Earlier this month, Japan’s veteran Hayabusa2 sample return spacecraft performed a surprisingly rapid and close-up flyby of a near Earth asteroid.

The Japan Aerospace Exploration Agency (JAXA) launched Hayabusa2 in 2014 to visit the asteroid Ryugu, arriving in June 2018, and returning samples back to Earth in December 2020. But after dropping off its precious cargo via reentry capsule, JAXA then sent the main spacecraft—designed for six years of deep space operations—on an extended mission, which includes this month’s flyby.

On 5 July the spacecraft made one of the closest asteroid flybys ever, even despite the aging and ailing spacecraft’s capabilities—having been designed for rendezvous and proximity operations around an asteroid, not high-speed flyby imaging.

Satoshi Tanaka of JAXA and its Institute of Space and Astronautical Science (ISAS), said that during the the 35th meeting of the NASA Small Bodies Assessment Group on 11 June that a standard flyby trajectory for the encounter would not suffice. He said that sending the craft within 100 kilometers of its target asteroid would still not provide enough resolution to reveal the asteroid’s global shape and major features. So the team opted for a more daring approach, one that required upgrading the probe from afar.

“By combining advanced navigation techniques and the engineering capabilities of Hayabusa2, we have made it possible to achieve a flyby at a distance of only about one kilometer,” Tanaka said.

Pushing Hayabusa2’s Flyby Limits on Just One Engine

Maneuvering Hayabusa2 very close to the asteroid meant that images captured during the encounter would reveal fascinating and valuable detail.

Yet, flashing past at a relative speed of 5.3 km per second leaves precious few moments to capture the target, especially considering the probe’s cameras were not designed for high-speed imaging. Hayabusa2 was also showing its age and revealing the side effects the mission’s previous successes. All of which required controlling the flyby on a second-by-second basis, Tanaka said.

“There’s only one ion engine left. It’s an old mission,” said Patrick Michel, principal investigator for the European Space Agency’s Hera asteroid mission and part of the Hayabusa2 science team. “It conducted two sampling operations, fired an impactor, came back, and its optics have been a little altered by dust from the sampling operation. All of that alone makes any operation challenging.”

Hayabusa2’s Optical Navigation Camera-Telescope (ONC-T) suffered significant degradation from the craft’s encounters with asteroid Ryugu. As described in a 2023 paper in the journal Earth, Planets and Space, ONC-T lost up to 15-20 percent of its sensitivity.

The telescope has also over time gradually accumulated “hot pixels,” a term describing cosmic ray damage to CCD sensors, which can lead the imaging elements to register false signals.

To compensate for Hayabusa2’s known shortcomings, the team uploaded an updated guidance, navigation, and control package, allowing the craft to make the most of its existing capabilities. The package also had Hayabusa2 perform an inflight calibration of its sensors.

The new flyby demanded optical guidance, which meant the probe’s telescope was consumed with navigation tasks until T-minus five minutes before closest approach. That’s when Hayabusa2 entered its science observation phase. This way the spacecraft could make the most of its limited field of view available during the high-speed maneuver, while still reducing the risk of unknowns during the spacecraft’s approach.

“It’s one thing to do celestial mechanics and define a trajectory,” Michel said. “But when you’re talking about flying by at a distance of one kilometer from an object that might be 400 meters or a kilometer along its longest axis, well, that’s interesting.”

Hayabusa2’s Science on the Fly(by)

In fact, uncertainty surroundingTorifune extended to whether it was a single object at all . “It could even be a contact binary,” Michel said, presciently, ahead of the flyby.

The returned images show Torifune as just such an asteroid, formed by two separate bodies that drifted together under gravity, reminiscent of comet 67P/Churyumov-Gerasimenko and the Kuiper Belt object 486958 Arrokoth.

JAXA released images of Torifune from ONC-T and from Hayabusa2’s Thermal Infrared Imager (TIR) camera. ONC-T’s image traces to approximately one second before the spacecraft’s closest approach. That was the image revealing a contact binary.

ONC-T’s image contains some blurring, but this was also predicted, with the nearer lobe moving across more pixels during the exposure than the more distant lobe. The TIR thermal emission image, taken a second earlier, when the probe was some 10 km away, also showed the same double-lobed structure. Further data, the agency said, is being analyzed and downlinked.

Beyond the wealth of scientific data generated, the ultra-close flyby was also a valuable test of rapid reconnaissance techniques for planetary defense. In a sense, Hayabusa2 provided a case study of conducting in situ investigations of a target asteroid. As demonstrated by NASA’s 2022 DART mission—a robotic probe sent to crash into a near Earth object to test how spacecraft impacts might one day deflect rogue asteroids—flyby reconnoiter missions like Hayabusa2’s could one day be crucial for studying asteroids that might ultimately be aimed at Earth. .

According to JAXA, the Torifune flyby is just the latest installment for Hayabusa2’s extended activities. If the spacecraft can continue functioning in its harsh deep-space environment, the probe will rendezvous with the tiny asteroid 1998 KY26 in 2031, an extended encounter more suited to Hayabusa2’s original design.