Gravitational waves are ripples in space-time, predicted by Einstein over a century ago. Earth-based detectors catch them with lasers, but are blind to low frequencies: hampered by the trembling of the Earth's crust and the thermal jitter of mirrors. The Moon is the perfect listener: no wind, no oceans, no tectonics. A passing low-frequency gravitational wave makes the Moon ring like a bell. The LILA mission will install laser sensors to capture this ringing.

LILA Pioneer will register mergers of black holes and neutron stars at frequencies from millihertz to decihertz. LILA Horizon will reach the edge of the visible Universe — to where the first galaxies were born. This is how we'll hear the echo of the expansion of the Universe. A half-century-old idea gains cosmic scale.