Ghosts in the Plasma: Tracking the Footprints of Orbital Debris

DENVER  –  Debris from decades of space missions remains in orbit around Earth, threatening thousands of satellites that provide critical infrastructure to our society. Orbital debris comes from fragmentation of worn-down spacecraft, unintentional collisions, tests of anti-satellite weaponry, and other more typical operations in space. With the ever-growing human presence in space, the hazard from orbital debris will only increase if left unchecked. 

The kinetic energy released in a head-on impact of a 50-gram (1.8-ounce) piece of debris with a satellite at a relative speed of 15 kilometers per second (9.3 miles per second) is equivalent to the explosion produced by one kilogram of TNT. This means that even a tiny errant object can disable a spacecraft. Currently, engineers use radar and optical observations to directly track more than a million pieces of debris larger than a few centimeters. However, there are tens of millions of debris smaller than this, which are untracked objects lurking in Earth orbit. Researchers from the Naval Research Laboratory (NRL) and the Institute for Plasma Research (IPR, India) are developing a novel way to detect these objects using the nonlinear interactions of debris with the background space plasma when moving at orbital speeds.

The uppermost part of the Earth’s atmosphere is a layer of plasma – a state of matter that consists of charged particles (electrons and ions). The charged particles in the plasma can oscillate collectively, which results in different waves. Any object immersed in a plasma will obtain an electric charge, which then enables that object to interact with its plasma environment through electric and magnetic fields. Because debris moves faster than characteristic velocities of some plasma waves, the interaction is nonlinear, creating large-amplitude long-lasting plasma structures that may be used for detection and tracking of debris too small to observe optically or with radar. 

For orbital debris speeds in low Earth orbit, the nonlinear interactions can produce structures like solitons, vortices, and shock waves, which can have a larger footprint than the debris itself. Solitons are stable solitary wave packets that preserve their shape as they move in space. For debris speeds just above the ion acoustic speed (the equivalent of sound speed in plasma), these solitons can even propagate ahead of the object, as seen in Figure 1. Sensing such plasma signatures can provide information about the piece of debris that generated it. 

While the concept has been demonstrated experimentally using dusty plasmas (collections of small grains of material suspended in plasma) at IPR, researchers at NRL are working towards a system to sense objects in plasmas typical of near-Earth space. The realistic space environment varies with natural space weather, where the debris is in orbit, and how the object moves relative to the Earth’s magnetic field. These complications can enhance or eliminate the debris-generated plasma signatures, depending on the situation. It is also necessary to differentiate debris-generated signatures from natural fluctuations in the plasma environment. The NRL researchers are incorporating each of these effects in theoretical models and computational simulations, and then testing against ground-based experiments in NRL’s Space Physics Simulation Chamber. 

Debris-generated nonlinear plasma structures provide a new way to sense and characterize unseen objects in near-Earth space. An operational system based on this concept could help characterize the population of small debris and ultimately help protect spacecraft from unanticipated impacts. 


About the U.S. Naval Research Laboratory

NRL is a scientific and engineering command dedicated to research that drives innovative advances for the U.S. Navy and Marine Corps from the seafloor to space and in the information domain. NRL is located in Washington, D.C. with major field sites in Stennis Space Center, Mississippi; Key West, Florida; Monterey, California, and employs approximately 3,000 civilian scientists, engineers and support personnel.