Abstract
This chapter introduces the challenge of space debris and the concepts of on-orbit servicing and active debris removal. The evolution of the debris population is put into a historical perspective, observation and modelling methods are described, and internationally agreed mitigation strategies are briefly introduced. Proposed mission types and the required technologies for servicing and removal activities are detailed and their challenges explained. Where applicable, past, current, and proposed activities are summarized to illustrate the concepts.
TopSpace Debris
Objects launched into orbits high enough that the drag of the residual atmosphere of Earth does not significantly impact their orbital motion require a very long time to fall back to Earth. Since the beginning of the Space Age, many objects were left in orbit, ranging from small ejectables1 to defunct satellites and burned-out upper stages of rockets. While the latter mostly fall back relatively quickly due to the higher atmospheric drag, there still are a significant number of satellites in orbit launched during the first years of space exploration. A good example is the Vanguard 1 satellite, the oldest satellite still in orbit, which was launched into a 658-km by 3,840-km elliptical orbit in 1958 and has a projected lifetime of 240 years (Easton & Votaw, 1959; McLaughlin Green & Lomask, 1970). Spacecraft in higher, more circular orbits have orbital lifetimes measured in thousands of years. Satellites in the highest orbits used today stay there practically forever.
The continued launching of spacecraft into orbit leads to a congestion of the orbital bands most useful for the current and future space industry: the low Earth orbit (LEO) and the geostationary orbit (GEO). In GEO, the availability of orbital slots is inherently limited, constrained not only by the minimum safe distance between two objects, but also because of possible radio interference between satellites operating in the same communication bands. The allocation of slots is overseen by the International Telecommunications Union (ITU), but in some cases the inter-satellite spacing is already well below the recommended limits2. Although remarkable gains in efficiencies and performances of communication satellites have been achieved in the past decades, many expect that the demand for new platforms in GEO will continue to rise (ASD-Eurospace, 2012). This is not only due to the rising global demand of satellite communications, but also because of the increasing utilization of inter-spacecraft communication for both manned and unmanned systems. The rise to fame of small satellites in recent years may, however, diminish the demand for large GEO platforms.
Key Terms in this Chapter
Geostationary Orbit: A spacecraft in geostationary orbit (GEO) orbits Earth at an altitude of 35,786 kilometers above the equator and appears at a fixed position in the sky. Its orbital period is equal to Earth’s rotational period.
Space Debris: Space debris is a term for the mass of defunct, artificially created objects in Earth orbit.
Active Debris Removal: The removal of a space debris object from orbit such that it either burns up in Earth’s atmosphere or is placed in an orbit of little operational value.
Low Earth Orbit: A low Earth orbit is an orbit around Earth with an altitude of less than 2,000 kilometers.
Chaser: The spacecraft “chasing” after a damaged satellite or space debris object that it shall repair or deorbit.
Target: The target of an on-orbit servicing or active debris removal mission can be either another spacecraft or a space debris object.
Satellite: An artificial satellite is a man-made object intentionally placed into orbit. Satellites are oftentimes manned or unmanned spacecraft.
Deorbiting Device: A device used to remove a spacecraft or debris object from orbit, which may be either active (propulsion unit) or passive (drag sail, electrodynamic tether).
On-Orbit Servicing: A spacecraft is used to repair, upgrade, refuel, or deorbit another spacecraft in orbit.