Complex sentence: Previous space missions to comets, such as "Giotto" and "Stardust", were fly-by missions, only able to photograph and examine the surfaces of cometary nuclei from a distance.
Split into simpler sentences: Previous space missions to comets, such as "Giotto" and "Stardust", were fly-by missions. These missions were only able to photograph and examine the surfaces of cometary nuclei from a distance.