147,000 (mph)

Fun Fact for the day: The fastest man made object ever created (and possibly the one which has traveled the farthest) is...a manhole cover.
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In 1957, an underground nuclear weapons test was conducted near Los Alamos. The manhole cover just happened to be covering an air shaft right above the site. When the nuke was detonated, the entire structure became a 12-megaton potato cannon, blasting the manhole cover into the air. 

A high-speed camera trained on the lid caught only one frame of it moving upward before it vanished—which means it was moving at a minimum of 66 km/s. That's about 147,000 miles per hour! Six times faster than would be needed to escape Earth's orbit.

The manhole cover was never found, it probably burned up in the atmosphere, but I like to think it's out there somewhere.

Too many factors working against it (like the fact that a manhole cover wasn't designed for space travel) but if it kept going and didn't burn up, at that speed, it would have passed the orbit of Neptune in 1962, entered interstellar space in 1972, and would now be well on its way to Alpha Centauri.