Nasa to order its OSIRIS-REx spacecraft to chase another asteroid

The probe began its two-year trip back to Earth from the 101955 Bennu asteroid last May. It is believed to be carrying a sample of Bennu weighing up to 1kg.

But Nasa has now decided to extend its mission, which will be renamed OSIRIS-APEX (short for OSIRIS-Apophis Explorer), to study the near-Earth asteroid Apophis for 18 months. Apophis will make a close approach to Earth in 2029.

The University of Arizona will lead the mission, which will make its first manoeuvre toward Apophis 30 days after the spacecraft delivers the sample it collected from Bennu. The extension adds another $200m (£153m) to the mission cost cap.

The mission team did an exhaustive search for potential asteroid targets. The OSIRIS-REx spacecraft was built for what’s called a rendezvous mission, meaning instead of making a single flyby of an object and quickly snapping images and collecting data, it was designed to “get up close and personal with the object.” DellaGiustina said. “Our spacecraft is really phenomenal at that.”

“Apophis is one of the most infamous asteroids,” DellaGiustina said. “When it was first discovered in 2004, there was concern that it would impact the Earth in 2029 during its close approach.

“That risk was retired after subsequent observations, but it will be the closest an asteroid of this size has gotten in the 50 or so years asteroids have been closely tracked, or for the next 100 years of asteroids we have discovered so far.

“It gets within one-tenth the distance between the Earth and Moon during the 2029 encounter. People in Europe and Africa will be able to see it with the naked eye, that’s how close it will get. We were stoked to find out the mission was extended.”

OSIRIS-APEX will not collect a sample, but when it reaches Apophis, it will study the asteroid for 18 months and collect data along the way. It also will make a manoeuvre similar to the one it made during sample collection at Bennu, by approaching the surface and firing its thrusters. This event will expose the asteroid’s subsurface, to allow mission scientists to learn more about the asteroid’s material properties.

The scientists also want to study how the asteroid will be physically affected by the gravitational pull of Earth as it makes its close approach in 2029.

They also want to learn more about the composition of the asteroid. Apophis is about the same size as Bennu – nearly 300m at its longest point.