Hayabusa2 Set for Super-Close Asteroid Flyby on July 5: ‘A New Beast in the Zoo of Asteroids’
Featured image: [Artist’s concept of Hayabusa2 approaching asteroid Torifune; credit: JAXA]
Japan’s Hayabusa2 spacecraft, already one of the most accomplished asteroid missions in history, is about to add a new feat to its resume. On July 5, the probe will execute one of the closest asteroid flybys ever attempted, passing just 1 to 10 kilometers from the surface of the stony near-Earth asteroid Torifune at a relative speed of 5.25 kilometers per second (roughly 11,740 miles per hour).
“This is one of the closest asteroid encounters ever attempted by a mission of this class,” said Satoshi Tanaka of the Japan Aerospace Exploration Agency (JAXA) during a presentation to NASA’s Small Bodies Assessment Group on June 11. “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 1 kilometer.”
The encounter represents an extraordinary engineering challenge. Hayabusa2 was designed for slow, careful rendezvous with its target, not high-speed flybys. It has no steerable telescopes, its cameras are fixed to the spacecraft body, and its telescopic camera resolves Torifune to more than a single pixel only about 800 seconds before closest approach. At 5.25 kilometers per second, the encounter will unfold in seconds.
A proven veteran of asteroid exploration
Hayabusa2 launched in December 2014 and arrived at the carbonaceous asteroid Ryugu in June 2018. During its 16-month stay, the spacecraft deployed four surface rovers, created an artificial crater with an impactor, and collected two samples from the surface and subsurface. In December 2020, its sample capsule parachuted to the South Australian desert carrying approximately 5.4 grams of material from Ryugu, the largest asteroid sample ever returned at that time. Analysis later confirmed more than 20 amino acids in the sample, including non-proteinogenic ones, pointing to a parent body that formed beyond Jupiter’s orbit.
After releasing the sample capsule, Hayabusa2 began an extended mission named Hayabusa2#, pronounced “Hayabusa2 Sharp,” standing for Small Hazardous Asteroid Reconnaissance Probe. The spacecraft has roughly 30 kilograms of xenon propellant remaining, about half its original supply, and all primary systems remain operational despite 11.5 years in deep space and some instrument degradation.
The target: Torifune
Asteroid (98943) Torifune was discovered in February 2001 by the LINEAR survey in New Mexico. It measures approximately 465 meters across, about the length of five football fields, and belongs to the S-complex of stony, silicate-rich asteroids. This makes it a fundamentally different object from Ryugu, which was a dark, water-rich carbonaceous asteroid. The contrast between the two will allow scientists to compare two major asteroid types studied at close range by the same spacecraft.
Torifune’s name comes from a public naming campaign held by JAXA between December 2023 and May 2024. The 60 public submissions were narrowed by a committee including Hayabusa2 team members and their children. “Torifune” is an abbreviation of Ame-no-torifune, a Japanese god and his ship, described as able to travel safely at high speed like a bird and steady as a rock. The name was approved by the International Astronomical Union’s Working Group on Small Bodies Nomenclature in September 2024.
Recent ground-based observations suggest Torifune has a highly elongated shape, possibly even a contact binary similar to comet 67P or the Kuiper Belt object Arrokoth. Its exact shape remains unknown, and that uncertainty is part of what makes the flyby scientifically compelling.
“We’re going to discover what it looks like,” said Patrick Michel, principal investigator of ESA’s Hera mission and a Hayabusa2 science team member. “And each time we have seen a new asteroid, we’ve been surprised. We’re going to discover another beast to put in the zoo of asteroids.”
Science at high speed
The flyby is scheduled for approximately 18:30 Japan Standard Time on July 5 (09:30 UTC). At closest approach, Hayabusa2 will be at a solar distance of 0.81 astronomical units. The spacecraft will use its telescopic and wide-angle cameras, thermal infrared imager, near-infrared spectrometer, and laser altimeter to study Torifune in a brief but intense observing campaign.
Six defined science objectives include determining the asteroid’s spin axis state and reflectance, capturing global surface features, measuring thermal properties, determining surface composition, obtaining at least one laser altimeter distance measurement, and reconstructing the three-dimensional shape from the flyby data.
Because the sun angle will be favorable before closest approach but unfavorable immediately afterward, most science observations must be completed in the hours leading up to the encounter. The spacecraft will switch from ground-based navigation to autonomous onboard navigation 12 hours before closest approach, with science instruments taking full priority in the final five minutes.
Planetary defense implications
The flyby has direct relevance to planetary defense. Demonstrating that a spacecraft not designed for fast flybys can navigate within a kilometer of a small asteroid at over 5 kilometers per second validates the precision navigation needed for kinetic impactor missions like NASA’s DART, which successfully deflected the asteroid Dimorphos in 2022.
JAXA established a Planetary Defense Team in April 2024, and the Torifune flyby demonstrates Japan’s independent capability for asteroid reconnaissance. The extended mission’s ultimate destination, a rendezvous with the tiny fast-rotating asteroid 1998 KY26 in 2031, will study an object roughly the size of the Chelyabinsk impactor that injured 1,500 people in 2013, making the mission’s planetary defense value clear.
Michel acknowledged the risk: “It’s still a risky operation, because they had not planned for this. The second thing is that we have a high uncertainty on the size of the object.” But the potential reward is a new close-up view of one of the estimated 27,000 near-Earth asteroids that pass within Earth’s neighborhood, a chance to see another beast in the zoo before it vanishes into the darkness of space.