Japan's Hayabusa2 spacecraft returned spectacular images of a near-Earth asteroid following a super-close flyby on July 5, but heated debate was needed before teams signed off on the daring attempt.

When images of the asteroid Torifune arrived on the morning of July 6 Japan time, Makoto Yoshikawa and his team at the Japan Aerospace Exploration Agency (JAXA) got two surprises at once. Torifune turned out to be a contact binary, in which two chunks of rock have come together under gravity, and the returned images were also larger than hoped.

"We did not imagine such a contact binary," Yoshikawa, former mission manager of Hayabusa2, told scientists gathered at the Asteroids, Comets and Meteors conference in Poznan, Poland, on July 10. "Originally, we didn't think we could have such a very big image. Maybe we will take a very small one, but the image was much larger than we expected."

The double surprise was the payoff of months of debate between science and engineering teams, and a last-minute proposal that alarmed some of the scientists it was meant to serve. In the end, the flyby was so close as to be at the very edge of what the aging spacecraft was designed to conduct.

Hayabusa2 launched in December 2014 and rendezvoused with the asteroid Ryugu four years later. The spacecraft collected samples and delivered them to Earth in 2020, completing its primary objectives. JAXA then made plans for an asteroid flyby and a rendezvous with the tiny asteroid 1998 KY26 in 2031.

Usually for flybys, the closest distance is 100 kilometers (62 miles), Yoshikawa said, but in Hayabusa2's case this would not be close enough to gather good images. Hayabusa2 was designed for rendezvous and proximity operations, including hovering, correcting and landing — not for a high-speed pass at 5.3 kilometers per second (3.3 miles per second). Its cameras were also not designed for high-speed slewing.

According to Yoshikawa, science team members pushed back. At a distance of 100 kilometers, Hayabusa2's cameras would barely resolve the asteroid's global shape. Engineering responded with a proposal of 10 kilometers (6.2 miles). Science said that was acceptable, but the team kept pushing. Eventually, engineers confirmed they could get to within 1 kilometer (0.6 miles) of the asteroid's center. "Science people were very happy, because they could have a nice photo," Yoshikawa said.

Then, just one month before the flyby, extended mission team leader Yuya Mimasu proposed going even closer. "Yuya Mimasu said the closest distance should be 800 meters," Yoshikawa recounted. "Some science people said 'No, it's too dangerous,' and a very heated discussion started."

One major issue was the unknown size and dimensions of Torifune and the safety of the spacecraft. The team had assumed a worst-case asteroid size of 1,400 meters by 400 meters (4,600 by 1,300 feet) based on ground-based observations. A pass 800 meters (2,625 feet) from the center of the target would sit just outside that exclusion zone. The spacecraft's optics had also been affected by dust from sampling Ryugu. The final navigation analysis put the targeting error ellipse at around 200 meters (656 feet). "The distance fixed is 800 meters," Yoshikawa said, "but this is quite a big challenge for us."

Hayabusa2 detected Torifune on June 19. The spacecraft used ground-based guidance up to three hours before the flyby, before switching to onboard guidance. "This is quite new," Yoshikawa said. "We developed software for this, and we sent it to the spacecraft."

The result was the stunning, up-close imagery of the dual-lobed Torifune, captured by the probe's Optical Navigation Camera Telescope (ONC-T). But all four of Hayabusa2's science instruments returned data. The Thermal Infrared Imager (TIR) captured nine seconds of thermal imaging between 09:29:50 and 09:29:59 GMT on July 5, just a second before closest approach, independently confirming the contact binary structure in heat emission.

The Near Infrared Spectrometer (NIRS3) and laser altimeter (LIDAR) also got data, with the latter delivering what Yoshikawa described as possibly the first successful LIDAR ranging measurement during an asteroid flyby. But while the most urgent 25 MB of data was downlinked, teams will need to wait months for the rest of the 300 MB of total science data. Hayabusa2's ion engine system restarted on July 9 to begin the cruise toward two Earth flybys in 2027 and 2028 and will fire for around four months. Only after this can the rest of the data be sent to Earth.

