The Solar System becomes more accessible each year, with an ever increasing number of spacecraft performing flybys, orbiting or landing on our closest neighbours. When it comes to our planetary neighbours though we have a responsibility not only to investigate, but also to protect. Every spacecraft must satisfy the rules of space exploration and ensure that it will neither expose other planets and moons to biological contamination from Earth, nor return to Earth harmful alien substances.
Neighbourhood Watch: Planetary protection for the Rosetta mission
The Solar System is not a lawless place. Since we first began our exploration of the Universe, humankind has established rules to preserve what future generations might later discover. The first set of planetary protection rules were put in place in the late 1950s by the International Council of Scientific Unions (ICSU) and subsequently covered by Article IX of the UN Outer Space Treaty in 1967.
The current version of these rules, maintained by the Committee of Space Research (COSPAR), presents a set of requirements for spacecraft to ensure they do not contaminate other planets and moons with biological substances from Earth. ESA has adopted this planetary protection policy and acts on behalf of its Member States to ensure that its requirements are met for all missions the Agency is flying or contributes to.
For every new mission beyond Earth orbit an assessment of risk to planetary protection is made. Missions that will encounter a target body with the potential for extant or extinct life (for example, Mars, Europa, Enceladus) must have a very high level of cleanliness, meaning a low level of microbiological contamination.
Cleaner still are those spacecraft searching for evidence of extraterrestrial life on such bodies to avoid compromising this search with terrestrial stowaways brought along from Earth.
There are also restrictions imposed on the mission operations.
Comets do not fall into the life-bearing categories, so the requirements for the Rosetta mission were not too onerous. However, since the spacecraft had a 12 year journey through the Solar System that included a Mars gravity assist manoeuvre before arriving at the comet, the mission did have to prove that either there was only a very small chance of the spacecraft impacting on Mars or that, in the event of a crash, the chance of biological contamination would be below an acceptable limit.
ESA successfully demonstrated that during the mission the probability of an impact on Mars was well within the given impact probability constraints by carefully evaluating variations of the spacecraft trajectory, including effects of the space environment and the reliability of the mission operations and flight hardware
The Agency also showed that comet 67P/Churyumov–Gerasimenko – the target of Rosetta's mission and where Rosetta will reside after the mission ends in September 2016 – will have no encounters with Mars closer than 94 million kilometres within fifty years after the launch of the spacecraft, despite crossing the orbit of Mars. The lander, Philae, also satisfies these constraints by virtue of the comet's low probability of impacting Mars.
This demonstration showed that the mission complied with the planetary protection policy and that Mars can be deemed safe from any contaminating effects from ESA's iconic Rosetta mission.