Complex sentence: On 14 July 2015, the "New Horizons" probe will fly by Pluto, the first time it will be visited by a spacecraft, during which "New Horizons" will take detailed measurements and images of Pluto and its moons.
Split into simpler sentences: On 14 July 2015, the "New Horizons" probe will fly by Pluto, the first spacecraft to do so. During this rendezvous "New Horizons" will take detailed measurements and images of Pluto and its moons.