Digital citizen science offers a low-cost way to strengthen the scientific infrastructure, and engage members of the public in science. It is based on two pillars:(1)a technological pillar, which involves developing computer systems to manage large amounts of distributed resources, and (2) a motivational pillar, which involves attracting and retaining volunteers who would contribute their skills, time, and effort to a scientific cause. While the technological dimension has been widely studied, the motivational dimension received little attention to date. To address this gap, we surveyed volunteers at Stardust@home a digital citizen science project, in which volunteers classify online images from NASA's Stardust spacecraft. We found that collective and intrinsic motivations are the most salient motivational factors, whereas reward motives seem to be less relevant. We also found that intrinsic and norm-oriented motives are most strongly associated with participation intentions, which were, in turn, found to be associated with participation effort. Implications for research and practice are discussed.