Due to limited downlink (satellite-to-ground) capacity, over 90% of the images captured by the earth-observation satellites are not downloaded to the ground. To overcome the downlink limitation, we present Earth+, a new on-board satellite imagery compression system that identifies and downloads only changed areas in each image compared to latest on-board reference images of the same location. The key of Earth+ is that it obtains latest on-board reference images by letting the ground stations upload images recently captured by all satellites in the constellation. To our best knowledge, Earth+ is the first system that leverages images across an entire satellite constellation to enable more images to be downloaded to the ground (by better satellite imagery compression). Our evaluation shows that to download images of the same area, Earth+ can reduce the downlink usage by 3.3× compared to state-of-the-art on-board image compression techniques without sacrificing imagery quality or using more resources (downlink, computation or storage).