and cost of implementing the debris reduction method. This includes not only the development cost of any new hardware, but also the ''opportunity cost" of any revenue lost or performance sacrificed in implementing the method. The choice of which methods to implement, when to implement them, and in what orbital regions they should be implemented typically involves a trade-off between these two factors.
As described in Chapter 1, there are three main types of mission-related debris: (1) objects released in spacecraft deployment and operations, (2) refuse from crewed missions, and (3) rocket exhaust products. Each of these debris types has very different orbital characteristics and size distributions. Together, they make up 13% of the total cataloged space object population; most of these objects are (as shown in Figures 3-4 and 3-5) located in orbital regions used by spacecraft. In addition, as discussed in Chapter 3, a large population of uncataloged mission-related debris also exists. Although ending the release of mission-related debris will obviously prevent the hazard from these objects from growing and further endangering future space operations, the balance between the costs and benefits of reduction actions varies greatly for the different types of mission-related debris.
Reducing the amount of mission-related debris released in spacecraft deployment and operations (e.g., clamps, covers for lenses or sensors, de-spin devices, pyrotechnic release hardware, wraparound cables) may be one of the easier ways of decreasing the future debris hazard to space operations. These objects make up the great majority of the cataloged mission-related debris population and typically have the longest orbital lifetimes of any mission-related debris. In the past, the practice has often been to simply jettison such items at separation from the launch vehicle or during appendage deployment. By using tethers or other simple devices, however, the release of most of these items can be avoided. Similarly, explosive bolts, which are commonly used to separate rocket upper stages, can be designed to not release large amounts of debris when activated. Because the parent spacecraft or rocket body would retain most objects, however, implementing such measures would not reduce the total mass of debris in orbit. (Chapter 8 discusses the significance of reducing mass in orbit.)
Measures to retain debris created during spacecraft deployment and operations are typically fairly easy to implement without affecting spacecraft operations. (Since the early 1980s, many such methods have been used on U.S. and other spacecraft.) The release of some types of mission-