An enormous number of near-Earth asteroid (NEA) orbit around the Sun, and among them 2072 NEAs, which are recorded in the Minor Planet Center (MPC) database, belong to the class of potential hazardous near-Earth asteroids (PHAs). These PHAs frequently make close approaches to Earth's orbit, and therefore, the hazard caused by PHAs is still a very real and ever-present threat. Faced with potential threats of PHA impacts, asteroid defense has been discussed with growing interest. Monitoring and early warning of those PHAs are the premise of planetary defense. In a research paper recently published in Space: Science and Technology,

Xiangyu Li, from School of Aerospace Engineering, Beijing Institute of Technology, proposed a novel concept of surveillance constellation of heterogeneous wide-field near-Earth asteroid (NEA) surveyors (CROWN), in which six space-based surveyors are loosely deployed in Venus-like orbits to detect the NEAs along the direction of the sunlight.

First of all, the author discussed the concept and overall design of the NEA surveillance constellation. Generally, an asteroid defense system should obey the circle of OODA, which included observation, orientation, decision, and action. The proposed CROWN was a key part of the asteroid defense system, which was mainly responsible for observation and orientation, i.e., the detection and early warning of the PHAs and tracking and orbit determination (OD) of the target.

The surveillance constellation contained a mothership and six daughter surveyors (space-based optical telescopes). The Mothership corresponded for providing the maneuverability required to transfer and deployment. Six daughter surveyors were loosely deployed on Venus-like orbits, which are equipped with the telescopes pointing the backward of the Sun and detect NEAs approaching from the Sun direction. Moreover, the methods and aspects for evaluating the performance of the proposed CROWN were presented.

In general, the performance of a surveillance constellation can be evaluated in aspects of the constellation, the surveyor, and the payload. For each aspect, both the ability and the cost were considered. For constellation, the abilities included the surveillance convergence and the OD performance. The costs of the constellation contained the energy required for transfer, deployment, and maintenance. The performance of a single surveyor can be evaluated by the life-circle costs and the relatively favorable cost/benefit ratios. The performance indexes of the payload included the sensitivity, accuracy, power consumption, and the price.

Afterwards, the author introduced the overall mission of the CROWN, which includes four stages. Firstly, the combination of the mothership and the daughter surveyors approached from the Earth to the Venus. Secondly, the combination entered the parking orbit (the Sun-Venus L1 halo orbit or the Sun-Venus L2 halo orbit) through the stable manifold.

In the third stage, deployment was realized with the help of the Sun-Venus manifold. The mothership released the daughter surveyors, and the daughter surveyors flied to the desired location through the unstable manifold. After releasing all the daughter surveyors, the mothership stayed on the Sun-Venus Lagrange point orbit for relay communication. In the final stage, a near-hexagon constellation was formed, and detection was carried out.

Then, the author detailed the design methods and results of the transfer and deployment. Two kinds of dynamic models, i.e., the dynamics in heliocentric inertial coordinate system and the dynamics in Sun-Venus rotating coordinate system, are proposed. Regarding the transfer trajectory design, the author employed the dynamics in heliocentric inertial coordinate system, and designed a three-impulses energy-saving transfer to the SVL1/SVL2 halo orbits. As for the constellation deployment, the author used the circular restricted three-body problem (CRTBP) dynamics to represent the motion of deployment in the Sun-Venus three-body system and proposed an energy-saving deployment strategy through the unstable manifolds.

Besides, the observation performance of the surveillance constellation was tested. A total of 2072 PHAs among 13916 NEAs were considered and an asteroid observation model was used to simulate observations. The author assumed that the mission began on January 1, 2031 and lasted for five years. Firstly, the visibility of the asteroids in the database was simulated. Then, the ability of the surveillance system to observe asteroids was analyzed.

Moreover, the influence of the limited apparent visual magnitude on the performance of the telescopes was investigated and the virtual PHAs were employed to test the performance of the proposed surveillance constellation. It could be seen that the surveillance constellation can provide effective early warning for almost all PHAs, and only 4 of 2072 PHAs were invisible during the 5-year mission.

Finally, the author studied the NEA orbit determination using the CROWN, and analyzed the accuracy results. Apart from detection or early warning, the proposed surveillance constellation can also be applied for tracking and determining the orbits of the PHAs because the telescopes can provide line-of-sight information. Simulation results showed that the OD accuracy was approximately 5km after 40 days of measurement using one surveyor or 10 days of measurement using two different surveyors. It was obviously shown that the OD using two observers showed better capability than OD using one observer with respect to the accuracy.

Research Report:Near-Earth Asteroid Surveillance Constellation in the Sun-Venus Three-Body System

Related Links
Beijing Institute of Technology
Asteroid and Comet Mission News, Science and Technology

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