By Ashley Strickland | CNN

After orbiting the near-Earth asteroid Bennu for nearly two years, NASA’s OSIRIS-REx spacecraft is ready to reach out its robotic arm and collect a sample from the asteroid’s surface on Tuesday. That sample will be returned to Earth in 2023.

A van-size spacecraft has to briefly touch down its arm in a landing site called Nightingale. The site is the width of a few parking spaces. The arm will collect a sample between 2 ounces and 2 kilograms before backing away to safety.

“It’s a historic first mission for NASA, returning an asteroid sample, and it’s hard,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate, during a Monday press conference.

The site itself is nestled within a crater the size of a tennis court and ringed in building-size boulders.

Located more than 200 million miles from Earth, Bennu is a boulder-studded asteroid shaped like a spinning top and as tall as the Empire State Building. It’s a “rubble pile” asteroid, which is a grouping of rocks held together by gravity rather than a single object.

The mission — which stands for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer — launched in September 2016.

Since arriving at Bennu, the spacecraft and its cameras have been collecting and sending back data and images to help the team learn more about the asteroid’s composition and map the best potential landing sites to collect samples.

The main event of the mission, called the Touch-and-Go sample collection event, or TAG, is scheduled for October 20 beginning at 5 p.m. ET.

Bennu has an orbit that brings it close to Earth, which is why it’s considered to be a near-Earth asteroid. One of its future approaches could bring it perilously close to Earth sometime in the next century; it has a one in 2,700 chance of impacting our planet.

The samples from Bennu could help scientists understand not only more about asteroids that could impact Earth but also about how planets formed and life began.

“Bennu is almost a Rosetta Stone out there, and it tells the history of our Earth and solar system during the last billions of years,” Zurbuchen said. “Bennu has presented a lot of challenges, but the ingenuity of the team has enabled us to get where we are.”

What to expect
Rather than the so-called “seven minutes of terror” of trying to land the Perseverance rover on Mars next year, the OSIRIS-REx team is anticipating “4.5 hours of mild anxiousness,” according to Beth Buck, the mission’s operations program manager at Lockheed Martin Space in Littleton, Colorado.

During this time, the spacecraft will descend from its orbit around the asteroid and eventually come close enough to touch it.

The asteroid and spacecraft are currently about 207 million miles from Earth, which will cause a communication delay of about 18.5 minutes.

The team at NASA will share animations on the mission’s Twitter account depicting what is occurring based on the commands that have already been sent to OSIRIS-REx hours ahead for the sample collection sequence.

A livestream will also be on NASA’s site beginning at 5 p.m. ET that shares animations and insight from the mission team as they explain the data they’re receiving.

The spacecraft will perform the entire sequence of approaching the asteroid and collecting the sample autonomously since live commands from Earth won’t be possible.

Touching down on an asteroid
The event will take about 4.5 hours to unfold and the spacecraft will execute three maneuvers to collect the sample.

The spacecraft will first fire thrusters to leave its safe orbit around the asteroid, which is about 2,500 feet away from the surface, and travel for four hours before reaching just 410 feet away. Then, the spacecraft will adjust for position and speed to continue descending.

Next, OSIRIS-REx will slow its descent to target a path so it matches the asteroid’s rotation during contact. Its solar panels will fold into a Y-wing configuration above the spacecraft to protect them.

At last, OSIRIS-REx will touch down for less than 16 seconds. The spacecraft will fire a pressurized nitrogen bottle into the asteroid, using the gas as a way to lift material off Bennu’s surface.

The spacecraft’s collector head will capture the stirred up material. This head, located on the 11-foot-long robotic sampling arm, is the only part of the spacecraft that will touch Bennu. The team compares it to an air filter in an older model car, perfect for collecting fine material.

Small discs, which can collect dust like sticky pads, are also located on the head in case part of the sampling maneuver doesn’t go according to plan.

After the event

A camera on the spacecraft will take footage of the collection event.

The OSIRIS-REx spacecraft will have to detect hazards and delay its own mission if any obstacles get in the way of the sample collection. Based on its simulations, the team estimates there is less than a 6% chance the spacecraft will abort the mission.

By Tuesday night, the team should be able to confirm if the touchdown occurred successfully. Imagery will be returned by the spacecraft on Wednesday, which will provide more details of the sample collection and how the spacecraft is faring.

The team estimates that they will have a mass measurement of the sample on Saturday. By October 30, NASA will confirm if the spacecraft collected enough of a sample or if it needs to make another sample collection attempt in January at another landing site called Osprey.

But if everything runs smoothly, the spacecraft and its prized sample will begin the long journey back to Earth next year and land the sample on Earth in 2023.