On 26 September, an act of targeted violence will unfold 11 million kilometers from Earth, as a spaceship about the size of a vending machine crashes into an asteroid. Dimorphos is an innocent bystander and poses no threat to our world. The DART is the first ever field test of a planetary defense mission.

The hope is that the collision will cause Dimorphos to go closer to Didymos. The idea that asteroids could be diverted onto safer courses is supported by a successful strike. New simulations and lab experiments show that the fate of the mission depends on the question of whether small asteroids are solid boulders or loose heaps of rubble.

When the exercise is not a test, the answer should be revealed from the crater and the ejected debris. The observation team for the DART mission will be led by a planetary scientist at Northern Arizona University.

Smaller asteroids are the top priority for planetary defense because they are thousands of times more likely to hit Earth than larger asteroids that have caused mass extinction events. They're not more than pinpricks of light to telescopes, so they're hard to see.

A photo of the surface of Ryugu
A Japanese mission to the asteroid Ryugu indicated such small bodies may be loosely held together piles of rock.R. Jaumann et al., Science (2019) 10.1126/science.aaw8627

The light dims when one body blocks the other. NASA scientists have been able to learn how fast Dimorphos and Didymos spin by observing small fluctuations in the light from them. They were able to design a navigation system that will steer DART as it closes in on its prey.

Who knows what will happen next. Thomas says that people assume that it is a solid rock and that we are playing a giant game of billiards in space. That isn't true because there's so many other things happening.

The strongest uncertainty is the strength of Dimorphos, according to Andy Cheng. That makes a big difference in the outcome.

There are a lot of clues that not all asteroids are solid. The Hayabusa2 probe shot a 2- kiloogram copper projectile into the asteroid, causing a crater 14 meters across. The experiment showed that the surface was held weakly. NASA's OSIRIS-REx probe sank on the Bennu asteroid after landing. The idea of weak asteroids was confirmed by these missions. The asteroids were held together by a piece of paper, according to scientists.

Researchers at the Astrobiology Center launched 20-millimeter projectiles into a bed of sand and porous ceramic balls to simulate the cratering process from the Double Asteroid Redirection Test’s impact on a rubble-pile asteroid (top). Similarly, a group at the Southwest Research Institute shot masses into rocks cemented together (bottom).(top to bottom) SwRI; Ormö et al. (2022) https://doi.org/10.1016/j.epsl.2022.117713

Cheng finds it difficult to believe that anything can be so weak. You cannot tell whether a rock is a rock by looking at it.

It is more difficult to forecast the consequences of an impact when the target is made up of a lot of loose rocks. It would take months or even years to model the crater if DART hits a weak rubble-pile target.

She and her colleague were able to speed up the calculation with the help of a computer code. In The Planetary Science Journal in June, they reported that the DART impact could transfer up to five times more force to a weak rubble pile target than a consolidated one. The loose rubble structure would allow more material to spurt from the impact and DART would get more bang for its buck.

A planetary scientist who is not involved in the DART mission says that themodeling is cutting-edge. You don't want to follow the same story.

A trailing CubeSat will help monitor the impact of the Double Asteroid Redirection Test.

graphic showing Dimorphos and DART

There's a push to model rubble-pile asteroids. James Walker has been shooting high-velocity projectiles at different targets at the Southwest Research Institute for more than a decade. The first makeshift rubble-pile asteroids for these tests have been constructed by Walker's team. Walker has been launching mass at a panel of rocks covered in cement and suspended on a pendulum, while Raducan has been firing directly down into a 7-meter-wide sand pit. One thing is clear, both teams have analyses waiting to be published, and the weaker targets show more dramatic explosions from the impacts.

The actual collision will be watched by a group of instruments. The LICIACube is a small device that will record the collision and its aftermath with two cameras. Four ground-based observatories and two space telescopes will take turns watching the dot of light. Thomas thinks that the observatories should be able to catch Dimorphos lighting up within hours after the crash.

The final cloud will tell us a lot about the target. It will take us more time to figure out what Dimorphos was saying.

The European Space Agency's Hera mission will arrive in 4 years to survey Dimorphos's surface and measure its mass. The asteroid's internal structure will be helped by this. In the event of a real asteroid threat, the goal is to hit the body just hard enough to divert it but not so hard to destroy it.

The bigger question is how the Solar System first took shape. Understanding what small bodies went through can help us understand planetary systems. The remnants of the process.

The scientists are waiting for DART to approach its bull's-eye, hoping their preparation work will be the key to unlocking its secrets.