Asteroid Ryugu shown to be loosely held together

NEW YORK • In April last year, a Japanese spacecraft launched a strike from above on an asteroid.

Japan's space agency was not declaring war. The bombardment was part of the work of Hayabusa2, a robotic space probe that is gathering hints about the origins of the solar system by studying the rocky object, Ryugu. It is a type of asteroid that is full of carbon molecules known as organics, including possibly amino acids, the building blocks of proteins.

The mission also provides information that could help defend our planet in the future. Ryugu, a diamond-shaped body more than 800m wide, is among the asteroids that swing inside the orbit of Earth as they travel around the Sun. Ryugu itself is not expected to collide with our planet anytime soon, but other similar asteroids might.

In the April experiment, the spacecraft released an apparatus, the Small Carry-on Impactor, and scurried to a safe location behind the asteroid. Plastic explosives accelerated a 1.8kg copper projectile 7,242kmh into its surface. A camera deployed by Hayabusa2 recorded the impact.

Three weeks later, Hayabusa2 returned to take a look at the scar, discovering it was larger than the scientists had expected. "That was wow, what a big surprise," said Professor Seiji Sugita, a planetary scientist at the University of Tokyo and one of the authors of a paper describing the results of the cratering experiment that was published last Thursday by the journal Science. From the size of the pockmark they made, scientists inferred that the asteroid Ryugu looked extremely young for its age.

Even though Ryugu is made of stuff dating back to the birth of the solar system 4.5 billion years ago, its surface is just nine million years old. The gargantuan difference in ages between Ryugu's materials and its surface appearance is not a contradiction. Ryugu - named after a magical undersea lair in a Japanese folk tale - likely coalesced out of debris knocked from a larger asteroid, and that collision and coalescence could have occurred nine million years ago.

The hole excavated by the blast was a semicircle, not a full circle, indicating that a large buried boulder deflected part of the impact's energy. The scientists had predicted that the hole would be up to 9m wide. Instead, the diameter of the crater was close to 18m, with a depth of a couple of metres. "The volume of the ejecta is about 10,000 hand buckets worth of sand and pebbles," Prof Sugita said.

If the impactor had hit rock, it would have carved a small crater; the energy would have dissipated into breaking apart the rock. But when it slammed into material that was loosely held together, the impactor kicked up far more debris, much like the difference between dropping a cannonball on a concrete sidewalk versus a beach.

The large crater size on Ryugu shifted the estimated age of the asteroid's surface. Smaller collisions occur more often, so the pattern of pockmarks on Ryugu could have been produced in just nine million years.

Ryugu is covered with boulders, and indeed, one large buried rock led to the semicircle shape of the crater. But the neighbouring material seems to be held in place just by the asteroid's weak gravity. Comparing the size of the crater with the results of laboratory experiments on Earth, scientists concluded that much of Ryugu was made of bits like coarse sand grains.

Results from a small German-French lander that Hayabusa2 had dropped onto Ryugu's surface months earlier also fit the picture of an asteroid only loosely held together. The fragile structure of the asteroid also helps explain why so few carbonaceous meteorites are found on Earth's surface, even though three-quarters of asteroids, including Ryugu, fall in this category: They largely disintegrate and burn up in the atmosphere before reaching the ground.

Hayabusa2, which arrived at Ryugu in June 2018, also picked up some dirt and small pebbles from the surface of the asteroid. After nearly 11/2 years of exploration, Hayabusa2 headed back towards Earth last November.

Near the end of this year, it will drop off a canister containing the samples, which will land in an empty part of Australia. Scientists will then be able to examine the composition of Ryugu in much more detail.

NYTIMES