Different neutron energies improve the derivation of the asteroid

A research collaboration between Lawrence Livermore National Laboratory (LLNL) and the Air Force Institute of Technology (AFIT) is investigating how neutron energy output through an explosion of nuclear devices can affect the deflection of an asteroid.

Scientists have compared the resulting asteroid deflection of two different neutron energy sources, representing cleavage and fusion neutrons, which allow comparisons with each other. The aim was to understand which neutron energy released from a nuclear explosion is better for bending an asteroid and why it might pave the way for optimal deflection performance.

The work is shown in Acta Astronautica and is led by Lansing Horan IV, as part of a collaboration with LLNL’s Planetary Defense and Weapon Output groups during its master’s program in nuclear engineering at AFIT. Co-authors of LLNL include Megan Bruck Syal and Joseph Wasem of LLNL’s Directorate of Arms and Complex Integration, and co-authors of AFIT include Darren Holland and Maj. James Bevins.

Horan said the research team focused on neutron radiation from a nuclear explosion, as neutrons can be more penetrating than X-rays.

“This means that a neutron yield could potentially heat larger amounts of asteroid surface material, and thus be more effective at deflecting asteroids than an X-ray yield,” he said.

Neutrons of different energies can interact with the same material through different interaction mechanisms. By changing the distribution and intensity of the deposited energy, the resulting asteroid deflection can also be affected.

The research shows that the energy deposition profiles – which map the spatial locations on and below the curved surface of the asteroid, where energy is deposited in different distributions – can completely differ between the two neutron energies compared in this work. When the deposited energy is distributed differently in the asteroid, it means that the molten / vaporized blow-off debris can change in quantity and speed, which ultimately determines the velocity change of the asteroid.

Defeating an Asteroid

Horan said there are two basic options for defeating an asteroid: disruption or deflection.

Disruption is the approach of giving so much energy to the asteroid that it breaks down strongly into many fragments moving at extreme speeds.

“From previous work, it was found that more than 99.5 percent of the mass of the original asteroid would miss the Earth,” he said. “This disruption path will probably be considered if the warning time before an asteroid impact is short and / or the asteroid is relatively small.”

Bending is the softer approach, which involves the asteroid applying a smaller amount of energy, keeping the object intact and pushing it at a slightly different speed on a slightly different orbit.

“Over time, with many years before the impact, even a small velocity change to a distance missing through the earth can be missing,” Horan said. “Deflection can generally be preferred as the safer and more ‘elegant’ option if we have sufficient alert time to institute this type of response. That is why our work has focused on deflection.”

Link energy deposition to asteroid reaction

The work was carried out in two primary phases which include neutron energy deposition and asteroid deflective reaction.

For the energy deposition phase, the Monte Carlo N-Particle (MCNP) radiation transport code from Los Alamos National Laboratory was used to simulate all the different case studies compared in this research. MCNP simulated a separate explosion of neutrons radiating to a 300 m SiO2 (silicon oxide) spherical asteroid. The asteroid was divided by hundreds of concentric spheres and encapsulated cones to form hundreds of thousands of cells, and the precipitation of energy was counted and detected for each individual cell to generate the energy deposition profiles or spatial distribution of energy through the asteroid.

For the asteroid deflection phase, LLNL’s 2D and 3D arbitrary Lagrangian-Eulerian (ALE3D) hydrodynamics code was used to simulate the response of the asteroid material to the considered energy deposits. The MCNP-generated energy deposition profiles were entered and mapped into the ALE3D asteroid to initialize the simulations. The resulting change in deflection velocity was obtained for different configurations of neutron yields and neutron energies, which could quantify the effect of the neutron energy on the resulting deflection.

One small step for deflection

Horan said the work is a small step forward for nuclear deflection simulations.

“One ultimate goal is to determine the optimal neutron energy spectrum, the distribution of neutron energy outputs that deposit their energy in the most ideal way to maximize the resulting velocity change or deflection,” he said. “This paper reveals that the specific neutron energy output can affect the performance of the asteroid deflection, and why it occurs, and serves as a stepping stone towards the larger goal.”

Horan said the research showed that precision and accuracy in the data on energy deposition are important. “If the input of the energy deposit is wrong, we should not have much confidence in the delivery of the asteroid,” he said. “We now know that the energy deposition profile is the most important for large yields that will be used to deflect large asteroids.”

He said if there was a plan to mitigate a large incoming asteroid, the spatial energy deposition profile should be taken into account to model the expected asteroid velocity change correctly.

“On the other hand, the efficiency of energy coupling is always important to consider, even for low yields against small asteroids,” he said. “We found that the magnitude of the precipitation of energy is the factor that most strongly predicts overall asteroid deflection, and affects the final velocity change more than the spatial distribution does.”

For planning an asteroid mitigation mission, it will be necessary to take these energy parameters into account in order to have accurate simulations and expectations.

“It is important that we explore and understand all asteroid mitigation technologies to maximize the tools in our toolkit,” Horan said. “In certain scenarios, using a nuclear device to deflect an asteroid may have several advantages over non-nuclear alternatives. If the warning time is short and / or the incident asteroid is large, a nuclear explosive is our only practical thing option for bending and / or disruption. ”