Hundreds of scientists and engineers have labored for the better part of two decades to reach this point. Now, their passenger-van-sized spacecraft is finally ready for its big moment, hovering near an asteroid about as long as the Empire State Building is tall.

Later today, this space drama will play out 333 million kilometers from Earth. NASA’s OSIRIS-REx spacecraft will approach an asteroid named Bennu and extend its sampling arm. The circular head at the end of this arm will essentially bump into the asteroid for about five seconds.

During this critical juncture, the spacecraft will expel nitrogen gas onto the surface of Bennu, aiming to drive small particles on the asteroid’s surface—with a width of 2cm or less—into a device akin to a catcher’s mitt.