Asteroid impacts pose a major threat to all life on Earth. Several serious impact events (i.e. Chixulub event, Tunguska event and Chelyabinsk event) have aroused people’s attention to the research of planetary defense. For defending large potentially hazardous asteroids (PHAs) with short warning times, the nuclear explosion seems to be the only feasible option. However, nuclear explosions may cause controversy. 

Despite of fragmentation risks, a kinetic impactor remains a promising strategy for asteroid deflection. In 2005, Deep Impact mission released an impactor weighing 370 kg to collide with comet Tempel 1 at a velocity of 10.2 km/s. This impact generated a 0.0001 mm/s velocity change in the comets orbital velocity and decreased its perihelion distance by 10 meters. DART (Double Asteroid Redirection Test) is currently planned to be implemented in October 2022, with a 555 kg spacecraft hitting Didymos' moon at a relative velocity of 6.65 km/s, generating a velocity change of 0.8-2 mm/s. Due to the limited weight of the artificial impactor, using a spacecraft to hit a large asteroid is similar to using an egg to hit a rock. Even at a higher impact velocity, the improvement of the deflection efficiency for large asteroids is limited.

The Enhanced Kinetic Impactor (EKI) concept is proposed to deflect large PHAs via maneuvering space rocks. The EKI concept is described in four key stages as follows. 1) Launch. An existing heavy launch vehicle, e.g., Long March 5, is used to launch an unmanned spacecraft from Earth. 2) Rendezvous with a Near-Earth Asteroid (NEA) & Collect Rocks. The unmanned spacecraft is used to rendezvous with an intermediate NEA. More than one hundred tons of rocks are collected from the NEA as the EKI, an entire NEA can also be captured as the EKI if the NEA is very small. 3) Maneuver. After the rocks are assembled into the EKI, the electric propulsion system begins to maneuver the EKI away from the original orbit toward the PHA. 4) Impact PHA. The EKI is maneuvered to impact the PHA at a high speed. After impact, the PHA will be farther away from Earth during the close encounter. 

A mission for deflecting Apophis is designed to demonstrate the feasibility and power of the EKI.  Its minimum geocentric distance will be 0.00025 AU in 2029, its diameter is about 350 m and its mass is about 6.1e10 kg. If the warning time is 10 years, the EKI can produce a velocity increment (∆v) of 39.81 mm/s in Apophis, thereby increasing the minimum geocentric distance during the close encounter in 2029 by 1,866.93 km, with a propellant cost of 2.98 t. Compared with a classic kinetic impactor, the deflection distance can be increased one order of magnitude. 

This idea was originally inspired by Asteroid Redirect Mission (ARM), which was designed to redirect a space rock into a stable orbit around the moon, once it's there, the astronauts will explore it and return with samples. Although this ambitious mission had been cancelled in 2017, we believe that the research results of ARM have more profound value. Here is an interesting question: what can we do with the captured rocks? Mazanek, D. D. et al. from NASA proposed to use captured rocks to form an Enhanced Gravitational Tractor (EGT), which uses the collected rocks to augment the mass of a gravitational tractor. In this paper, we propose the Enhanced Kinetic Impactor (EKI) concept, which uses the collected rocks to impact PHAs.

By using space rocks as the impactor, the EKI concept has the following main advantages: 1) breaking through the limitation of the ground-based launch constraints, which means that the mass of the impactor can be increased from several tons to more than one hundred tons; 2) combining the areas of science, planetary defense and exploration, which can generate more scientific returns compared with the classic kinetic impactor. In the future, for defending large potentially hazardous asteroids (PHAs) with short warning times, the EKI concept provides another option besides the nuclear explosion.

At the current stage, the EKI is a conceptual study that requires in-depth research on key technologies. We welcome further discussion/cooperation about EKI or other planetary defense related topics.

 The full article Enhanced Kinetic Impactor for Deflecting Large Potentially Hazardous Asteroids via Maneuvering Space Rocks by Mingtao Li et al. is available at 

www.nature.com/articles/s41598...

Acknowledgments

The paper is co-authored with Yirui Wang, Youliang Wang, Binghong Zhou and Wei Zheng. This research is supported by the Beijing Municipal Science and Technology Commission (Z181100002918004), Strategic Priority Program on Space Science (XDA1502030502, XDA15014900). Mingtao Li is also supported by Youth Innovation Promotion Association CAS and CAS Interdisciplinary Innovation Team.