Launched in January 2006, the NASA probe New Horizons received a gravity assist from Jupiter during a flyby in February 2007, and is now just days away from reaching Pluto after a journey of approximately five billion km.

When New Horizons set out, Pluto was still classed as the ninth planet in the solar system. It was discovered by American astronomer Clyde Tombaugh in 1930. But compared to the other planets, Pluto was always an odd-ball.

To begin with, it’s tiny (at 2300 km in diameter, it’s two-thirds the size of our moon). Its orbit around the Sun is also wacky. Whereas other planets orbit on a relatively flat plane called the ecliptic, Pluto’s orbit is tilted at a 17 degree angle. Its orbit is also quite eccentric, ranging between 29.6 and 48.8 AU. For part of its orbit, in fact, it’s actually closer to the Sun than Neptune.

Pluto is known to have five satellites — the largest being Charon. But because of its peculiar character, the International Astronomy Union took a second look at the planet in 2006 and decided to reclassify it as a Trans-Neptunian Kuiper Belt Object. Similar to the asteroid belt between Mars and Jupiter, the Kuiper Belt is a vast swath of asteroids that exists between 30 and 50 AU from the Sun.

This mission is a fly-by, so New Horizons won’t be able to do an in-depth study of Pluto from orbit. Instead, on July 14 it will pass within 10,000 km of Pluto and 27,000 km of Charon before continuing on its way in the direction of the constellation Sagittarius.

One thing scientists are curious about is if Pluto’s tidal interaction with Charon (which is half its size, and only 19,570 km away) generates enough heat to create a subsurface “ocean” on the ice-laden body. Such oceans are rare in the solar system, and they’re seen as sites where primitive life could possibly form.

You can find out more on the mission on the NASA website. And below is a short NASA video that recaps efforts humanity has made over the last 50 years to visit via probes other planets in our solar system.