Pluto’s moon Hydra was detected twice by the New Horizons spacecraft LORRI (LOng Range Reconnaisance Imager) camera. The detections were made on July 18, 2014 and a few days later on July 20, from a distance of 430 million kilometers / 267 million miles. Analysis of those July images came in September 2014 by New Horizons Science Team members John Spencer of the Southwest Research Institute and Hal Weaver of Johns Hopkins.
New Horizons spied Hydra while conducting long exposures to try a method of detecting any possible smaller moons and rings – and possibly hazardous debris – when the spacecraft starts long-range encounter sequences in January 2015.
Forty ten-second exposures were made and the outermost known Pluto moon Hydra – only 160 kilometers / 100 miles wide – was detected.
Bottom line: The New Horizons spacecraft has spotted Pluto’s small, faint, outermost known moon, Hydra, in July 2014. The photos were analyzed, and Hydra seen, in September. Hydra is one of five known moons for Pluto. New Horizons will begin its long-range encounter sequences in January 2015 and will sweep past Pluto in July 2015,.