  The Laser Interferometer Space Antenna (LISA) will open three decades of
gravitational wave (GW) spectrum between 0.1 and 100 mHz, the mHz band. This
band is expected to be the richest part of the GW spectrum, in types of
sources, numbers of sources, signal-to-noise ratios and discovery potential.
When LISA opens the low-frequency window of the gravitational wave spectrum,
around 2034, the surge of gravitational-wave astronomy will strongly compel a
subsequent mission to further explore the frequency bands of the GW spectrum
that can only be accessed from space. The 2020s is the time to start developing
technology and studying mission concepts for a large-scale mission to be
launched in the 2040s. The mission concept would then be proposed to Astro2030.
Only space based missions can access the GW spectrum between 10 nHz and 1 Hz
because of the Earths seismic noise. This white paper surveys the science in
this band and mission concepts that could accomplish that science. The proposed
small scale activity is a technology development program that would support a
range of concepts and a mission concept study to choose a specific mission
concept for Astro2030. In this white paper, we will refer to a generic GW
mission beyond LISA as bLISA.