Yoshikawa explained that the Torifune flyby was more than just a bonus milestone and photo and science op. The successful super-close flyby means that "JAXA has acquired the technology to collide spacecraft with a small celestial body," he said in his closing remarks at ACM, drawing a parallel to NASA's DART mission. "This flyby mission can be said to serve as a demonstration of the fast reconnaissance concept in planetary defense," verifying the ability to characterize an unknown asteroid rapidly — a capability that could provide crucial information ahead of an impactor mission.

This is not the end for Hayabusa2. The ultimate destination for the spacecraft's extended mission is the tiny asteroid 1998 KY26, a roughly 36-foot-wide (11 m), rapidly rotating rock with which it is scheduled to rendezvous in 2031.

A Japanese spacecraft has gotten up close and personal with yet another asteroid, beaming home stunning new imagery of the distant space rock.

On Sunday (July 5), the Japan Aerospace Exploration Agency (JAXA)'s Hayabusa2 probe performed a close flyby of asteroid Torifune, a 1,475-foot (450-meter) space rock currently traveling through space some 62 million miles (100 million kilometers) from Earth. It was expected to be one of the closest-ever high-speed passes a spacecraft has had with an asteroid.

During the flyby, Hayabusa2 captured this breathtaking new image of Torifune using its optical camera, and was able to transmit it back to JAXA controllers. The probe captured additional scientific data about the asteroid, but will beam those results home at a later date, according to JAXA.

Hayabusa2 also imaged asteroid Torifune using its Mid-Infrared Camera (TIR), which allows scientists to measure asteroids' surface temperatures, thermal inertia and surface roughness, according to JAXA. This mid-infrared image reveals Torifune to be much cooler in what appear to be shadowed regions seen in the optical image, and much warmer where the surface faces the sun.

Torifune orbits the sun every 383 days and rotates every 5 hours. It belongs to the Apollo group, a classification of near-Earth asteroids whose orbits cross Earth's as they make their way around the sun.

Hayabusa2's flyby of Torifune was not part of its original mission, and one of the members of the probe's scientific team previously told Space.com the flyby was a "risky operation" due to the unknowns surrounding the asteroid.

These new images add to the growing list of incredible accomplishments the Hayabusa2 probe has notched on its now nearly 12-year mission.

Hayabusa2 launched in December 2014 on an ambitious mission to collect asteroid samples and return them back to Earth. The probe did just that in December 2020 when JAXA successfully landed samples of asteroid Ryugu in the Australian desert.

Since then, scientists have used the samples to peer back in cosmic time at the history of our solar system and have even discovered that Ryugu contains all five nucleobases found within DNA and RNA.

After collecting its precious samples, Hayabusa2 left Ryugu in 2019. It's ultimate goal is now to flyby asteroid 1998 KY26, which could become the smallest asteroid ever visited by a spacecraft. 1998 KY26 is just 36 feet (11 meters) across, close to the size of the asteroid that exploded above Chelyabinsk, Russia in 2013.

Hayabusa2 is expected to reach 1998 KY26 in the year 2031. Once there, the probe will orbit the space rock before attempting to touch down on its surface. JAXA hopes the mission will help scientists more about the structure and composition of small asteroids.

OSIRIS-REx was the third mission in history to collect a sample from an asteroid and featured the most comprehensive suite of instruments ever flown on an asteroid research mission. The spacecraft's two years of scanning the surface of asteroid Bennu and its brief sample-collection touchdown showed scientists how little they know about space rocks.

Prior to OSIRIS-REx's 2016 launch, two other missions studied asteroids in detail: Japan's Hayabusa 1 that delivered sand and dust grains from asteroid Itokawa to Earth in 2010; and NASA's NEAR-Shoemaker, which explored asteroid Eros in the late 1990s and landed on its surface in 2001. A handful of earlier missions had made brief flybys of several asteroids before that. 

