After a two-year chase, a NASA spacecraft has arrived at the ancient asteroid Bennu, its first visitor in billions of years.

The robotic explorer Osiris-Rex, right on cue, pulled within 19 kilometres of the diamond-shaped space rock early on Tuesday AEDT.

It will get even closer in the days ahead and actually go into orbit around Bennu on December 31. No spacecraft has ever orbited such a small cosmic body.

Flight controllers applauded and exchanged high-fives once confirmation came through – exactly one week after NASA landed a spacecraft on Mars.

“Relieved, proud, and anxious to start exploring!” tweeted lead scientist Dante Lauretta of the University of Arizona.

With Bennu more than 130 million kilometres away, it took seven minutes for word to get from the spacecraft to flight controllers at Lockheed Martin in Littleton, Colorado. The company built the spacecraft there.

#WelcomeToBennu! After two years of travel – and more than a decade of planning and work by my team – I’m here. But Arrival is just the beginning… https://t.co/0bQPUwqUCp

Credit: NASA/Goddard/University of Arizona pic.twitter.com/VyPG3gRRdw

— NASA's OSIRIS-REx (@OSIRISREx) December 3, 2018

Bennu is estimated to be just over 500 metres across. Researchers will provide a more precise description at a scientific meeting next Monday in Washington.

About the size of an SUV, the spacecraft will shadow the asteroid for a year, before scooping up some gravel for return to earth in 2023.

Scientists are eager to study material from a carbon-rich asteroid like dark Bennu, which could hold evidence dating back to the beginning of our solar system 4.5 billion years ago. As such, it’s an astronomical time capsule.

A Japanese spacecraft, meanwhile, has been hanging out at another near-earth asteroid since June, also for samples. It is Japan’s second asteroid mission. This latest rock is named Ryugu and about double the size of Bennu.

Ryugu’s specks should be here by December 2020, but will be far less than Osiris-Rex’s promised booty.

Osiris-Rex aims to collect at least 60 grams of dust and gravel from Bennu.

The spacecraft won’t land, but rather use a 3m mechanical arm in 2020 to momentarily touch down and vacuum up particles.

The sample container would break loose and head towards earth in 2021.

The collection – parachuting down to Utah – would represent the biggest cosmic haul since the Apollo astronauts hand-delivered moon rocks to earth in the late 1960s and early 1970s.

Both Bennu and Ryugu are considered potentially hazardous asteroids. That means they could smack earth years from now.

At worst, Bennu would carve out a crater during a projected close call 150 years from now.

Scientists contend the more they learn about asteroids, the better equipped earth will be in heading off a truly catastrophic strike.

-AP