Nasa's first asteroid sample-collector nears target, Bennu

In the coming days, the spacecraft will get a little closer to begin its preliminary survey.

Scientists look forward to having some real bits of asteroid back here on Earth to analyse. Over the past several months, OSIRIS REx has been creeping toward Bennu, and finally reached the space rock when it was about 80 million miles (129 million kilometers) from Earth. By the end of the probe's mission, Bennu will have become the smallest known object ever orbited by a spacecraft and by 2020 will be able to connect and collect material samples.

OSIRIS-REx has a camera suite, a laser altimeter for 3D mapping, a thermal emission spectrometer to take temperature and mineral content and spectrometers to measure X-rays, almost infrared and visible light.

It has been chasing the diamond-shaped body for two years, and will orbit it for another year before scooping some gravel from the asteroid's surface and sending it back to Earth.

"This does look a challenging surface", said Lauretta, gesturing to a blown-up image of Bennu.

The spacecraft has enough gas to attempt three sample collections from the surface.

The arm has a full range of motion, with joints capable of movement comparable to shoulder, elbow and wrist joints.

Using a circular device much like a car's air filter, and a reverse vacuum to stir up and collect dust, the device aims to grab about two ounces (60 grams) of material from the asteroid's surface, and return it to Earth for further study. At the time of the mission's asteroid selection in 2005, there were only 192 asteroids classified as Near Earth Objects that met NASA's proximity requirements.

The spacecraft reached Bennu shortly after noon United States eastern time today, after a two-year journey. He expects those pictures to be back on Earth around 10 December, when a press conference is scheduled at the AGU fall meeting in Washington, D.C. There, the OSIRIS-REx team plans to present early Bennu science results.

For two years after the return, the sample will be cataloged and analyzed. The craft will also scan the asteroid to find a safe and scientifically interesting location to land and sample.

The asteroid is of particular interest to NASA because it may contain water and clays from the protoplasmic disc that formed the Sun and the planets in our Solar System. Only five asteroids met all criteria. Once OSIRIS-REx arrives at Bennu, the craft will stay about 12 miles (20 kilometers) away from the asteroid. Both Bennu and Ryugu are near-earth asteroids, with a diameter of about 500 meters and 900 meters respectively.

OSIRIS-REx is in no hurry.

The asteroid could pass close to Earth, closer than the moon, in 2135, with even closer approaches possible in 2175 and 2195. Carbon is the key to the organic molecules needed for life, so finding organic molecules on a sample from Bennu would help to answer a big question about the origin of life.

"We know the burn is occurring", said Javi Cerna, an OSIRIS-REx telecom engineer, during a NASA TV broadcast from Lockheed Martin in Littleton, Colorado. "You have a pristine sample of what the solar system was like billions of years ago", said Michelle Thaller, a spokeswoman for NASA's Goddard Space Flight Center. And Bennu is believed to be a grouping of rocks held together by gravity rather than a single object.

OSIRIS-REx will slowly approach Bennu, at a speed of only 0.2 meters per second. When will asteroid hit Earth?

The agency describes Bennu as "a carbonaceous asteroid whose regolith may record the earliest history of our solar system". "When we understand Bennu, we will understand something fundamental about our solar system".

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