  This document introduces the exciting and fundamentally new science and
astronomy that the European New Gravitational Wave Observatory (NGO) mission
(derived from the previous LISA proposal) will deliver. The mission (which we
will refer to by its informal name "eLISA") will survey for the first time the
low-frequency gravitational wave band (about 0.1 mHz to 1 Hz), with sufficient
sensitivity to detect interesting individual astrophysical sources out to z =
15. The eLISA mission will discover and study a variety of cosmic events and
systems with high sensitivity: coalescences of massive black holes binaries,
brought together by galaxy mergers; mergers of earlier, less-massive black
holes during the epoch of hierarchical galaxy and black-hole growth;
stellar-mass black holes and compact stars in orbits just skimming the horizons
of massive black holes in galactic nuclei of the present era; extremely compact
white dwarf binaries in our Galaxy, a rich source of information about binary
evolution and about future Type Ia supernovae; and possibly most interesting of
all, the uncertain and unpredicted sources, for example relics of inflation and
of the symmetry-breaking epoch directly after the Big Bang. eLISA's
measurements will allow detailed studies of these signals with high
signal-to-noise ratio, addressing most of the key scientific questions raised
by ESA's Cosmic Vision programme in the areas of astrophysics and cosmology.
They will also provide stringent tests of general relativity in the
strong-field dynamical regime, which cannot be probed in any other way. This
document not only describes the science but also gives an overview on the
mission design and orbits.
