in the Mars Design Reference Architecture (DRA). In 2017, NASA continued research and development on NTRs, designing for space applications with civilian approved materials, with a three-year, US$18.8 million contract. In 2019, an appropriation bill passed by the U.S. Congress included US$125 million in funding for nuclear thermal propulsion research, including planning for a flight demonstration mission by 2024. As of 2021, there has been much interest in nuclear thermal rockets by the United States Space Force and DARPA for orbital and cis-lunar uses. In addition to the U.S. military, NASA administrator Jim Bridenstine has also expressed interest in the project and its potential applications for a future mission to Mars. DARPA has awarded 2 contracts for their Demonstration Rocket for Agile Cislunar Operations (DRACO) program, which aims to demonstrate a nuclear thermal propulsion system in orbit: one award in September 2020 to Gryphon Technologies for US$14 million, and another award in April 2021 to General Atomics for US$22 million, both for preliminary designs for the reactor. Currently, DARPA is in the next phase of soliciting proposals for the design, development, fabrication, and assembly of the engine. Two conceptual spacecraft designs by Blue Origin and Lockheed Martin were selected. Proposals for a flight demonstration of nuclear thermal propulsion in the fiscal year 2026 are due August 5, 2022. Risks An atmospheric or orbital rocket failure could result in the dispersal of radioactive material into the environment. A collision with orbital debris, material failure due to uncontrolled fission, material imperfections or fatigue, or human design flaws could cause a containment breach of the fissile material. Such a catastrophic failure while in flight could release radioactive material over the Earth in a wide and unpredictable area. The amount of contamination would depend on the size of the nuclear thermal rocket engine, while the zone of contamination and its concentration would be dependent on prevailing weather and orbital parameters at the time of re-entry. It is considered unlikely that a reactor's fuel elements would be spread over a wide area, as they are composed of materials such as carbon composites or carbides and are normally coated with zirconium hydride. Before criticality occurs, solid core NTR fuel is not particularly hazardous. Once the reactor has been started for the first time, extremely radioactive short-life fission products are produced, as well as less radioactive but extremely long-lived fission products. Additionally, all engine structures are exposed