New Horizons made its closest approach to Pluto at 11:48:45 UTC on 14 July 2015, passing about 13,700 km from Pluto's centre while some 32 AU from Earth. The chart for that second has a Cancer Sun and a Cancer Moon, with Mercury and Mars in Cancer as well: four bodies in one sign, all of them across the zodiac from Pluto in Capricorn.
At the flyby, the Sun, Moon, Mercury and Mars were all in Cancer, the sign directly opposite Pluto's own sign of Capricorn. Mars comes closest to an exact opposition, eight tenths of a degree away. An opposition is two points looking straight at each other, and here it's Mars, the planet of drive and courage, facing the very world being visited. Mercury, the messenger planet of notes and records, sits a couple of degrees from Mars in the same sign, so curiosity and nerve travel together.
Both lights were in Cancer at closest approach, the Sun at 21° and the Moon at 3°. The Moon was a slim waning crescent, close to new, so the moment sits at the quiet end of one lunar cycle with the next about to begin. Cancer is the sign of belonging, memory and looking after what matters, and with the Sun and Moon both there the whole chart has a tender, homeward feel, even with the spacecraft so far from home. The Sun also makes a square to Uranus in Aries, the planet of breakthroughs, which gives the sky a bright, inventive edge.
Venus and Jupiter were both in Leo during the flyby, a warm and openhanded pairing in the sign of light and pride. Venus sits at 28° and Jupiter at 24°, close enough to read as one glow. Venus also makes an exact square to Saturn in Scorpio, a tenth of a degree from perfect, which grounds that warmth in patience and long effort. Mercury makes an easy trine to Neptune, the planet of distance and wonder, linking careful thought with a sense of the vast. You can see what Pluto's sign means in the Pluto in Capricorn guide.