Japan’s Experiment to Calculate an Asteroid’s Age Was a Smashing Success

The spacecraft Hayabusa2 hurled a four-pound copper ball toward the asteroid’s surface at about 4,500 miles an hour to create an artificial crater

Hayabusa2 deployed a camera to film the plume of regolith thrown up by the impact. Courtesy of JAXA

Last April, Japan’s Hayabusa2 spacecraft created an artificial crater on the asteroid, Ryugu, by hurling a four-pound copper ball, called SCI, toward the asteroid's surface at about 4,500 miles per hour in order to calculate Ryugu's age. Now, results from the out-of-this-world experiment are in.

Previous calculations suggested the surface of the asteroid might be anywhere from a few million to 200 million years old. The new study, published on March 19 in the journal Science, uses the results of the artificial impact to give a more accurate estimate of Ryugu's age: between six and 11 million years old.

The Hayabusa2 spacecraft got out of the way for the cannonball impact to protect itself from debris, but it left behind a camera to film the event. Three weeks later, the spacecraft returned to the site of the impact to measure the brand new cavity.

"I was so surprised that the SCI crater was so large," lead author and Kobe University planetary scientist Masahiko Arakawa tells Charles Choi at Space. At 47 feet wide, the crater was larger than the team expected. The mark is about seven times larger than it would have been if a similar experiment was performed on Earth.

The size and shape of a crater on Ryugu mostly depends on two things: the gravitational force of the asteroid, and the strength of the soil, called regolith, that covers the asteroid’s surface. Ryugu is covered in scars from big and small impacts that have occurred during its millions of years in the solar system. But without an example crater caused by a meteorite of known size and speed, it was difficult to work backwards from the asteroid’s existing pocks to determine the asteroid’s age.

Hayabusa2’s projectile was about the size of a tennis ball and, with the help of explosives, flew at Ryugu at about 4,475 miles per hour, near double the speed of sound on Earth. The plume of sand it left behind was enough to fill some 10,000 buckets worth of material, University of Tokyo planetary scientist and co-author Seiji Sugita tells Kenneth Chang at the New York Times.

“This is the first time that we have observed the crater formation process in a microgravity environment,” Arakawa tells New Scientist’s Leah Crane. Most studies of crater formation have been done on in laboratories on Earth.

The resulting crater is semicircular in shape and has a slightly deeper ten-foot-wide pit at the point of impact, which suggests that Ryugu has a loose top layer that covers a denser core. The crater's large size also supports evidence, recently published in Nature, that the asteroid is made of loosely packed sand, not solid rock.

The researchers also note a caveat to their results: although the surface of Ryugu may be under 11 million years old, the asteroid itself might actually be older. Sugita tells the New York Times that most asteroids of the same size are closer to 100 million years old. But when the asteroid speeds up its spin, it can smooth out old craters and reset the surface’s apparent age. Landslides that Hayabusa2 has spotted on the surface suggest that at some point, the asteroid slowed down from a higher speed in its past.

Hayabusa2 left Ryugu in November carrying samples taken from the center of the crater. The spacecraft will drop the samples in Australia near the end of this year, giving scientists a chance to study the asteroid’s makeup in more detail.

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