Current characteristics and trends of the tracked satellite population in the human space flight regime

Since the end of the Apollo program in 1972, human space flight has been restricted to altitudes below 620 km above the Earth's surface with most missions restricted to a ceiling below 400 km. An investigation of the tracked satellite population transiting and influencing the human space flight regime during the past 11 years (equivalent to a full solar cycle) has recently been completed. The overall effects of satellite breakups and solar activity are typically less pronounced in the human space flight regime than other regions of low Earth orbit. As of January 2006 nearly 1500 tracked objects resided in or traversed the human space flight regime, although two-thirds of these objects were in orbits of moderate to high eccentricity. Since the beginning of the International Space Station era, the spatial density of tracked objects in the 350-400 km altitude regime has demonstrated a general decline, decreasing by 40% by the beginning of 2006. On the other hand, the region immediately above 600km experienced a significant increase in its population density. This regime is important for future risk assessments, since this region represents the reservoir of debris which will influence human space flight safety in the future. The paper seeks to put into sharper perspective the risks posed to human space flight by the tracked satellite population, as well as the influences of solar activity and the effects of compliance with orbital debris mitigation guidelines on human space flight missions. Finally, the methods and successes of characterizing the population of smaller debris in human space flight regimes are addressed.