The strange binary Asteroid 65803 Didymos-Dimorphos could be held together like dust bunnies, says an international team of researchers that includes queen rock guitarist and astrophysicist Brian May.
The study, published in the journal Icarus, details how the same forces responsible for building dust bunnies under our beds may be responsible for holding the asteroid Didymos together, says the European space agency (ESA).
Based on the team’s calculations, Didymos requires a surface cohesion of between 3 to 6 Pascals (the standard unit of pressure), and an internal ‘bulk’ cohesion of between roughly 11 to 17 Pascals, depending on particle arrangement and size distribution, says ESA.
Our new study predicted the required amount of cohesion for keeping it stable, Yun Zhang, the paper’s lead author and a postdoctoral researcher in astrophysics at the Observatoire de la cote d’Azur in Nice, France, told me.
“It turned out that the required cohesion (less than 20 Pascals) is very small, which can be attributed to the Van der Waals cohesive force between fine grains,” said Zhang. Therefore, she says, the Didymos primary is very likely to be a rubble pile held together by small Van der Waals forces.
Van der Waals forces are produced due to how electrical charges are distributed within atoms or molecules, says ESA; it’s the force that builds ‘dust bunnies’ in neglected corners of our houses and under our beds. On Earth, however, the force is only strong enough to have an effect on particles a few fractions of a millimeter across, but in the ultra-low-gravity environment of Didymos, such forces could really make a difference.
This kind of study has real-world implications for earth’s planetary defense from near-Earth impactors.
To that end, Didymos-Dimorphos is the target of the first asteroid deflection test (NASA’s Double Asteroid Redirection Test, DART) and the first binary asteroid system that will be characterized by a rendezvous mission (ESA’s Hera) in early 2027, the authors note.
This year NASA’s DART spacecraft lifts off on a journey to impact Dimorphos and shift its orbit, says ESA. Then in 2024 ESA’s Hera mission will perform close-up measurements of the asteroid’s impact crater, composition and mass, as well as precisely measuring its deflection, notes the space agency.
Didymos, the focus of this particular study, is the target of the first asteroid deflection test with the NASA DART and ESA Hera missions, says Zhang. These two missions will use the so-called kinetic impactor technique as a mean to test the effectiveness of deflecting an asteroid’s orbit by impact, she says.
“Would the target be disrupted or just be cratered by the impact?” Zhang wonders. “We don’t know yet.”
The team carried out its research using advanced supercomputer simulations to spin the asteroid apart, says ESA. Then, Brian May and his collaborator, Claudia Manzoni from the London Stereoscopic Company made stereo 3D movies of the disruption event, says ESA.
The researchers will obtain more data about how to deflect these type asteroids when the dart spacecraft impacts a small asteroidal moon of this binary system and then in 2027 when the hera spacecraft makes its rendezvous with the system.
But ESA says it’s expected that bodies the size of Didymos would be accumulated ‘rubble piles’ of material. Even so, Didymos has such fast spin orbit, rotating once every 2.26 hours, theoretically it would logically seem prone to breaking apart.
How could it possibly hold together?
“Cohesion serves as a glue between the different pieces,” Zhang said in a statement. “It originates from the so-called Van der Waals forces, most likely caused by the presence of very small grains between the boulders in the rubble pile.”
Even such low-density asteroids can pose threats to Earth.
The impact threat to Earth posed by such an asteroid would be significant and multifold, says Zhang. The material strength of a rubble pile is 3 to 6 orders of magnitude lower than that of a monolithic body, she says.
If they were impacting Earth, this inherent weak structure could make them more vulnerable to being torn apart by Earth’s gravity and atmosphere, which in turn would make their potential impact locations hard to predict, says Zhang.
As for deflecting such asteroids?
The rubble-pile structure increases more uncertainties, says Zhang, since the researchers don’t yet know about how they would respond to such external deflection forces. Thus, at this point in the research, it’s impossible to know whether the asteroids would crumble like yesterday’s cake or possibly even still hold together.
But it’s clear that even the tiniest of nature’s forces can make a huge difference in deep space.
“It is fascinating to discover that even a very tiny amount of cohesion makes a huge difference in the structural stability of an asteroid,” said Brian May in a statement. “The equivalent of a small leaf on the top of my hand is what makes the difference.”