Spheral hydro simulation that was used to calculate the hydro model: 1 Megaton at just a few meters from an asteroid measuring 100 meters in diameter (with Bennu form). Colors denote velocities. The legend is cm/us which is equal to 10 km/s. Credit to Lawrence Livermore National Laboratory
Scientists will often stage a deflection if an asteroid is on an Earth-impacting path. This is where the asteroid's velocity is slightly decreased while the bulk of it is kept together.
A deflection can be achieved by either a kinetic impactor, or a standoff nuclear explosion. If the time required to deflect an asteroid is too short, you can add a lot of energy and disperse it into smaller pieces. This is disruption, and is what many people associate with planetary defense. Scientists would like to be able to predict more, but they must also be ready for any scenario. There are still many undiscovered near-Earth asteroids.
New research now examines how different asteroid orbits impact the fate of fragments. Using initial conditions from hydrodynamics calculations, a 1-megaton yield device was placed a few meters below the surface of an asteroid with a Bennu-shaped and 100-meter diameter (1/5 of Bennu's scale, a near-Earth object discovered in 1999).
This work was published in Acta Astronautica by Patrick King, a former Lawrence Livermore National Laboratory Graduate Scholar Program Fellow who collaborated with LLNL’s Planetary Defense group to complete his Ph.D thesis. King is currently a physicist for the Space Exploration Sector at Johns Hopkins University Applied Physics Laboratory. Megan Bruck Syal and David Dearborn are co-authors.
The paper's results are encouraging: The disruption was carried out just two months prior to the Earth impact date for all five asteroid orbits. This allowed the fraction of impacting material to be reduced by a factor of 1,000 or greater (99.9% of the mass is missing Earth). A larger asteroid would have a less robust dispersal, but even a reduction of an order of magnitude in dispersal velocity would result in 99.9% of the mass being lost to Earth, provided disruption is performed at least six months before the impact date.
King stated that one of the difficulties in assessing disruption is the need to model all fragment orbits. This is usually far more difficult than modeling a simple deflection. These are the challenges we must face if disruption is to be considered as a strategy.
King stated that the main finding of the research was that nuclear disruption can be a very effective defense against last resort. He said that the study of 'late' disruptions meant that the impacting body has been disintegrated shortly before it impacts. "Kenetic impactors can be used to deflect the impacting bodies when you have a lot of time, typically a decade.
One of the many benefits of Kinetic Impactors is that they are well-known and being tested on real missions such as the DART mission. It can also handle a wide variety of threats if you have enough time. They do have limitations so be aware that there are many options available in an emergency.
Owen stated that this paper is crucial for understanding the implications and requirements of disrupting an asteroid near Earth. Spheral was Owen's software. It was used to simulate the nuclear destruction of the original asteroids. This model followed the detailed physics of breaking up and shocking the original rocky asteroid. Then, it captured the properties of the fragments. The team then used Spheral for tracking the evolution of the fragment cloud's gravitational structure, taking into account the effects of fragments on each other and the gravitational influences of the sun, planets, and moons.
He said, "If we spot a dangerous object that is likely to strike the Earth too soon to safely divert it," he added. This is a complex orbital question, but if you break down an asteroid into smaller pieces, each fragment will follow its own path around the sun and interact with the other planets. The cloud will form a curving stream of fragments that follows the original path taken by the asteroid. The speed at which these pieces spread (along with the time it takes for the cloud to cross Earth's path) will tell us how many will hit the Earth.
Bruck Syal stated that the work addresses a key goal set by the White House OSTP's National Near-Earth Object Preparedness Strategy (NEO) Strategy and Action Plan: To improve NEO modeling and prediction, and information integration.
She said that "our group continues to refine its modeling approaches for nuclear disruption and deflection, including ongoing improvements in X-ray Energy Deposition modeling, which sets initial blowoff conditions and shock conditions for nuclear disruption problems." This paper shows how modern multiphysics tools can be used for simulating this problem across multiple relevant physics regimes.
Continue reading
More information: Patrick K. King and colleagues, Late-time small bodies disruptions for planet defense, Acta Astronautica (2021). Information from the Journal: Acta Astronautica Patrick K. King and colleagues, Late-time small bodies disruptions for planet defense, (2021). DOI: 10.1016/j.actaastro.2021.07.034
