Solitons for Orbital Debris Detection and Tracking

Small sized orbital debris objects (<5 cms) are hard to detect optically from ground based instruments and pose a major threat to space assets in the Low Earth Orbit (LEO) and Geosynchronous Orbit (GEO) regions. A novel idea of exploiting plasma effects to provide an indirect means of detecting and tracking such objects was first proposed in 2015 [1]. The basic concept is as follows. Since the debris objects are moving in the ionospheric plasma medium they can get highly charged due to the flow of electron and ion currents on them. As charged objects their motion can create electrostatic and/or electromagnetic wakes in a manner analogous to wakes created by objects moving in a neutral fluid. However, when the object speed exceeds a critical velocity, like the phase speed of a collective mode of the medium, it can excite nonlinear fore-wake structures in the form of solitons that move away from it in the forward direction. Such a phenomenon, that is well known in hydrodynamics [2], was shown to be theoretically possible in plasmas when the speed of the charged object exceeded the ion acoustic velocity [1], a condition that is easily fulfilled by debris objects in the LEO region. Two kinds of solitons were predicted – forward moving precursor solitons and stationary pinned solitons that remained attached to the debris object as an enveloping nonlinear electrostatic structure. The calculated scale sizes and amplitudes of these nonlinear structures were found to be comparable to those of ionospheric irregularities and hence within the detection capabilities of ground based radars or in-situ sensors. It was therefore suggested that these solitons could provide an indirect means of detecting and tracking small sized debris objects. This basic idea was subsequently strengthened and developed in a number of later works. An experimental verification of the existence of precursor solitons was provided in a laboratory experiment where upstreaming dust acoustic solitons were observed when a dusty plasma was made to flow supersonically over a stationary electrostatic potential hill [3]. The experimental results were well validated by model calculations based on a forced Korteweg-de Vries (fKdV) equation [3] as well as fluid and molecular dynamic simulations [4]. The original calculations of [1], were further extended and refined in a detailed feasibility study [5], to work out the amplitude, width, and production frequency of ion acoustic solitons that may be produced by millimeter and centimeter-scale orbital debris as a function of the debris’ size, velocity, and location (altitude, latitude, and longitude) about the Earth. Since electrostatic structures can suffer strong Landau damping in the Low Earth Orbit (LEO) region they can be quite short lived. Electromagnetic nonlinear structures might provide a better alternative by creating longer life time and larger spatial extent solitons. Particle-in-cell (PIC) simulations [6] and fluid model studies [7] have delineated the existence regimes and characteristic of such electromagnetic structures. Presently there are efforts to establish their existence in a controlled laboratory experiment in the large plasma device at the Naval Research Laboratory (NRL), U.S.A. More realistic theoretical modeling of the soliton dynamics that accounts for three dimensional effects as well as inhomogeneous wake flow effects are currently in progress. In view of all these developments the concept of soliton detection of debris objects has been identified as an attractive option in the recent IARPA inititiative [8] inviting proposals based on novel techniques for debris detection and tracking.