The hardest push on asteroid Dimorphos in 2022 was not providedby NASA’s half-ton Double Asteroid Redirection Test (DART) spacecraft but instead came from the asteroid itself. As the spacecraft collided with the 160-meter-wide moonlet at 6.15 km/s, the material expelled from the asteroid in the resulting plume provided momentum that was actually greater than that of the impact itself and changed the orbit of Dimorphos around its parent asteroid, Didymos.
The remarkable results were recorded by the Italian LICIACube, a cubeSat built by the Italian space agency, which was ejected from DART just 15 days before the impact, Catalina Albornoz Vasquez says. The spacecraft LICIACube zoomed by the area of the impact at a speed of 21,140 km/h and had just 60 seconds to collect its most important data. During this time, the LEIA and LUKE cameras on board LICIACube took 18 images of the Moon from different angles, the last one as close as 85.3 kilometers to the surface. The images revealed the plume illuminated by sunlight in the head-on photos, with sunlight shining through a cloud of millimeter particles in the images taken from behind.
Analysis of these images, along with data from ground- and orbiting-based telescopes, showed that this asteroid impact had ejected only 16 million kilograms of rock and dust less than 0.5% of Dimorphos’ mass but 30,000 times as massive as DART. This ejecting of mass served as a rocket engine’s nozzle, providing a “short burst” of propulsion, as NASA’s Ramin Lolachi described it. This shortened Dimorphos’ orbital period by 33 minutes, which greatly surpassed the minimum success requirement of 73 seconds.
This is due to the dynamics of the binary asteroid, and was an important factor in the deflection process. Dimorphos, the ‘rubble pile’ asteroid, orbits about Didymos in the same way the Moon orbits our Earth. This ‘rubble pile’ was so weakly bound that the ejected debris had no problem escaping, resulting in ‘bonus momentum’ to the deflection process. This had been predicted for many years in ‘shock physics’ simulations of the impact process using the Spheral code developed by the Livermore Laboratory.
Also, the images from LICIACube helped in understanding the composition and mechanical characteristics of the surface of the asteroid. Results from boulder morphology analysis of the impact area suggest the average equivalent diameter of the boulders is 2.15 meters, Sub-rounded boulders, and the angle of internal friction of the boulders is approximately 32.7°. All these characteristics suggest the boulders in Dimorphos may have come from the catastrophic breakup of a parent body which eventually accumulated into the present structure of the loosely packed Dimorphos.
“Numerical models matched to the cratering data provided by DART’s impacts” suggest that Dimorphos has low surface cohesion (less than 0.03 Pa) and macroporosity of about 35% of the surface. This gives insight about the way shock waves will be transmitted through Dimorphos and eject material. Weakly boulder-packed targets (packing fractions below 30% by volume) are easily shattered, resulting in a large ejecta fraction, while highly packed (packing fraction above 40%) targets efficiently absorb the energy of the shock and reduce mass loss. In the case of Dimorphos, low boulder packing allowed for effective excavation.
The ultimate destiny of the ejecta is a current area of active research. Models have indicated that some meter-sized pieces may impact Mars in as little as 7-13 years or even the Earth. These ejected bodies would be traveling from a few hundred to a few thousand meters per second and would disintegrate in the Earth’s atmosphere but could make it to Mars’ atmosphere. Results from the deflection demonstration directly affect planetary protection. Having demonstrated that a modestly sized spacecraft capable of changing the orbit of a “rubble-pile” asteroid is a possibility, DART has shown kinetic impact to be a viable option, so long as it is sufficient in advance.
The beta momentum gain factor, indicated to be in the range 2.4-4.9, is a reminder that ejecta recoil momentum must be factored in for any missions moving forward. ESA’s Hera mission, which is expected to launch in 2024 and arrive in 2026, will undertake a thorough “crash scene analysis” on the asteroid pair Dimorphos and Didymos. It will determine the mass properties and characterize the exact area affected by the strike. Until then, only the close-up perspective offered by LICIACube currently documents humanity’s first effort to affect the path of a celestial body a accomplishment that occurred not only because of the spacecraft’s impact but also because of the asteroid itself.
