WASHINGTON: After a four-year journey, NASA's robotic spacecraft Osiris-Rex will descend to asteroid Bennu's boulder-strewn surface on Tuesday, touching down for a few seconds to collect rock and dust samples in a precision operation 200 million miles (330 million kilometers) from Earth.

Last year, Japan managed with its Hayabusa2 probe to collect some dust from another asteroid, Ryugu, and is now on its way home. With Osiris-Rex, NASA hopes to collect a much larger sample - at least 60 grams - which it hopes will reveal the original ingredients of the solar system.

The spacecraft, about the size of a large van, is at this moment in position about a kilometer above Bennu, which is 490 meters (1,600 feet) in diameter. Engineers from NASA and Lockheed Martin sent it its final commands on Tuesday to carry out the sampling operation, which will be fully automated.

"We aren't able to 'joystick' the spacecraft in real time," said Kenneth Getzandanner, flight dynamics manager for the mission.

At this distance, it will take about 18.5 minutes for signals to travel between them.

The first confirmation signal for the operation will arrive on Earth at 6:12 pm Eastern Time (2212 GMT) on Tuesday. The first images will reach us on Wednesday, but we will have to wait until Saturday to find out if Osiris-Rex has succeeded in collecting the desired amount of dust. "It's not easy to navigate around a small body," said Heather Enos, the project's deputy principal investigator, who has spent 12 years on the mission preparing for this moment.