# Sources of Extraterrestrial Rare Earth Elements: To the Moon and Beyond
## 2 The Criticality of REEs in Our Society  
The REEs have thus emerged as a critical materials group over the past 20 years and the uses, market demands, and holders of these materials have played integral roles in the growth of the global economy over this time period [10]. Specifically, REEs are deemed critical to the growing global green economy [9]. Generally, the demand for REEs ultimately relates to the unique orbital structures and transitions of electrons in orbitals of the REEs such as transitions involving intra-4f or 4f-5d that produce fluorescent light emissions, oxidation state changes in reactions or the magnetic properties of REEs. In a 2011 USGS report, REE applications market sectors were classified as metallurgy neodymium-iron-boron magnets catalysts glass including polishing and additives and other uses and detailed consumption in these areas are based on available reviewed 2008 data [11]. These general resource sectors have permeated nearly every aspect of the global economy with a wide variety of uses. The REEs are also a critical material in clean energy technologies and within major areas of utility including wind turbines, lighting, magnets, and vehicles [9]. Several industry sectors are heavily dependent on the REEs owing to their magnetic, electrical, and optical properties. These industry sectors include but are not limited to defense, power generation, energy efficiency, catalysis, medicine [10]. Figure 2: Figure modified from the United States Department of Energy [9] highlighting the criticality of rare-earth metals, starting in 2015. The star symbols for each element are colored to correspond with the criticality index in the legend. Note that the REEs, particularly dysprosium (Dy), Eu, Nd, terbium (Tb), and Y, are associated with both a high supply risk, and are considered highly valuable to clean energy within the next 13 years. The advent of REE based magnets in the 1960s revolutionized the utility of wind power [12]. Wind turbines utilize REE magnets in their dynamos. The REEs functionally provide stronger magnetic field in smaller volumes compared to their traditional Fe-based counterparts [12]. Thus, wind turbines have become an increasing important renewable energy source in the last 10 years and this technological evolution has been directly driven by REEs [9]. In addition, the REEs are used in a multitude of defense technologies in the US and are considered a strategic resource. Examples of some specific defense uses of REEs include motors for disk drives in numerous aircraft, land vehicles and command and control facilities, optical technologies including laser weapon detections and countermeasures and other equipment, satellite communications, radar and sonar components, as well as other technologies [13]. The REEs are also well recognized catalysts. For example, Ce is well recognized as an oxidative catalyst [14; 15] and Eu has been used as a catalyst in molecular synthesis [16; 17]. This is an area of use that is expected to grow as catalysis is directly tied to energy efficiency [9]. In a recent review, luminescence, medical imaging, and therapeutic applications were highlighted as the most important microcrystal and nanocrystal application areas [18]. In the context of medical technology, optical imaging, X-ray tomography (CT), positron emission tomography (PET) and magnetic resonance imaging (MRI) techniques (as stated earlier) can be enhanced by use of REE nanoparticles typically involving Dy, Gd, Tb, and Eu [18]. REE nanoparticles have also been investigated and used in the treatment of cancer in photodynamic therapies and photothermal therapies, in addition to the simple delivery of chemotherapy components [18]. All of the above uses are dependent on essentially mined resources. Recycling of REEs is challenging and not practiced widely [11]. What recycling does occur is primarily from fluorescent lamps, batteries, and magnets but is of limited scope, estimated at perhaps only 250 tons [19].