In April 2019, a Japanese spacecraft launched a strike from above on an asteroid.
The bombardment from Japan’s space agency 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 half a mile wide, is among the asteroids that swing inside the orbit of Earth. Ryugu is not expected to collide with our planet any time soon, but other 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 4-pound copper projectile to 4,500 mph into the asteroid’s surface. A camera deployed by Hayabusa2 recorded the impact.
Three weeks later, Hayabusa2 returned and found the scar was larger than the scientists had expected. “That was wow, what a big surprise,” said Seiji Sugita, a planetary scientist at the University of Tokyo and one of the authors of a paper describing the results of the experiment that was published by the journal Science. From the size of the pockmark they made, scientists infer that the asteroid Ryugu looks 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 9 million years old. The gargantuan difference in ages between the materials of Ryugu and its surface appearance is not a contradiction. Ryugu probably coalesced out of debris knocked off a larger asteroid, and that collision and coalescence could have occurred just 9 million years ago.
Sugita thinks that Ryugu is somewhat older than that. The expected age of asteroids the size of Ryugu is about 100 million years, he said. Some event that occurred 9 million years ago, like a speeding up of the asteroid’s spin, could have erased and filled in the older craters. (Think of it like Botox for the solar system.)
Ryugu does appear to have been spinning much faster at that time, which would explain the bulge around its equator. Hayabusa2 has also spotted landslides on the surface, which could have occurred as the asteroid later slowed.
The hole excavated by the blast was a semicircle, not a full circle. That indicates that a large buried boulder deflected part of the impact’s energy.
The scientists had predicted a hole would be up to 30 feet wide. Instead, the diameter of the crater was close to 60 feet.
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.
Ryugu is covered with boulders, and indeed, one large buried rock led to the semicircle shape of the crater. But the neighboring 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.
“In hindsight, everything makes sense,” Sugita said.
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 9 million years.
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.