The predicted growth of the low-Earth orbit space debris environment — an assessment of future risk for spacecraft

Space debris is a worldwide-recognized issue concerning the safety of commercial, military, and exploration spacecraft. The space debris environment includes both naturally occuring meteoroids and objects in Earth orbit that are generated by human activity, termed orbital debris. Space agencies around the world are addressing the dangers of debris collisions to both crewed and robotic spacecraft. In the United States, the Orbital Debris Program Office at the NASA Johnson Space Center leads the effort to categorize debris, predict its growth, and formulate mitigation policy for the environment from low-Earth orbit (LEO) through geosynchronous orbit. The current paper presents recent results derived from the NASA long-term debris environment model, LEGEND. It includes the revised NASA sodium potassium droplet model, newly corrected for a factor of two over-estimation of the droplet population. The study indicates a LEO environment that is already collisionally active among orbital debris larger than 1 cm in size. Most of the modelled collision events are non-catastrophic (i.e. they lead to a cratering of the target, but no large scale fragmentation). They take place between impactors smaller than 10 cm and targets larger than 10 cm. Given the small size of the impactor these events would likely be undetectable by present-day measurement means. The activity continues into the future as would be expected. Impact rates of about four per year are predicted by the current study within the next 30 years, with the majority of targets being abandoned intacts (spent upper stages and spacecraft). Still, operational spacecraft do showa small collisional activity that increases over time as the small fragment population increases. And such an event would be potentially mission-ending for the spacecraft.