  Asteroids and comets are of strategic importance for science in an effort to
understand the formation, evolution and composition of the Solar System.
Near-Earth Objects (NEOs) are of particular interest because of their
accessibility from Earth, but also because of their speculated wealth of
material resources. The exploitation of these resources has long been discussed
as a means to lower the cost of future space endeavours. In this paper, we
consider the currently known NEO population and define a family of so-called
Easily Retrievable Objects (EROs), objects that can be transported from
accessible heliocentric orbits into the Earth's neighbourhood at affordable
costs. The asteroid retrieval transfers are sought from the continuum of low
energy transfers enabled by the dynamics of invariant manifolds; specifically,
the retrieval transfers target planar, vertical Lyapunov and halo orbit
families associated with the collinear equilibrium points of the Sun-Earth
Circular Restricted Three Body problem. The judicious use of these dynamical
features provides the best opportunity to find extremely low energy Earth
transfers for asteroid material. A catalogue of asteroid retrieval candidates
is then presented. Despite the highly incomplete census of very small
asteroids, the ERO catalogue can already be populated with 12 different objects
retrievable with less than 500 m/s of {\Delta}v. Moreover, the approach
proposed represents a robust search and ranking methodology for future
retrieval candidates that can be automatically applied to the growing survey of
NEOs.
