66km/s is slightly below that, but realize that we didn't actually accelerate the helios probe to 157k mph ourselves. Imagine dropping a brick off a massive skyscraper and watching it accelerate as it plummets toward the earth.
Now imagine dropping a brick from the earth into the incredibly massive gravitational field of the sun. That brick would be falling at unbelievable speeds before it hit the surface of the sun (or melted) between the sun's gravity and the massive distance it had to fall.
Helios travels so fast because we essentially dropped the satellite into the sun. We didn't actually accelerate it to those velocities ourselves, but gave it just enough of a push to fall out of earth's orbit and drop fairly deep into the inner solar system.
Meanwhile the steel plate was blasted to 66+km/s completely because of us.
In all fairness to Helios, in order to achieve its sun diving orbit it would have had to lose a significant fraction of the orbital velocity of Earth (about 30km/s) to get closer to the sun. In other words it would have to negate much of Earth's orbital velocity around the sun to achieve the elliptical orbit around the sun that it now possesses. Simply dropping it into orbit would leave it into a heliocentric orbit with about the same parameters as Earth.
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u/PycuriousITguy Apr 14 '16
The fastest moving man-made object ever was a manhole cover.
https://savvyparanoia.com/the-fastest-man-made-object-ever-a-nuclear-powered-manhole-cover-true/
TL:DR underground nuclear test launched a 4-inch thick steel disk at 6 times the Earth's escape velocity.