Lawrence Livermore National Laboratory



Richard London

Overview

The goal of this project was to use radiation-hydrodynamic simulations to analyze the hazards posed to spacecraft by collisions with interstellar dust grains. Dust grains pass through thin structures, such as light sails, with little damage. For thicker structures, all the grain's impact energy will be imparted, possibly causing significant damage. We conducted a quantitative assessment of damage caused by grain–vehicle collisions in order to develop designs to reduce damage. We performed computer simulations of grain–vehicle collisions that used a radiation-hydrodynamic code to model the deposition of grain energy in the material, energy transport by conduction and radiation, expansion of the heated volume due to melting, and other processes that would damage the surrounding material.

We found that the energy imparted to a given material during a dust-grain impact is deposited in a long, thin cylindrical volume, creating temperatures higher than 108 K. The outward spreading distribution of this energy can produce a large volume of damage, but radiation and evaporation can remove energy, thereby limiting the damage. We investigated the feasibility of reducing impact damage by placing shielding material on a spacecraft's leading surface, as well as by placing a thin shield in front of the vehicle to atomize and disperse the grains before they strike the main body of the craft.

Background and Research Objectives

For travel to nearby stars, the preferred mode of propulsion will be a light sail driven by an Earth-based high-powered laser (Forward 1962 and 1984). Spacecraft velocities of approximately one-tenth the speed of light (0.1c) will be required for mission durations that do not exceed a single human lifetime. At such high velocities, damage to the sail and the spacecraft by collisions with interstellar dust grains are a significant challenge to mission success.

Previous work (Early and London 2000 and 2015) to estimate the damage to a light sail by dust-grain collisions concluded that the area destroyed by a single collision would be limited to the cross-sectional area of the dust grain itself since only a small fraction of the grain's energy is imparted to the sail's thin material. Given the anticipated dust-grain density and size distribution in the medium of interstellar space, the total damaged area would be small. However, dust-grain damage to the bulk of a spacecraft might be significant.

Recently, the privately funded Breakthrough Starshot Initiative was announced (Oberbye 2016), the goal of which is to study and describe the technological challenges related to sending a miniature spacecraft to the nearest star, Alpha Centauri, and sending back imaging and other data (Lubin 2016). Several technical challenges that require research were identified (Starshot 2018). Among them is damage to the spacecraft caused by collisions with interplanetary and interstellar dust.

Describing the nature of the interaction of a dust grain with a high-speed spacecraft is essentially high-energy-density (HED) science. The kinetic energy of a collision with a dust grain will be largely transferred to the spacecraft material, creating an initial super-hot cylinder-shaped volume with a radius approximately that of the dust grain (1 µm) and a depth approximately that of the stopping length (10–1,000 µm) of the dust grain. The temperature of this heated cylinder is about 10 keV. To determine the volume of material that is ultimately eroded and/or damaged by this enormous energy deposition, one must determine how much of the initial energy is removed from the spacecraft by radiation and hydrodynamic flow and how much remains to create damage.

We used Lawrence Livermore National Laboratory's unique expertise in HED science to demonstrate the application of quantitative numerical simulations of dust-grain damage and to begin to explore damage-mitigation strategies. These simulations showed that the stopping distance of a grain upon impact was substantially increased by the high temperatures created in the spacecraft's material. Other simulations were used to study the energy transport from the initial elongated region directly heated by a grain's impact and to estimate the mass of material that was melted. A two-dimensional simulation was performed to show the size and shape of the crater resulting from a grain impact. These simulations indicated that the actual mass of the spacecraft's material that was melted was approximately 25 percent of the maximum expected value, where all of the dust grain's imparted energy caused melting.

Our final analysis of all simulations indicated that dust-grain damage can be reduced by (1) using an optimal geometry for the design of the spacecraft, (2) placing shielding material on the front surfaces and leading edges of the spacecraft, and (3) by mounting a thin foil at some distance ahead of the craft to ionize and explode dust grains.

Impact on Mission

This research supports the NNSA goal of strengthening the science, technology, and engineering base by broadening our understanding of future security needs. It also supports the Laboratory's research and development challenge in space security, as well as its core competencies in HED science and computer-based simulation and data science. The accomplishment of the goals of this project was facilitated by the use of the Laboratory's unique expertise in computational simulations in this field.

Conclusion

This small project produced valuable preliminary results on the effects of dust-grain collisions on interstellar spacecraft. The publication of the results and the personal connections made with external researchers has placed the Laboratory in a good position to compete for future external funding in this field of study. The connections with university scientists may lead to future collaborative work and will aid in the promotion and advancement of HED science in the academic community.

References

"Starshot," Breakthrough Initiatives (website), accessed 20 December 2018, https://breakthroughinitiatives.org/initiative/3.

Early, J. T. and R. A. London. 2000. "Dust Grain Damage to Interstellar Laser-Pushed Lightsail." Journal of Spacecraft and Rockets 37(4): 526–531. doi: 10.2514/2.3595.

——— . 2015. "Dust Grain Damage to Interstellar Vehicles and Lightsails." Journal of the British Interplanetary Society 68: 205–210.

Forward, R. L., 1962. "Pluto - The Gateway to the Stars." Missiles and Rockets 10, 26–28.

——— . 1984. "Round-Trip Interstellar Travel Using Laser-Pushed Lightsails." Journal of Spacecraft and Rockets 21(2): 187–195. doi: 10.2514/3.8632.

Lubin, P. 2016. "A Roadmap to Interstellar Flight." Journal of the British Interplanetary Society 69(2): 40–72.

Overbye, D. 2016. "Reaching for the Stars, Across 4.37 Light-Years," New York Times, April 12, 2016, http://www.nytimes.com/2016/04/13/science/alpha-centauri-breakthrough-starshot-yuri-milner-stephen-hawking.html.

Publications and Presentations

London, R. A. and J. T. Early. 2018. "Evaluation of the Hazard of Dust Impacts on Interstellar Spacecraft.," Journal of the British Interplanetary Society 71(4): 133–139. LLNL-JRNL-758538.