1. Pluto Kuiper Express
As we saw with the Mars Telecommunications Orbiter, a canceled NASA mission is often just a little ahead of its time. The Pluto Kuiper Express (PKE) was a similar case.
This spacecraft was originally slated to launch in 2004 for a 2012 arrival at the dwarf planet Pluto, where it would study the far-flung Kuiper Belt of space beyond Neptune. With a year 2000 price tag of $350 million, can you guess why NASA canceled its first mission to Pluto? That's right: budgetary concerns.
When news spread that the PKE was destined for the scrap heap, the Planetary Society, the world's largest space interest group, launched a letter-writing campaign to try to save the project.
Naturally, efforts to win over members of U.S. Congress failed and, in 2006, Pluto fans suffered another blow as the International Astronomical Union took away its planet status. Yet, that same year, NASA successfully launched PKE's successor, the New Horizons Pluto Kuiper Belt Flyby.
In 2015, the spacecraft will indeed cross paths with the black sheep of our planet's solar family.