained and analysis is advancing, so please look forward to it."

Through updating the closest approach record in flyby exploration and the success of laser range-finding, "major results were obtained that increase the degree of freedom in future exploration," said Takeuchi. In the field of planetary defense, high-precision control technology was demonstrated to collide a spacecraft into celestial bodies that pose a risk of colliding with Earth to divert their orbits. If a celestial body colliding with Earth is found and the time margin is short, dispatching a spacecraft already navigating for another purpose to conduct emergency exploration can be considered. The research confirms that effectiveness as well.

Lead Research Manager Yuya Mimasu, who serves as team leader, stated: "I am proud of the hard-working team. The spacecraft delivered wonderful results again."

Hayabusa2 delivered samples of Ryugu to Earth in 2020, and as of the 14th of August after departing again, it has already navigated over 5.6 billion kilometers. It is located approximately 68 million kilometers away from Earth. Going forward, through two events of "swing-by," which changes the trajectory by utilizing Earth's gravity, in December 2027 and June 2028, it is planned to arrive at the final destination asteroid 1998 KY26 in July 2031. It is a one-way ticket, and it will not return to Earth again.