Near Earth Asteroids have a possibility of impacting with the Earth and always have a thread on the Earth. This paper proposes a way of changing the trajectory of the asteroid to avoid the impaction. Solar sail evolving in a H-reversal trajectory is utilized for asteroid deflection. Firstly, the dynamics of solar sail and the characteristics of the H-reversal trajectory are analyzed. Then, the attitude of the solar sail is optimized to guide the sail to impact with the object asteroid along a H-reversal trajectory. The impact velocity depends on two important parameters: the minimum solar distance along the trajectory and lightness number. A larger lightness number and a smaller solar distance lead to a higher impact velocity. Finally, the deflection capability of a solar sail impacting with the asteroid along the H-reversal is discussed. The results show that a 10 kg solar sail with a lead-time of one year can move Apophis out of a 600-m keyhole area in 2029 to eliminate the possibility of its resonant return in 2036.
Compared with a regular spacecraft, solar sail using H-reversal trajectory requires less mass. The impact velocity of a typical spacecraft along a Kepler orbit is about 30 km per second. A solar sail evolving a retrograde Kepler orbit can raise the impact velocity to about 60 km per second. The solar sail utilizing a H-reversal trajectory raises the impact velocity further to about 90 km per second. Therefore, the impact energy can be greatly enhanced for unit mass and the impact efficiency is much higher. One week point of a H-reversal is that a high performance solar sail is required. It means that much larger area of sail film is required for the same mass of spacecraft. The solar sail leaves the Sun at a high velocity. To impact with the asteroid has a high demand on the navigation, guidance and control system since a small error may make the sail miss the asteroid.
A high performance solar sail can evolve in a H-reversal trajectory. A typical
H-reversal trajectory is realized by reducing the angular momentum of solar sail until it is reversed. Then, the solar sail approaches the Sun to gain energy and leaves the Sun along a hyperbolic trajectory and impacts with the asteroid head-to-head. An optimization method is utilized to maximize the impact velocity. The impact velocity is dependent on the minimum solar distance along the trajectory and lightness number. For minimum distance less than 0.3 AU, the impact velocity is above 90 km/s. For this impact velocity, a solar sail of 10 kg with a lead-time of one year can move the Apophis out of its 600-m keyhole area.