Hayabusa2's MINERVA-II1 Rovers: The First Hops on Asteroid Ryugu

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Hayabusa2’s Hopping Rovers Made History on Asteroid Ryugu

An artist's impression of the two MINERVA-II1 rovers on the surface of asteroid Ryugu. Image: JAXA.

On 21 September 2018, Japan's Hayabusa2 spacecraft dropped two small, drum-shaped robots onto the asteroid Ryugu. Both landed, both sent back pictures, and at least one was soon confirmed to be moving. The pair, known together as MINERVA-II1, became the first rovers ever to land on an asteroid and travel across its surface on their own, and they did it by hopping rather than rolling.

The two machines were officially Rover-1A and Rover-1B, later nicknamed HIBOU and OWL. HIBOU kept working for 36 Earth days and returned 609 images; OWL lasted three days and returned 39. Below is how the landing unfolded, why hopping was the only sensible way to get around, what the earlier Hayabusa mission had taught the team, and what the wider mission went on to achieve.

How the landing unfolded

Hayabusa2 launched from Tanegashima in Japan on 3 December 2014 and reached Ryugu on 27 June 2018, after a journey of more than three years. It spent the summer mapping the asteroid from about 20 kilometres away before risking anything on the surface.

The rovers left the spacecraft at 13:06 Japan time on 21 September, released from an altitude of roughly 55 metres. Hayabusa2 then climbed back to a safe distance, so the first evidence of success was indirect. During a press briefing that afternoon, the team explained that the rovers' power readings had dropped at exactly the moment they should have passed into shadow if they were sitting on the ground, a strong hint that both had touched down.



Full confirmation came the next day, once Hayabusa2 had relayed enough data. JAXA announced that both rovers were in good condition, had transmitted photos and data, and were moving on the surface. One of the first images, taken by Rover-1B a minute after separation, shows Ryugu's dark terrain in one corner and a smear of reflected sunlight in another; the rover was turning slowly as it fell, which helped keep the picture sharp.



Why the rovers hopped instead of driving

Ryugu is a small world by any measure. It is about 900 metres across, shaped like a spinning top with a ridge around its equator, and appears to be a rubble pile: a loose heap of boulders and gravel held together by its own weak pull, with perhaps half of its volume made up of empty space. Gravity at the equator is roughly one eighty-thousandth of Earth's.

In those conditions, wheels are almost useless. A wheeled rover needs its weight to press the tyres against the ground so they can grip; on Ryugu there is so little weight that a wheel would mostly spin, or push the vehicle off into space. The MINERVA-II1 designers solved this with a motor-driven mass inside each rover. Spinning that mass and then stopping it makes the whole body react, so the rover tumbles or springs off the ground and drifts back down in a long, slow arc.

The hops had to be gentle. Push too hard and a rover could exceed the asteroid's tiny escape velocity and never come back. That is also why a common description of the rovers is misleading. Each one has a mass of about 1.1 kilograms on Earth and exactly the same mass on Ryugu; what changes is its weight, the force gravity exerts on it. Reports that the rovers "weighed less than a quarter of a gram" on the asteroid confuse those two quantities.


The picture above, taken by Rover-1A on 22 September during a hop, is a good illustration: half of the frame is Ryugu's surface and the other half is washed out by sunlight. JAXA's team said the image proved that the movement mechanism worked on a small body, which had never been demonstrated before.

What the little rovers carried

Each rover is about 18 centimetres wide and 7 centimetres tall, roughly the size of a biscuit tin. Despite that, they carried colour cameras along with temperature sensors to record how hot different patches of ground became as Ryugu rotated. The asteroid turns once every 7.6 hours or so, so the surface heats and cools quickly.

The rovers could not talk to Earth directly. They sent their data up to Hayabusa2, which stored it and relayed it home, a link that worked only when the spacecraft was in view. That arrangement is one reason the first results trickled out over a day rather than arriving instantly.


As the news broke, space fans online shared the first images and mission diagrams alongside JAXA's own posts, and the hashtag #asteroidlanding spread quickly.

