This NASA SBIR Phase I project will develop high temperature ALLVAR Alloys to improve the reliability of structures and sensors necessary for NASAs future planetary body exploration. Different thermal expansion mismatch between materials and components can push sensors out of specification and reduce sensor life after thermal cycling potentially leading to to mission failure. Negative thermal expansion ALLVAR Alloys can compensate for the thermal expansion mismatch between components by shrinking when heated and expanding when cooled; the opposite of other materials. Commercially available ALLVAR Alloy 30 is ideal for athermalizing infrared optics, telescope assemblies, and mechanical fasteners, but it is currently limited to applications below 100 degrees Celsius. New alloy development is necessary to push the maximum operating temperature of ALLVAR Alloys to the higher temperatures experienced on the Moon and Venus. The Phase I development will identify alloys and produce a new cost effective negative thermal expansion alloy for these high temperature environments. A follow-on Phase II will scale manufacturing processes and explore specific sensor athermalizing applications such as metering structures for optics or washers for constant force fasteners. Potential NASA Applications (Limit 1500 characters, approximately 150 words) A new high temperature material with negative thermal expansion can potentially improve the thermal stability of hot environment sensors, probes, landers, and rovers critical to NASAs Artemis, Discovery, and New Frontiers Programs. If successful, high temperature ALLVAR Alloy HT could: 1) Athermalize optics and sensors used in hot environments and 2) Improve reliability and structure reusability of mechanically fastened joints. Potential Non-NASA Applications (Limit 1500 characters, approximately 150 words) High temperature ALLVAR Alloys unique negative thermal expansion properties can compensate for thermal expansion mismatch in telescopes, satellites, high temperature infrared optics, and down hole oil and gas applications.