maneuver several hours after separation from the spacecraft.) Both rocket bodies and spacecraft would require sufficient fuel to perform these maneuvers.

Figure 7-2 shows the change in velocity (ΔV) and propellant mass fraction required to perform a deorbiting or lifetime reduction maneuver from various circular low Earth orbits. (The propellant mass fraction is the mass of the propellant divided by the total mass of the space vehicle, including propellant.) As can be seen, the mass of fuel required to deorbit a spacecraft or rocket body is greater than the amount needed to reduce its orbital lifetime. A rocket body with a ratio of cross-sectional area to mass of 0.01 m2/kg in an 800-km circular orbit, for example, would require about half the amount of propellant to reduce its orbital lifetime to 10 years than it would require to deorbit. Performing either maneuver would remove a large, long-lived (up to hundreds of years) hazard from LEO, but the extra fuel required for either maneuver would directly reduce the launch vehicle's or spacecraft's payload capacity, making it less capable and putting it at a disadvantage with competitors that are not carrying out such maneuvers.

Natural perturbing forces can sometimes be used to reduce rocket body orbital lifetimes. Atmospheric drag is obviously a perturbing force with a major effect on the orbital lifetimes of objects that pass through low-altitude regions. Figure 1-6 illustrates how initial altitude can affect the orbital lifetime of various space objects in circular orbits. Figure 7-3 illustrates how orbital lifetimes for objects in elliptical orbits can vary even more sharply, depending on their initial perigee altitude. Clearly, rocket bodies launched into transfer orbits with low perigees experience much more rapid orbital decay than those launched into orbits with higher perigees; when possible, this can be a very effective means of limiting the orbital lifetimes of rocket bodies in highly elliptical orbits.

More subtle gravitational perturbations can also affect the orbital lifetime of objects in geostationary transfer orbits with perigees below about 300 km. Careful selection of the orbit's orientation with respect to the Sun and Moon (by launching at a particular time of day) can cause lunarsolar perturbations to lower the orbit's perigee. Figure 7-4 shows how the orbital lifetime of a rocket body varies depending on the initial sun angle. This technique could be a low-cost option to accelerate orbital decay from certain missions, but it can require major design changes for other missions; a comprehensive analysis is needed for each particular mission to examine possible conflicts with other requirements.

Finally, drag augmentation devices can be used to accelerate the orbital decay of rocket bodies or spacecraft. Drag augmentation, which would be effective only in low-altitude orbits, would involve deploying a device to increase the surface area, and thus the drag, of a space object. Figures



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