Lessons from the first Hayabusa mission

Hayabusa2 was a second attempt at something Japan had already tried. The original Hayabusa reached the asteroid Itokawa in September 2005 and carried its own small hopping lander, also called MINERVA. It is often said that MINERVA was lost on the way to the asteroid, but that is not quite what happened. It was released at Itokawa on 12 November 2005, but at the wrong moment: the spacecraft had begun climbing automatically before the command arrived, so the lander was let go too high and too fast, escaped Itokawa's gravity and tumbled into space.

Hayabusa's sampling system also misfired. The projectile meant to kick up surface material does not appear to have been fired during either touchdown. Even so, when the capsule landed in the Australian outback in June 2010, scientists found about 1,500 tiny rock grains inside, most of them around ten micrometres across. It was the first material ever brought back from an asteroid, and it showed the concept could work.

Hayabusa2 carried improved versions of everything: two MINERVA-II rovers in one package, a second rover from a different team, the German-French MASCOT lander, and a far more ambitious plan for collecting samples.

What Hayabusa2 did after the rovers landed

The rovers were only the opening act. MASCOT, a 9.6-kilogram box, landed on 3 October 2018 and worked for about 17 hours on its non-rechargeable battery, even flipping itself once to take measurements at a second spot.

On 21 February 2019, Hayabusa2 made its first brief touchdown, firing a small tantalum bullet into the ground and catching the debris. On 5 April it released the Small Carry-on Impactor, a projectile that blasted a crater about ten metres wide. On 11 July the spacecraft touched down again next to that crater to collect material dug up from below the surface, which had been shielded from space weathering. The final rover, MINERVA-II2, developed problems before deployment; it was released into orbit on 2 October 2019 to help measure Ryugu's gravity and struck the surface a few days later.

Hayabusa2 left Ryugu on 13 November 2019. On 5 December 2020 (6 December in Japan) it dropped its capsule over the Woomera range in South Australia, carrying about 5.4 grams of asteroid samples, many times more than its predecessor managed. The landing drama was often compared with that of the European probe Philae, which reached Comet 67P in November 2014 but bounced and came to rest in shadow; this time the robots worked as planned.

What the Ryugu samples revealed

Ryugu is a dark, carbon-rich asteroid of a rare type, and its samples have turned out to be among the most pristine material available to science. Laboratory teams have reported that the grains contain several percent water locked inside minerals, a tiny pocket of carbonated liquid water trapped in a crystal, around 20 different amino acids, and the nucleobase uracil together with vitamin B3. Because the samples were sealed when collected, contamination from Earth has been ruled out for those findings, although microbes later seen on one grain were judged to be contamination picked up in the lab.

The chemistry suggests Ryugu's parent body formed in the cold outer Solar System, and it supports the idea that asteroids helped deliver water and the building blocks of life to the early Earth.



The spacecraft itself is still flying. It passed the asteroid 98943 Torifune in July 2026 and is heading for a rendezvous with the small, fast-spinning asteroid 1998 KY26, planned for July 2031.

Why two tiny rovers mattered

An orbiting spacecraft can map a whole asteroid, but a robot on the ground can see individual pebbles, feel how warm a boulder gets and show how loose the surface really is. MINERVA-II1 proved that small, cheap machines can move around on bodies with almost no gravity, a technique that future missions to asteroids, comets and small moons can build on.

It is worth being precise about what "first" means here. NASA's NEAR Shoemaker spacecraft touched down on the asteroid Eros in 2001, and the original Hayabusa briefly touched Itokawa in 2005, so neither landing on an asteroid nor touching one was new. What MINERVA-II1 achieved for the first time was a working rover: a separate robot that landed, moved under its own control and photographed an asteroid from its surface.

Sources: JAXA: MINERVA-II1 landing and hopping confirmed (22 September 2018); Wikipedia: Hayabusa2; Wikipedia: 162173 Ryugu; Wikipedia: Hayabusa. Lead illustration and embedded images: JAXA and the Hayabusa2 project.

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