But it was Hayabusa 1 that for the first time confirmed what space rock researchers had long theorized: That many asteroids are not solid blocks of rock but conglomerations of boulders, sand and gravel that have arisen from collisions between larger bodies throughout the history of the solar system. These conglomerations, held together only by the feeble forces of the rock's gravitational forces, became known as rubble piles.

"When the Japanese Hayabusa 1 mission visited asteroid Itokawa, it looked exactly like what everyone's intuition said a rubble pile should look like," Kevin Walsh, a planetary scientist at Southwest Research Institute and lead scientist of the Regolith Development Working Group of the OSIRIS-REx mission, told Space.com. "It was literally just a pile of rocks in space."

Related: The greatest asteroid missions of all time!

Rubble piles are not alike 

Looking at OSIRIS-REx's target asteroid Bennu through telescopes, astronomers thought that the rock would be rather similar to Itokawa. They were in for a surprise. 

Prior to OSIRIS-REx's arrival at Bennu, scientists knew that the 1,614-foot-wide (492 meters) rock has a different chemical composition than Itokawa. While Itokawa is mostly made of quartz-like silica-rich material, Bennu is rich in carbon. But Bennu is a rare carbon-rich kind of asteroid, the so-called B-type that is believed to contain chemical compounds from the early epochs of the solar system.

But other than that, the way the surface reflected light indicated that just like Itokawa, Bennu  would be made of conglomerations of boulders interspersed with wide ponds filled with sand, on which the spacecraft could comfortably land. 

But when OSIRIS-REx took its first close look at Bennu, it saw a "hellscape" of rocks rising against the feeble gravity of the tiny Bennu into unexpected heights, according to Walsh

Odd rocks 

These rocks were oddly porous, which, the researchers realized, confused their telescope measurements and convinced them that Bennu will be sandy just like Itokawa. The material these rocks were made of was somewhat similar to clays that can be found on Earth, but the asteroid's weak gravity allowed the rocks to create unexpected formations. 

"The physical structure of that material is totally unlike what we have on Earth," Edward Beau Bierhaus, an OSIRIS-REx research scientist at Lockheed Martin, which built the spacecraft for NASA, told Space.com. "We see boulders on the surface of Bennu that are fairly large, tens of meters in size. If such boulders were to exist on Earth, the material would have to have a fairly significant amount of strength to support its weight, but that doesn't appear to be the case on Bennu. These boulders actually appear extremely porous and very weak."

Thanks to their odd structure, these boulders behave in unexpected ways. A study led by Bierhous and published in 2022 suggested that the sponge-like nature of these rocks  protects the lumpy asteroid from the effects of impacts of other bodies. These porous rocks, scattered all over Bennu's surface, seem to be absorbing energy of smaller impacting bodies, acting like a crumple zone in a car and protecting the surface of the asteroid. As a result, scientists see far fewer smaller craters on the asteroid's surface than they would statistically expect. 

"You can have an impact or strike a boulder and that porous boulder might potentially totally disrupt," said Bierhaus. "But in doing so, it absorbs the energy of that impact so that it doesn't transmit into the rest of the body."

Like a swamp 

OSIRIS-REx is the third spacecraft in history to snatch a sample from an asteroid after Japan's missions Hayabusa 1 and Hayabusa 2 (Hayabusa 2 landed on asteroid Ryugu in 2019). OSIRIS-REx's approach to the sample collection, however, was radically different from the Hayabusa missions. Thanks to that the OSIRIS-REx team gained a whole new level of understanding of what asteroids are all about. 

"The Hayabusa touchdowns were much softer," said Walsh. "We brought the entire force of the spacecraft down and really pushed hard into the surface to get the entire sampler head flat on the surface. Then we blew gas into the surface that helped to get our sample into our collection mechanism."

The asteroid's response to the touchdown was unexpected. First, the spacecraft sank some 20 inches (50 centimeters) deep into the surface, as if swallowed by a swamp. It was only thanks to the power of its backaway thrusters that OSIRIS-REx' life didn't end on Bennu. But the firing of the thrusters sent a cloud of gravel and dust up into space, threatening the spacecraft during its retreat. OSIRIS-REx survived this incident too. The team was relieved but at the same time, they were scratching their heads. According to expectations, the surface should have been quite resistant to the spacecraft's weight, like a heap of gravel 

Missing sand grains 

Walsh says that the touchdown shows that Bennu's surface layer must have a much lower density than the rest of the asteroid.

