Gravity Probe B is finally getting ready to launch — after 45 years. What, might you ask, is Gravity Probe B? Basically, it’s a really weird experiment intended to prove some aspects of Einstein’s theory of relativity.

One of Einstein’s predictions is that if you put an object in orbit around the earth, after a long enough time zipping around space-time, it would begin to experience a different orientation than the same object would if left stationary on Earth. Essentially, it’d have a different sense of “north”. It’s an effect called “frame dragging,” which you can now officially use to trump the pseuds at the next cocktail party.

The problem is, at satellite speed — which is way, way slower than the speed of light — the effect of frame dragging is incredibly minute. The shift in perspective for an object would be 1/3 of a milli-arc-second a year — which is, like, well, just kind of incredibly tiny. (It’s what a piece of hair would look like if you peered at it from 20 miles away.) So how do you measure such a tiny shift in degree?

With Gravity Probe B! It’s essentially an enormous flying thermos that contains four ping-pong-sized balls of quartz, which are the roundest things ever made by humans. They’re so smooth that the only thing rounder is a neutron star. As a cool story about Gravity Probe B puts it:

That’s equivalent to the Earth’s surface being polished so round that mountain peaks and ocean trenches are within 5 meters (16 ft) of the same level.