What happens when you fire a cannonball at an asteroid?
Thanks to a wild new study published Thursday in the journal Science, we now know the answer.
Scientists used the Japanese-made spacecraft Hayabusa2 to fire a 2-kilogram copper cannonball at a speed of 2 kilometers-per-second into the asteroid Ryugu, a tiny, rocky body orbiting nearby, between Earth and Mars.
The cannonball, though tiny, managed to blast a 14-meter-wide, semicircular crater into the asteroid’s crisp surface. But aside from making an impact in the real sense, the shot transformed scientists' understanding of the asteroid's age, composition, and other properties.
The unprecedented results pave the way for researchers to better estimate these same details for the myriad other asteroids strewn across the universe.
“All this is super exciting,” Patrick Michel, director of research at the French Scientific Research National Center, tells Inverse.
“And in this crazy and dangerous world (...), it gives hope that we can fight big challenges! The SCI was an operation done on a tiny body, at several hundred million km from Earth. This is amazing and gives a lot of hope.”
Wrecking an asteroid
The collision enabled scientists to capture images and samples from the asteroid that would be otherwise impossible.
“The surface of the asteroid is heated by the solar radiation and irradiated by the solar wind and the cosmic ray, so the outermost layer of the surface could be altered,” Masahiko Arakawa, professor of planetology at Kobe University, tells Inverse.
He explains that the organic materials and hydrated minerals on the surface of the asteroid can change dramatically over time.
“We wanted to have a fresh material without alteration, so we developed and operated a Small Carry-on Impactor, SCI, to form an artificial impact crater to expose the asteroid subsurface or interior,” Arakawa says.
The team then developed a microsatellite to observe the crater formation process and the continuous ejection of the subsurface material.
“Collisions play a fundamental role in the formation and history of our Solar System, starting with planet formation through collisional accretion,” Michel explains.
“So far, models developed to understand the collisional history of our Solar System rely on ad-hoc parameters to determine the outcome of each collision.”
This means researchers’ understanding of cosmic collisions are usually obtained from laboratory simulations, and not first-hand observations.
“Scaling the lab results to asteroid scales is not trivial,” Michel says.
“The SCI experiment is the first hyper-velocity impact experiment on such an asteroid, and it is a super complex operation, and yet achieved with high success.”
Making holes, answering questions
Firing the cannonball enabled researchers to directly observe the formation of a crater in microgravity for the first time, answering how much the microgravity affects crater formation.
The crater growth process appears to be controlled by the surface gravity, not by the surface material strength. In fact, the diameter of the new crater on Ryugu is about 7 times larger than if it had formed on sand under the Earth’s gravity.
“We found that the crater’s formation and dimensions are such that the only way to explain them is that the process is controlled by the gravity of the asteroid, despite its extremely low value,” Michel says.
“This was totally unexpected, and I must say that I still have a hard time to interpret this.”
“How can such small gravity be the only contributor? And we see many boulders, which should have strength, so how is this possible? This is fascinating and opens a totally new window in our understanding of cratering physics in such different regimes than on Earth,” Michel says.
“If we want to deflect an asteroid by an impact, we need a much better understanding of this process.”
Aside from this finding, Arakawa and his colleague were also able to put a number on the asteroid's age — previously estimates had pegged it at between 9 million and 160 million years. Turns out Ryugu is on the younger side of the spectrum.
“Our results suggest that 9 million years is better for the surface age of Ryugu.”
Knowing what Ryugu's surface is made of and how impact craters form there is critical for defending our own planet from incoming collisions from the cosmos, Michel says.
One of the main strategies scientists propose for any near-Earth object that looks like it might actually hit us is to send a projectile to knock it off course.
“If we want to deflect an asteroid by an impact, we need a much better understanding of this process,” Michel says.
“So, even if the SCI was not meant to deflect Ryugu because the impact energy is too small, its results will definitely feed planetary defense studies.”
But in these missions' cases, the asteroid target is very different from Ryugu, and probably much less porous, so the results won't apply entirely. However the new study does offer a guide for using this "wrecking ball" method of data collection to understand how asteroids form, their key features, and whether they might do damage to Earth.
Abstract: The Hayabusa2 spacecraft investigated the small asteroid Ryugu, which has a rubble pile structure. We describe an impact experiment on Ryugu using Hayabusa2’s Small Carry-on Impactor (SCI). The impact produced an artificial crater with a diameter >10 m, which has a semicircular shape, an elevated rim and a central pit. Images of the impact and resulting ejecta were recorded by the Deployable CAMera 3 (DCAM3) for >8 min, showing the growth of an ejecta curtain (the outer edge of the ejecta) and deposition of ejecta onto the surface. The ejecta curtain was asymmetric, heterogeneous, and never fully detached from the surface. The crater formed in the gravity-dominated regime i.e., crater growth was limited by gravity, not surface strength. We discuss implications for Ryugu’s surface age.