"When we did our calculations, initially we were taking the density of all of Bennu, which is 1.1 grams per cubic centimeter," said Walsh. "But our models show afterwards that to be able to compress the surface so much and drive the tag head so deep into the surface, we must have been  driving through material that was like 0.4 grams per cubic centimeter: less than half as dense as the entire body."

Scientists are still looking for an explanation. Walsh thinks the low density might mean that there are empty spaces between rocks in the surface layer. In the past, those voids may have been filled with fine material, but over the billions of years that Bennu has traveled through the solar system, these fine grains may have filtered through the rocks into the asteroid's interior, which would, as a result, be much more dense than the surface. 

To know for sure, scientists would have to be able to slice up the asteroid. While that's still an impossibility, they hope instead that they might glean some information about the interior of rubble pile asteroids from the upcoming European Space Agency's HERA mission, which will study the aftermath of the experimental impact of NASA's DART probe into asteroid Dimorphos, which took place in September 2022. 

Surprisingly complex 

Bierhaus said that the OSIRIS-REx mission showed that asteroids are much more than dead rocks floating in space. They might be only a few hundreds of feet wide, but their geology appears surprisingly complex.

"What we're finding is that the physical processes that create and evolve these bodies are very diverse and very complicated," said Bierhaus. "We think that Bennu is the product of a collision from a larger asteroid. The evolution from that body to where Bennu is still a mystery."

Walsh thinks that the analysis of OSIRIS-REx's sample might shed some light on the asteroid's eventful past. Records of past collisions may be seared into those rock fragments and scientists hope to decode them in their labs.

"We have tools to track collisional histories of different grains and in different meteorites," said Walsh. "So we should be able to apply those to the samples of Bennu." 

OSIRIS-REx is not yet done teaching us about asteroids. 

A space capsule that may contain the first-ever samplesfrom an asteroid in deep space has begun one last trek from itsAustralia landing site to Japan ? its final destination after abillion-mile voyage.

The capsule, which parachutedback to Earth Sunday in the Australian outback, is all that remains ofJapan's seven-year Hayabusa mission that visited the asteroid Itokawa.The basketball-sized capsule is being flown to Japan for its long-awaitedopening to see, once and for all, if it actually grabbed samples of anasteroid.

"JAXA has commenced to transport the retrievedcapsule to Japan," the Japan Aerospace Exploration Agency (JAXA) said in astatement.

The capsule is expected to arrive at JAXA facility inSagahimara, Japan, on Friday for a grand opening that will cap a 3.75billion-mile (6 billion-km) to a near-Earth asteroid. An international team ofscientists from JAXA, NASA and Australia will be on hand to begin catalogingany asteroid samples that may be inside. [Photos:Hayabusa's fiery Earth return.]

"Certainly, any samples retrieved from Itokawa willprovide exciting new insights to understanding the early history of the solarsystem," Tommy Thompson, NASA's Hayabusa project manager from the JetPropulsion Laboratory in Pasadena, Calif., has said. "This will be theicing on the cake, as this mission has already taught us so much."

Japan launched the Hayabusamission in 2003 to visit the asteroid Itokawa, which the probereached in 2005. The ambitious $200 million expedition is the first missionever to attempt to return samples of an asteroid.

But the Itokawa rendezvous was not entirely smooth.

Hayabusa landed on the asteroid several times to try andgrab samples in its return capsule after a projectile device designed to kickup surface material. A small lander, called Minerva, also failed in its bid toset down on Itokawa.

A fuel leak and harrowing ? but temporary ? loss ofcommunication, reaction control wheel malfunctions and ion engine failures alsoplagued the mission, adding three years onto its round trip to Itokawa.

