There are many asteroids that cross the orbit of the Earth. Occasionally, one passes near the Earth. If a large asteroid hits the Earth, the consequences could be catastrophic. There have been a lot of discussions and debates about what could be done if astronomers detect an asteroid on a collision course with the Earth.
I have mentioned before that bacteria in spacecraft can develop into more virulent strains with antibiotic resistance. This adds to the biological problems that humans face when living in space beyond the atmosphere and gravity of the Earth. A recent experiment on the International Space Station provided additional detail on how these changes occur.
The Outer Space Treaty was signed in 1976 by one hundred and seven countries. Among its provisions is a ban on putting any nuclear, biological, or chemical weapons in Earth orbit. Of course there is the problem of being able to actually know exactly what is being launched into orbit by different countries and private firms.