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In 1989, Voyager 2 did a fly-by of the planet Neptune. The orignal plan for the Grand Tour would have included Pluto, but the Voyager series was reduced from four craft to just two.

The New Horizons craft, launched in 2006, is expected to arrive in the Pluto-Charon system in July of 2015. Technically, Neptune will still be the most distant visited "planet" since Pluto has been reclassified as a dwarf planet.

More generally, regarding the furthest explored astronomical "feature", in 2013, it was determined that Voyager 1 crossed the heliopause and entered interstellar space. The nature of the heliopause is more subtle, though, and probably not directly observable without instrumentation.

In 1989, Voyager 2 did a fly-by of the planet Neptune. The orignal plan for the Grand Tour would have included Pluto, but the Voyager series was reduced from four craft to just two.

The New Horizons craft, launched in 2006, made it's closes approach to Pluto on July 14, 2015, at 11:49 UTC (2,500 km/7,800 mi). Technically, Neptune is still be the most distant visited "planet" since Pluto has been reclassified as a dwarf planet.

More generally, regarding the furthest explored astronomical "feature", in 2013, it was determined that Voyager 1 crossed the heliopause and entered interstellar space. The nature of the heliopause is more subtle, though, and probably not directly observable without instrumentation.

In 1989, Voyager 2 did a fly-by of the planet Neptune. The orignal plan for the Grand Tour would have included Pluto, but the Voyager series was reduced from four craft to just two.

More generally, regarding the furthest explored astronomical "feature", in 2013, it was determined that Voyager 1 crossed the heliopause and entered interstellar space. The nature of the heliopause is more subtle, though, and probably not directly observable without instrumentation.

In 1989, Voyager 2 did a fly-by of the planet Neptune. The orignal plan for the Grand Tour would have included Pluto, but the Voyager series was reduced from four craft to just two.

The New Horizons craft, launched in 2006, is expected to arrive in the Pluto-Charon system in July of 2015. Technically, Neptune will still be the most distant visited "planet" since Pluto has been reclassified as a dwarf planet.

More generally, regarding the furthest explored astronomical "feature", in 2013, it was determined that Voyager 1 crossed the heliopause and entered interstellar space. The nature of the heliopause is more subtle, though, and probably not directly observable without instrumentation.

That depends on how you define "object".

In 1989, Voyager 2 did a fly-by of the planet Neptune.

In 2013, it was determined that Voyager 1 crossed the heliopause and entered interstellar space. The nature of the heliopause is more subtle, though, and probably not directly observable without instrumentation.

In 1989, Voyager 2 did a fly-by of the planet Neptune. The orignal plan for the Grand Tour would have included Pluto, but the Voyager series was reduced from four craft to just two.

More generally, regarding the furthest explored astronomical "feature", in 2013, it was determined that Voyager 1 crossed the heliopause and entered interstellar space. The nature of the heliopause is more subtle, though, and probably not directly observable without instrumentation.