however, is not constant and can vary significantly (particularly at less than 1,000 km) due to atmospheric heating associated with the 11-year solar cycle. This natural phenomenon has the effect of accelerating the orbital decay of debris during periods of solar maximum (increased sunspot activity and energy emissions). During the last two peaks in the solar cycle, the total cataloged space object population actually declined, because the rate of orbital decay exceeded the rate of space object generation via new launches and fragmentations.
Figure 1-6, which displays the predicted orbital lifetimes for a number of different objects in circular LEOs at different periods in the solar cycle, illustrates the importance of cross-sectional-area-to-mass ratio, altitude, and solar activity in determining orbital lifetimes in LEO. First, objects with low ratios of cross-sectional area to mass decay much more slowly than objects with high area-to-mass ratios. Second, objects at low altitude experience more rapid orbital decay than objects at high altitude. Finally, objects decay much more rapidly during periods of solar maximum than during the solar minimum.
Solar-lunar gravitational perturbations primarily affect the orbital lifetimes of space objects in highly elliptical orbits (e.g., Molniya-class or Geostationary Transfer Orbits [GTO]). Depending on the alignments of the space object's orbital plane with the Moon and the Sun, these forces can either accelerate or decelerate the orbital decay process substantially. For example, a GTO rocket body could reenter the Earth's atmosphere within a few months or remain in orbit for more than a century, depending on the position of the Sun and the Moon at the time of its injection into transfer orbit. GEO missions launched by the CIS routinely take advantage of these forces to limit the lifetime of GTO debris to less than three years, with many objects decaying in less than six months.
Solar radiation pressure normally induces a noticeable effect on a space object's orbit if that object possesses a large area-to-mass ratio. These effects can lead to an increase in the eccentricity of the orbit, which in turn leads to more rapid decay if the resulting lower perigee exposes the space object to significantly greater atmospheric density levels. Insulation materials and inflatable space objects are often strongly affected by solar radiation pressure. Debris from the ruptured Pageos balloon, for example, exhibited strong orbital perturbations due to solar radiation pressure, as has some debris from more conventional fragmentations.
The combination of all of these forces has caused approximately 16,000 cataloged objects to reenter the atmosphere since the beginning of the space era. In recent years, an average of two to three space objects large enough to be cataloged (as well as numerous smaller debris particles) reenter the Earth's atmosphere each day. Over the course of a year, this amounts to hundreds of metric tons of material. This material is