101955 Bennu is an asteroid the size of the Empire State building and it could be heading straight for us.

Every six years, Bennu hurtles closer to the Earth than our own moon. And it could come closer still. Scientists fear it a possible collision with our planet some time in the 22nd century.

Which is why the OSIRIS-Rex space mission is trying to getting a better grasp of the risk and how it might be averted.

"If it really comes close to the Earth and we have to remove it before it hits our planet, we need to know as much about it as possible,"  Dr. Harald Michaelis of the DLR Institute for Planetary Research in Berlin, who was on the team preparing the OSIRIS-Rex mission, told DW. "This can help us predict how to remove such a body away from its original orbit."

One option would be to hit the asteroid with a missile, nudging it into just enough of a change to its orbit around the sun to avoid our planet.

Collecting clues to the past

We don't yet know if Bennu poses a threat to Earth. But that's not the only reason scientists want to study the asteroid.

They curious as to what it's made of, and the mission's primary goal is to collect samples from the asteroid's surface.

"Everything was mixed up during the formation of our planet, but these bodies [asteroids] are building blocks of our Earth and other planets," Michaelis explains. "We would like to know what these 'raw materials' are in order to understand the whole process of planet formation and planet development."

Scientists think Bennu stands out from millions of other minor planets it its composition: It's one of the most primitive asteroids, largely made up of carbon, and has not changed significantly over the last 4 billion years, they believe.

The stuff of myth and legend

Both Bennu and Osiris take their names from ancient Egyptian mythology.

A third-grade student from North Carolina in the United States won a "Name That Asteroid!" competition with the name Bennu, an Egyptian mythological bird and symbol of rebirth.

The spacecraft's moniker is an acronym of its main goals: Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer.

Grab and go

Scientists have long been interested in the composition of asteroids.

The first space probes were orbiting missions. Then in 2010, the Japanese probe Hayabusa brought home the first samples from the near-Earth asteroid Itokawa.

This time, scientists are aiming for a bigger sample of between 60 grams and 2 kilograms – far larger than previously possible.

Having journeyed through space for the last two years, OSIRIS-Rex will now orbit Bennu for about a year, snapping pictures and taking measurements.

It will also look for a suitable landing spot to collect the samples.

A delicate operation

Reaching the asteroid was relatively easy. But getting up close and collecting the sample is a real challenge, Michaelis says: "The most critical phase is to go down: You have to move very slowly, otherwise the spacecraft hits the surface and gets damaged."

The probe is equipped with a robotic arm that will make contact with the surface of the asteroid for approximately five seconds.

The plan is for it shoot liquid nitrogen into the rock, breaking it up and collecting it in a container. It will have three attempts. This part of the mission is scheduled for July 2020, before OSIRIS-REx heads home.

If everything goes to plan, it should arrive back on planet Earth in September 2023, carrying extraterrestrial material that will help scientists understand more about the origins of our solar system.

Named for the king of the underworld and god of the afterlife, the mission could even reach back into our own distant past to reveal something about the very beginnings of life.