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 with The predicted growth of the low Earth orbit space debris environment - an assessment of future risk for spacecraft P.H. Krisko ESCG/Jacobs, 2224 Bay Area Blvd., Houston, TX, 77058 paula.krisko-1@nasa.gov Abstract: Space debris is a worldwide-recognized is sue concerning the safety of commercial, military, and exploration spacecraft. The space debris environment includes both naturally occuring meteoroids and objects in Earth orbit that ar e generated by human activity, termed orbital debris. Space ag encies 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 growt h, and formulate mitigation policy for the environment from low Earth orbit (LEO) through geosynchronous orbit (GEO). This paper presents recent results de rived 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-estima tion of the droplet population. The study indicates a LEO envi ronment that is already highly collisi onally active among orbital debris larger than 1 cm in size. Most of the modeled collision events are non-catastrophic (i.e., They lead to a crat ering of the target, but no large scale fragmentation.). But they are potentially mission-ending, and take place between impactors smaller than 10 cm and targets orbital debris impacts have grown around thes e research efforts [28-31]. Recognizing the issue as international has led to coopera tion in the study of space debris through the Inter-agency Space Debris Coordination Committee (IADC) and the United Nations (UN), and to international debris mitigation standards [32,33]. This paper describes the issue of orbital debris and past work on modeling efforts of the LEO debris environment. It presents cu rrent results of a study encompassing the historical period through the near future. It confirms that the inclusion of smaller debris objects (down to 1 cm) in the calculation of collision rates am ong objects in LEO may be necessary to better understand the dangers of the environment. 2.0 Debris Identification -- Known Sources The space debris environment includes both naturally occuring meteoroids and objects in Earth orbit that are generated by hum an activity. The spatial extent of the environment ranges from LEO (200 km through 2000 km) well past GEO (33,000 km through 39,000 km), though the current interest extends generally up to the GEO region. These orbiting human-made objects are termed orbital debris and include objects from sub-microns to meters in cross section. Known sources of orbital debris in LEO include the following, • spent intact satellites past end-of-life, • mission-related debris (i.e ., objects released in th e course of spacecraft deployment and operations), • fragments of intact satellites resulting from accidental or intentional explosions and collisions, • radiator coolant droplets (i.e., sodium potassium, NaK) from re-orbited and ejected RORSAT nuclear cores, • solid rocket motor exhaust products, • ejecta from micro-particle impacts with intact satellite and fragment surfaces, • and paint flakes (i.e., intact satellite and fragment surface degradation products). For the most part the sources of space debris span the size regimes noted in Table 1. Naturally occurring meteoroids display a terrific range in sizes due to their sources, asteroidal and cometary dust and frag ments [34]. But it must be remembered that for both meteoroids and orbital de bris the particle size and fl ux are negatively correlated. As for meteoroids larger than 1 cm, they are much less populous in LEO than orbital debris as shown in Figure 1. The fragmentation or breakup process is re sponsible for over 60% of all cataloged objects in LEO today (i.e., the greatest sour ce of larger than 10-cm debris). In particular, accidental propulsi on-related explosive events involving upper stages have been confirmed in 85 breakup events of a total of 190 to date, and are the most important source of the present-day fragment population. Other sources of fragmentation include spacecraft battery explosions, deliberate explosions or collisions, accidental on-orbit collisions, anomalous breakups, and breakups with unknown causes 2