How can the probability of an Earth swingby reentry accident be so low?
• The Cassini mission has been designed to ensure than an inadvertent swingby accident does not occur. Mission rules state that the chance of such an accident occurring must be less than one in one million. JPL has conducted an in-depth analysis, which incorporated human error and historical JPL spacecraft data, to determine the probability of an inadvertent reentry. This analysis determined that the probability of an inadvertent Earth reentry is less than one in one million. This result may be surprising to some people (at first) since it is difficult to prove that failures of any system, particularly spacecraft, can be that small. The result is driven by two factors. • First, for most of the Cassini trajectory it is very hard to hit the Earth. In fact, until about 50 days before Earth swingby, the probability of hitting the Earth is much less than one in one million regardless of the spacecraft failure (this is because of the vastness of space, the smallness of the Earth as a target,
