For the past two years, the OSIRIS-REx spacecraft has sailed across the solar system by the light of the stars. Like ancient mariners and the Apollo astronauts, it needed the constancy of the constellations to navigate the dark unknown.

All that changed Monday, when the NASA probe finally reached its target, an Empire State Building-sized asteroid called Bennu.

Now OSIRIS-REx faces a whole new kind of challenge: exploring the smallest object ever orbited by a spacecraft.

Sitting at mission control at the Denver offices of Lockheed Martin, which operates the spacecraft for NASA, engineer Javi Cerna waited for the signal indicating OSIRIS-REx had begun the burn needed to bring it close to its target.

He fidgeted in his chair, then stood. The room was silent.

Then Cerna grinned and spread his arms out wide.

“We have arrived!”

OSIRIS-REx was within 12 miles of Bennu’s surface — about the distance between the White House and NASA’s Goddard Space Flight Center, which manages the spacecraft.

Soon an image of the asteroid appeared on the mission control screens: a diamond-shaped body with a rough, speckled exterior. OSIRIS-REx was at the doorstep of its new home.

Bennu is a carbonaceous asteroid — a primitive, carbon-rich piece of debris left over from the process that formed the solar system 4.6 billion years ago. OSIRIS-REx will spend the next 18 months surveying the landscape and probing Bennu’s chemical makeup before selecting what piece of the asteroid it wants to bring back home. It will be the largest planetary sample retrieved since the Apollo era, when astronauts brought rocks back from the moon.

Scientists hope to uncover clues about the birth of the planets and the origins of Earth’s water and life. They may also uncover potentially useful natural resources such as organic molecules and precious metals.

And since Bennu has a 1-in-2,700 chance of hitting Earth about 200 years from now, researchers figured it would be good to glean some insights about the asteroid’s fate — and how it might intersect with ours.

Bennu is so small, dark and distant (about 75 million miles from Earth at the moment) that scientists could only theorize about what it might look like when they launched OSIRIS-REx two years ago. To their delight, newly acquired close-ups of the asteroid closely match their predictions.

In the forthcoming months, University of Arizona Planetary Scientist Bashar Rizk, who oversees three of OSIRIS-REx’s cameras, and his team will try to get measurements of the asteroid’s shape, density and gravity that will allow scientists to fine-tune how they orbit it.

Bennu is so small (about 0.05 percent of the mass of Mount Everest) that its gravity is nearly negligible. If you stood at Bennu’s North Pole and jumped, you would achieve escape velocity and go soaring off into the void.

That makes orbiting — which relies on a delicate balance between a spacecraft’s velocity and an object’s gravity — especially hard. “It will really be record-breaking in terms of the precision, the navigation, compared to anything we’ve done before,” said flight navigator Coralie Adam, an engineer at aerospace company KinetX.

Yet Bennu’s small size also makes it possible for OSIRIS-REx to perform carefully choreographed hairpin maneuvers around the asteroid. Engineers will uplink new instructions to the spacecraft every day instead of the typical update of once a week, Adam said.

In 2020, after 18 months of observations, OSIRIS-REx will swoop close to Bennu and extend a long robotic arm, and with a puff of nitrogen gas, blow some material off the asteroid’s surface, gathering as much as 4.4 pounds of rock in the head of the sample. Then it must turn around and retrace its path back home. It is expected to streak through Earth’s atmosphere and land in the Utah desert Sept. 24, 2023.