* Different levels of study: Some missions have flown by comets quickly for a brief study, while others have orbited comets for extended periods, conducting detailed analysis.
* Definition of "studied": What constitutes a "study" can be subjective. A flyby with a single instrument could be considered a study, but it's far less comprehensive than a multi-year orbital mission.
However, we can say that over a dozen comets have been studied by spacecraft in some capacity. This includes:
* Flybys: Numerous spacecraft have performed flybys, including the famous Halley's Comet encounter by Giotto in 1986.
* Orbital missions: The Rosetta mission, which orbited comet 67P/Churyumov-Gerasimenko for two years, is a prime example of a comprehensive study.
* Sample return: The Stardust mission captured dust particles from comet Wild 2 and returned them to Earth for study.
This list only includes missions with primary objectives focused on comets. Additionally, many other spacecraft have had the opportunity to observe comets during their missions, even if the comet wasn't the primary target.
Therefore, the number of comets studied by spacecraft continues to grow as new missions are launched and existing ones continue to gather data.
