  This white paper proposes that AMBITION, a Comet Nucleus Sample Return
mission, be a cornerstone of ESA's Voyage 2050 programme. We summarise some of
the most important questions still open in cometary science after the successes
of the Rosetta mission, many of which require sample analysis using techniques
that are only possible in laboratories on Earth. We then summarise
measurements, instrumentation and mission scenarios that can address these
questions, with a recommendation that ESA select an ambitious cryogenic sample
return mission. Rendezvous missions to Main Belt comets and Centaurs are
compelling cases for M-class missions, expanding our knowledge by exploring new
classes of comets. AMBITION would engage a wide community, drawing expertise
from a vast range of disciplines within planetary science and astrophysics.
With AMBITION, Europe will continue its leadership in the exploration of the
most primitive Solar System bodies.
