Escape from Earth

Projectile motion has a limit. Throw something fast enough — about 11.2 km/s — and Earth's curvature falls away beneath it faster than gravity can pull it back, so it never returns. This is escape velocity, the speed needed to break free of a planet's gravity for good. (At everyday speeds the path looks like a parabola; on planetary scales gravity bends it into an ellipse, or at escape speed a hyperbola.)

Below that threshold, every path curves back to Earth eventually:

• A pitcher's 150 km/h throw: a few dozen meters.
• A cannon: a few kilometers.
• An intercontinental missile: thousands of kilometers — but still a curved arc, still coming back.