Impact assessments (IAs) have become a standard practice for major human actions that may affect humans and Earth’s terrestrial, aquatic, and air environments. However, we have failed to recognize that our extraterrestrial actions, i.e., actions affecting celestial bodies including the Moon, asteroids, and other planets and their moons, may affect those other bodies based on learning from IA of our actions here on Earth. We should expect, for example, that mining for water ice on Mars may have similar impacts to mining on Earth, yet there are currently few regulations or practices favoring the use of IAs for extraterrestrial activities. The 1967 United Nations Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies (the Outer Space Treaty) limits the proliferation or testing of weapons of mass destruction in outer space, cautions states to avoid harmful contamination of space and celestial bodies, and sets guidelines for forward biological contamination (transmission of Earth organisms to other celestial bodies) but is silent on other potentially adverse impacts. No enforcement provisions are provided. Until recently, extraterrestrial impacts have not been a significant concern because of the small number of mostly scientific investigations on the Moon, Mars, and elsewhere in outer space over the past 50 years and their minimal impact on affected environments. That is rapidly changing. National space programs (National Aeronautics and Space Administra tion in the US, European Space Agency, China National Space Administration, Indian Space Research Organization, and others) have active plans for placing humans on the Moon and undertaking actions to support lengthy stays that require construction of infrastructure. Some are planning expeditions to Mars. Commercial private sector space enterprises such as the Space Explorations Technologies Corporation (SpaceX), Blue Origin, and others are planning on accessing the resources of the Moon, Mars, and asteroids. Their actions will, by design, affect those environments. International best practice principles (IBPP) are required for IA of extraterrestrial activities if we hope to sustain continued use of those areas in an efficient manner, preserve options for future generations, protect human health and well-being, and respect humanity’s beliefs and diversities.