Planetary protection is a notion that presents scientific, legal, and ethical questions of the highest importance.
It covers contamination of celestial bodies other than the Earth. Biological contamination could make it difficult to determine if life was originally present on the celestial body, hence the importance of preventing it. States must also avoid contaminating the Earth’s environment due to the introduction of extraterrestrial matter.
According to Article IX of the Outer Space Treaty, States Parties shall avoid adverse changes and harmful contamination of both the terrestrial and space environments, and they shall adopt appropriate measures when necessary. Practical guidance for compliance is found in the COSPAR Planetary Protection Policy. However, future challenges abound, and we can expect the further evolution of planetary protection as we increase our knowledge of celestial bodies and the presence -or not- of life on them.