In April of last year, a Japanese spacecraft launched a strike from above on an asteroid.
The Japanese space agency did not declare war. The bombing was part of the work of Hayabusa2, a robotic space probe that collects clues 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 organic matter, possibly including amino acids, the building blocks of protein.
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 one of the asteroids that sway inside Earth’s orbit as they move around the sun. Ryugu itself shouldn’t collide with our planet anytime soon, but other similar asteroids could.
In the April experiment, the spacecraft pulled out a device, the small cabin impactor, and rushed to a safe place behind the asteroid. The plastic explosives accelerated a four-pound copper projectile at 4,500 miles per hour into its surface. A camera deployed by Hayabusa2 recorded the impact.
Three weeks later, Hayabusa2 returned to take a look at the scar, discovering that it was larger than what scientists had expected.
“It was wow, what a big surprise,” said Seiji Sugita, a planetary scientist at the University of Tokyo and one of the authors of an article describing the results of the craterization experiment that was published Thursday by the journal Science. From the size of the brand they made, scientists deduce that the asteroid Ryugu looks extremely young for its age.
Even though Ryugu is made of things dating back to the birth of the solar system 4.5 billion years ago, its surface is only nine million years old. The gargantuan difference in age between Ryugu’s materials and its surface appearance is not a contradiction. Ryugu probably fused with debris that made a larger asteroid fall, and this collision and coalescence could have happened only nine million years ago.
Dr. Sugita thinks that Ryugu is a little older than that. The expected age of asteroids the size of Ryugu is around 100 million years, he said. An event that happened nine million years ago, such as an acceleration of the rotation of the asteroid, could have erased and filled the old craters. (Think of it as Botox for the solar system.)
Ryugu seems to have turned much faster at that time, which would explain the bulge around its equator. Hayabusa2 also spotted landslides at the surface, which could have happened when the asteroid later slowed down.
Here’s how scientists analyzed the crater.
The hole dug by the explosion was a semicircle, not a full circle. This indicates that a large buried rock diverted some of the impact energy.
Scientists had predicted that a hole would be up to 30 feet wide. Instead, the diameter of the crater was close to 60 feet, with a depth of a few meters.
“The volume of the ejecta is approximately 10,000 hand buckets of sand and pebbles,” said Dr. Sugita.
If the impactor had hit the rock, it would have carved a small crater; the energy would have dissipated to break the rock. But when it collided with materials that were held loose together, the impactor raised much more debris. (Think about the difference between dropping a cannonball on a concrete sidewalk and a beach.)
Ryugu is covered with rocks, and indeed, a large buried rock led to the semicircle shape of the crater. But the neighboring material seems to be held in place only by the low gravity of the asteroid. By comparing the size of the crater with the results of laboratory experiments on Earth, the scientists concluded that a large part of Ryugu consisted of pieces like coarse grains of sand.
“With hindsight, everything makes sense,” said Dr. Sugita.
The large size of the crater on Ryugu changed the estimated age of the asteroid’s surface. Smaller collisions occur more often, so the pattern of eyelid marks on Ryugu could have been produced in just nine million years.
The results of a small German-French lander that Hayabusa2 had dropped on the surface of Ryugu a few months earlier also corresponded to the photo of an asteroid which was only slightly held together. In an article published last week in the journal Nature, the researchers working on the lander describe infrared images taken at the surface, which show that the asteroid is made almost entirely of highly porous material – fragments of a larger asteroid that was broken by the impacts.
The fragile structure of the asteroid also explains why so few carbon meteorites are on the surface of the Earth, even if three quarters of the asteroids, including Ryugu, fall into this category: they disintegrate and burn large part in the atmosphere before reaching the ground.
Hayabusa2, who arrived in Ryugu in June 2018, also picked up dirt and small stones from the surface of the asteroid, which is named Ryugu-jo, or palace of the dragon – a magical underwater lair in a tale popular Japanese.
After almost a year and a half of exploration, Hayabusa2 returned to Earth last November. Towards the end of this year, he will drop a box 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.
A NASA mission, OSIRIS-REx, explores another carbon-rich asteroid, Bennu, and he too, will try to collect pieces to bring them back to Earth, although he will not return with his samples until September 2023.