Despite those hurdles, the probe's actual re-entry and capsulelanding on Sunday went off without a hitch. Even without an asteroidsample, the mission has been an amazing achievement, scientists said.

"It's remarkable that they are managing to get thisspacecraft back," said Don Yeomans, NASA's project scientist for theHayabusa mission.

Yeomans said even a few small grains from the asteroidItokawa would be a critical find for scientists hoping to understand how suchspace rocks formed about 4.6 billion years ago when our solar system was young.

• Photos: Japan Asteroid Probe's Fiery Finale, Video
• Graphic: How Japan's Hayabusa Asteroid Mission Worked
• 5 Reasons to Care About Asteroids

This story was updated at 4:38 p.m. ET.

The triumphant Sunday landing of a Japanese space capsulethat visited an asteroid and returned to Earth despite near-overwhelming?malfunctions now has scientists around the world asking one final question: Didit actually capture a piece of asteroid Itokawa?

"First, the sample container will be inspected, andthen the content will be extracted," said Keiji Tachikawa, president ofJapan's space agency JAXA, in a Monday statement.? "We hope to find theItokawa's surface material in the capsule, and contribute to understanding theorigin and evolution of the solar system."

Japan's 950-pound (510-kg) Hayabusaasteroid probe lit up the night sky Sunday as it re-entered the atmosphereabove the remote Woomera Prohibited Area in the Australian outback. [Videoof asteroid probe's fiery re-entry.]

?The spacecraft's sample return capsule, which separated asplanned three hours before re-entry, appeared as a bright light leading themain fireball created by the demise of its mother ship.

The dazzling spectacle was a fiery finale for the Hayabusaprobe, but just the beginning for scientists on Earth.?

Hayabusa's sample return capsule, a 16-inch (40-cm)container about the size of a basketball, survived re-entry using a heat shieldand successfully parachuted to the Earth's surface. It has since been recoveredby JAXA scientists.?

But it is unclear if Hayabusa's sample capsule actuallycontains traces of the asteroid Itokawa.

"The hope is that there will be at least some dustparticles inside the space capsule," Don Yeomans, the U.S. projectscientist for the Hayabusa mission, told SPACE.com.

Japan launched the $200 million Hayabusa mission in 2003 andit reached the silicon-rich asteroidItokawa in 2005, where it landed multiple times in attempts to capture samples.But the probe's projectile device designed to kick up samples so they could becollected failed to work properly.

Yeomans said that scientists know Hayabusa's target asteroidItokawa is a silicone-rich space rock with the consistency of arubble pile. But actual samples, however small, will give researchers concreteevidence of the fundamental characteristics for such asteroids.

"Even if only a couple of particles muchsmaller than a grain of sand were collected, we would be able to get moreinformation," Yeomans said.

For now, scientists will have to wait until the Hayabusasample capsule makes one last journey from the Australian outback to JAXA'sSagamihara Campus in Kanagawa, Japan. Only then will the capsule be opened in aclean room to reveal, once and for all, if it captured the first-ever sample ofan asteroid.

In addition to its asteroid mission, Hayabusa has alsoalready contributed to a different branch of space science.

An international team caught the spacecraft's re-entry onvideo from a NASA chase plane as part of an experiment to study how spacecraftheat shields perform during re-entry. Hayabusa's heat shield, too, was laterrecovered.

"It was incredibly fortuitous to see the capsule andbus so well separated. We got data on both phenomena," said PeterJenniskens, who led a NASA-sponsored observation campaign and is a researcherfrom the SETI Institute in Mountain View, Calif. "For years to come, this video will serve as a demonstration of why we build thermalprotection systems, as it shows the capsule surviving the entry and the unprotected rest of thespacecraft being torn apart bythermal and mechanical stresses."

• Gallery - Japan's Asteroid Probe Returns to Earth
• 5 Reasons to Care About Asteroids, Hayabusa graphic
• Video - Brilliant Fireball From Japan's Asteroid Probe

SPACE.com's Space Insider ColumnistLeonard David contributed to this report from Golden, Colo.