ALPACA brings together the technologies of liquid-mirrors, lightweight conventional mirrors, and advanced detectors, to make a novel telescope with uniquely-powerful capabilities. Scanning a long strip of sky every night, it will detect a billion galaxies, quasars and stars. At the same time, it will be particularly sensitive to moving and variable objects. This will allow it to detect potential planet-killing asteroids and comets, as well as stars exploding in distant galaxies. In addition to providing an important near-Earth asteroid patrol, it will map the distant Universe, studying the distribution of luminous and dark matter and the nature of dark energy.