On September 26, an act of targeted violence will occur 11 million kilometers from Earth, as a spacecraft the size of a vending machine collides with a small asteroid at 6 kilometers per second. Unlike some asteroids that are alarmingly far from Earth’s orbit, Demorphos – the 160-meter-long moon of a larger body – is an innocent bystander, posing no threat to our world. But the looming attack marks humanity’s first-ever field test of a planetary defense mission: NASA’s Double Asteroid Redirection Test, or DART.
The hope is that the collision will push Demorphos into a closer orbit around its 780-meter partner Didymos, reducing its orbital time of nearly 12 hours by a few minutes. A successful strike would support the idea that in the future, similar efforts could divert threatened asteroids onto safer paths. But new lab simulations and experiments show that the mission’s fate depends largely on a crucial question: Are these tiny asteroids solid rock or – as astronomers increasingly believe – piles of rubble?
The answer, to be revealed from the crater and ejection from the DART collision, could determine how hard an asteroid would hit when the exercise wasn’t a test. “It’s going to be exciting — and very stressful — but in the end, I think we’ll learn a lot,” says Christina Thomas, a planetary scientist at Northern Arizona University who leads the observation team for the DART mission.
Asteroids the size of Demorphos are likely to hit Earth thousands of times more likely than larger ones that caused previous mass extinction events, and still have the potential to devastate a small nation or country, making these smaller objects a top priority for planetary defense efforts. But they are nothing more than the light stings of ground-based telescopes, making them difficult to detect, let alone study.
When observed, binary asteroid systems are more revealing, because their light dims when one blocks the other. By observing small fluctuations in light from Dimorphos and Didymos, NASA scientists and others were able to work out how fast they rotate and the frequency of the smaller object’s orbit. This knowledge allowed them to design an autonomous navigation system that, aided by a new solar-powered ion engine, will guide DART as it approaches its prey.
What will happen next is anyone’s guess. “People assume it’s rock solid, and we have a solid spacecraft, and we’re basically playing a giant game of pool in space… and you basically solve that as a simple physics equation,” Thomas says. “But there’s a lot going on that makes that not true.”
The biggest uncertainty is the “strength” of Dimorphos, according to DART principal investigator Andy Cheng of the Johns Hopkins University Applied Physics Laboratory. “And that makes a huge difference in terms of the outcome,” he says.
Evidence that not all asteroids are solid, homogeneous rocks have accumulated in recent years. In 2019, the Japanese Hayabusa 2 probe launched a 2-kg copper shell at the asteroid Ryugu, surprisingly exploding a large crater, 14 meters in diameter. Experiment indicated that Ryugu’s surface held together much weaker than expected. The following year, NASA’s OSIRIS-REx probe landed on the asteroid Bennu and sank directly without resistance. These missions emphasized the idea of asteroids’ weakness and set limits on their surface strength – the amount of force needed to deform objects. Scientists estimated that these asteroids were held together by about 1 Pa – the pressure from a piece of paper placed on your hand.
“I’m quite old, and I find it hard to believe that anything could be so weak,” Cheng says. “It reinforced a lesson we’ve learned from planetary geology for decades: You can’t tell if something is rock by looking at pictures.”
Unfortunately, the consequences of a collision are more difficult to predict when the target is made of thousands of loosely connected rocks than when it is solid rock. If DART hits a weak target from the pile of rubble, the resulting crater would evolve over the course of a few hours, a process that could take months or even years to model using conventional computer simulations, says Sabina Radokan, a planetary scientist at the university. Bern.
Recently, she and a colleague optimized computer code to model a 3D shock wave to speed up the computation to a few weeks. Unexpectedly, the new simulations show that the DART effect can transfer four to five times more momentum to a weak target from the rubble pile than a uniform target — enough to reshape the entire asteroid rather than leaving a small impact crater, as they report in Planetary Science Journal in June. DART will get more bang for their buck on a weak target because the bulk structure of the rubble will allow more material to explode from the impact – pushing the asteroid forward like a thrust rocket.
“this is [modeling] Planetary scientist Julie Brisset of the Florida Space Institute, who was not involved in the DART mission, says. “You don’t want to hold on to the old bonding [rock] a story.”
The push to model rubble pile asteroids extends beyond computer screens. Scientists often simulate their impact with lab experiments — firing high-velocity projectiles at different targets, as James Walker at the Southwest Research Institute has done for more than a decade. Recently, the Walker and Radokan team built the first temporary asteroids from the rubble pile for these tests. Walker was shooting horizontal blocks at a slab of rock covered in cement and suspended from a pendulum, while Raducan was shooting straight down into a 7-meter-wide sandpit embedded with small blocks. Both teams have analyzes awaiting publication, and one thing is clear: weaker targets show more dramatic explosions of effects.
A crew of instruments will monitor how well the actual collision matches the simulations. On Sunday, DART deployed a toaster-sized CubeSat that will record the collision and its aftermath with two optical cameras. Meanwhile, the James Webb and Hubble Space Telescopes, along with four ground-based observatories, will take turns observing the point of light. If the Dimorphos is a weak pile of rubble and its output shaft is as large as Raducan predicted, Thomas thinks observatories should be able to catch it light up within hours after the collapse.
“That final cloud … will tell us a lot about the actual physical properties of the target,” Thomas says. “It wouldn’t take long for Demorphos to give us the answers; it would take us more time to figure out what he was telling us.”
The full picture will not appear for another 4 years, when the European Space Agency’s HERA mission arrives to survey the surface of Dimorphos and measure its mass. This will help diagnose the asteroid’s internal structure and aid in future planetary defense missions. In the event of a real threat to an asteroid, the goal is to hit the object with enough force to turn it but not too difficult to vaporize it and send a hailstorm of small rocky shards toward Earth.
Confirmation that Demorphos has a rubble-mound structure would also shed light on a larger question: how the solar system first formed. “Understanding what these small bodies have gone through helps us understand how planetary systems formed,” Brissett says. “They are the remnants of this process.”
For now, however, scientists must wait impatiently while DART approaches Taurus Point, hoping that their preparatory work will be the key to unraveling its secrets.