What are the limits to space travel?
The distances between the stars are vast and are measured using a unit called "light years". One light year is the distance travelled by light in one year. The speed of light is 299,792,458 meters per second (671,000,000 miles per hour), so one light year is the same as 9,460,730,472,580.8 kilometers (5,878,625,373,183.608 miles).
Most remote man-made object
As of 12 February 2012 NASA's Voyager 1, launched in 1977 to help us study the outer solar system, was 17,960,000,000 kilometres from the Sun. Should it ever be found by aliens, it contains a gold-plated disc carrying data on the human race, including photographs and music.
Proxima Centauri (Stage 1)
At 4.24 light years away, this red dwarf is the nearest star to the Sun. Even traveling at the fastest spacecraft speed yet achieved it would take some 18,000 years to reach. To put this into perspective, 18,000 years ago our ancestors were creating cave art and using stone tools - 4.24 light years
Pluto (Stage 2)
Despite its demotion from planet to dwarf planet, Pluto is the most distant Solar System object currently targeted by a robotic mission. NASA's New Horizons spacecraft is en route and will perform a flyby of Pluto and its four moons in July 2015 - 5,900,000,000 kilometers
Europa (Stage 3)
The Galileo orbiter suggested that, with its icy surface and the potential of liquid water oceans, this Jovian moon is under discussion as a concept but will launch no earlier than around 2030 - 778,000,000 kilometers
Mars (Stage 4)
The USSR Mars 2 and Mars 3 missions were the first spacecraft to reach the surface of Mars, in 1971. We have yet to see a successful sample return mission, but the first visit by humans should happen within the next few decades - 225,000,000 kilometers
Itokawa asteroid (Stage 5)
On 13 July 2010, the unnamed spacecraft Hayabusa (Japan) landed on Earth with its cargo of tiny grains of material collected form the surface of the asteroid Itokawa. It was the first spacecraft to lift off from an asteroid and the farthest land-and-return sample mission launched - 105,600,000 kilometers
Moon (Stage 6)
Apollo 11, which landed on the Moon on 29 July 1969, was the first mission to reach Stage 6; also counted as the first sample return mission - 405,410 kilometers