Comets and asteroids, remnants of planet formation, have been subject to various processes in the 4.5 billion years since their formation. Studying their activity leads us to better understand how these bodies evolve with time, and what the properties of their surfaces and sub-surface layers are.
Dawn was NASA's mission to the asteroid belt. In 2011 the spacecraft reached the asteroid Vesta and accompanied it for more than a year. Dawn reached its second destination (dwarf planet Ceres) in 2015 where the mission ended in November 2018.
Rosetta was ESA's mission to comet 67P/Churyumov-Gerasimenko. Rosetta consisted of an orbiter and a lander called Philae. Rosetta reached the comet in 2014. In November 2014, Philae successfully landed on the comet's surface.