Today, one of NASA’s deep-space probes, OSIRIS-REx, arrived at the space rock it’s been traveling toward for the last two years, an asteroid named Bennu. At noon ET, OSIRIS-REx came within about 12 miles (20 kilometers) of the asteroid, which is closer than ever before. The arrival means that OSIRIS-REx is now starting a new phase of its mission that entails extensively mapping the surface of the asteroid to find the best place to grab a sample of material.

“We have arrived!” Javi Cerna, the OSIRIS-REx telecom engineer at Lockheed Martin, jubilantly proclaimed on NASA TV when the mission team received word that the spacecraft had made it to the asteroid. The announcement was followed by cheers and applause by the team members at the Denver headquarters of Lockheed Martin, the company that built the spacecraft.

“We have arrived!”

OSIRIS-REx launched on top of an Atlas V rocket in September 2016, and it has been hurtling through space ever since. The spacecraft’s mission is relatively straightforward: meet up with an asteroid, scoop up some rocks off of its surface, and then come back to Earth. Known as an asteroid sample return, this kind of mission is exciting for scientists since studying the materials from an asteroid in depth could tell us a great deal about how our Solar System came to be 4.5 billion years ago.

Asteroids are thought to be primitive remnants of our cosmic neighborhood, containing the building blocks that made up the Solar System when it was being born. In that way, they act as a snapshot of what the early Solar System was like. Analyzing the rocks from an asteroid can tell us what types of materials were around at the beginning of the Solar System and potentially how life on Earth came to be. Scientists think that asteroids may be part of the reason why we’re here; it’s possible that these rocks brought the precursors of life to Earth when they slammed into our planet’s surface.

The mission team behind OSIRIS-REx picked Bennu to sample because of its composition and orbit. Bennu is what’s known as a carbonaceous asteroid, meaning it contains compounds rich in carbon, which is an essential element for organic life. Carbonaceous asteroids are thought to be relatively unchanged since the early days of the Solar System, and it’s possible that Bennu contains organic compounds similar to the ones that helped life grow here on Earth. Additionally, Bennu is considered a potentially hazardous asteroid since there’s a slight chance it will come into contact with Earth in the 2100s. That also made the asteroid an attractive target for OSIRIS-REx, as the spacecraft will attempt to study how the object’s orbit might change over time.

“It’s been a long time coming to the arrival.”

OSIRIS-REx’s journey to Bennu and back will be a long one, spanning seven years. It’s already taken a long and circuitous route to get to the asteroid. The spacecraft did a flyby of Earth in September 2017, using our planet’s gravity to shift position and put the vehicle on the right path to the asteroid. OSIRIS-REx has also been firing its thrusters in order to brake and match Bennu’s speed and orbit through space. At around 12PM ET today, OSIRIS-REx did one last burn of its thrusters to arrive at Bennu, bringing the spacecraft over the north pole of the asteroid.

“It’s been a long time coming to the arrival, and we’re really looking forward to the next chapter of this mission,” Heather Enos, the deputy principal investigator for OSIRIS-REx at the University of Arizona, said on NASA TV.

Now that OSIRIS-REx is within reach of Bennu, the next step for the spacecraft is to insert itself into the asteroid’s orbit. That’s slated to happen on December 31st. When that occurs, the OSIRIS-REx team will make history, as Bennu will be the smallest Solar System object to ever be orbited by a spacecraft. Bennu is less than 1,700 feet across, which is only slightly longer than the height of the Empire State Building. Its petite presence makes it a difficult object to orbit, as the object doesn’t have a lot of gravity to pull the spacecraft closer to its surface. OSIRIS-REx will regularly perform course correction maneuvers to stay in formation around the asteroid. And it will even bring itself out of orbit a few times to get a comprehensive view of the rock.

It will take a full year of studying Bennu to figure out where the spacecraft should scrape up a sample. Using its instruments, OSIRIS-REx will be mapping the entirety of the asteroid’s surface, and researchers will use those maps to come up with the best candidate sampling sites. Once the best location is picked, OSIRIS-REx will slowly maneuver into position and quickly “tap” the asteroid with its sampling instrument, potentially scooping up to 4.4 pounds (2 kilograms) of material. Once those precious rocks are stowed away safely, the spacecraft will begin its return to Earth in 2021. Eventually, the mission will wrap up with those samples landing in the Utah desert in September 2023.