The lengthy two-year journey of NASA's OSIRIS-REx toward the asteroid Bennu has entered its final leg, with the unmanned probe officially beginning the approach phase of its mission. This will see it slowly close in on the chunk of space rock, snapping its first direct images along they way before snatching a sample for scientists to study here on Earth.

OSIRIS-REx took off atop an Atlas V rocket in September 2016 and spent a year quietly orbiting the Sun, before using Earth's gravity to slingshot off into space for its rendezvous with the asteroid.

Now around 2 million km (1.2 million miles) away from Bennu, OSIRIS-REx is moving in for the kill. During this approach phase, the probe will enter orbit around Bennu for the first time and use its suite of scientific instruments to scan its surroundings for hazards. It will also visually locate Bennu for the first time, gathering images so that scientists can build a detailed model of the asteroid and understand how it spins through space.

The OSIRIS-REx spacecraft takes off aboard an Atlas V rocket United Launch Alliance

Because Bennu moves around the Sun at an average speed of 101,000 km/h (63,000 mph), OSIRIS-REx will approach carefully, using a series of braking maneuvers as it sidles up alongside before finally latching onto it on December 3.

This phase of the mission is called the Touch-And-Go (TAG) maneuver, and will see the spacecraft use its sampling arm to scoop up a sample of regolith from Bennu's surface. To make things easier, the spacecraft will blast the surface with nitrogen gas to kick up dust and small pebbles for collection, with the entire maneuver to take all of five seconds.

Impression of OSIRIS-REx extending its sampling arm toward the asteroid Bennu NASA Goddard Space Flight Center

OSIRIS-REx will then depart Bennu, but not before mission control confirms that it has got the goods. This involves a clever maneuver that has the probe spin around with its sampling arm extended as it blasts off from the asteroid. By comparing the spacecraft's inertia during this move with an empty-handed spin conducted previously, the scientists will be able to measure the size of the sample.

The spacecraft will gently drift away from Bennu and then fire its main engines again in March 2021 to set itself on a trajectory for Earth's orbit. The asteroid dust and rock, held safely inside the Sample Return Capsule (SRC), will then be released into the Earth's atmosphere on a trajectory toward the surface, while OSIRIS-REx performs a deflection maneuver to enter a stable orbit around the Sun.

When the SRC parachutes into the Utah desert in September 2023, it will mark the end of a momentous seven-year journey, but will only be the beginning for scientists interested in the composition of Bennu and asteroids in general.

Primitive asteroids like this one have gone largely unchanged since they formed nearly 4.5 billion years ago and scientists will promptly get to work analyzing the sample down to the atomic level to investigate its chemical composition. They'll be on the lookout for potential resources for commercial exploitation, clues as to the origins of the solar system and organic compounds that can serve as the building blocks of life, such as sugars and amino acids.

Source: NASA