7 facts: NASA's OSIRIS-REx mission to Bennu

There's always someone or something waiting in the wings to steal the crown… or so the saying (sort of) goes. It's just days after the European Space Agency announced it had finally found its lost Philae lander on Comet 67P/Churyumov–Gerasimenko. A bitter-sweet image was sent around the world of the prodigal probe, stuffed in a "dark crack," two years after it became the first to successfully (sort of) land on a comet.

The idea was for Philae to conduct a series of scientific experiments on comet 67P, and it managed about 80 percent of those before its battery power died. Its carrier, Rosetta, orbited the comet to study its nucleus and environment. With that done, and Philae spotted a month before the end of the mission, Rosetta is due to crash land onto the comet on September 30. What a way to go.

Now it's NASA's turn. Only the American space agency wants to take things a step further.

NASA's OSIRIS-REx will fly to a near-Earth asteroid called Bennu. And - if a lot of things go right - it will bring a sample back for scientists to study on Earth.

Here's what you need to know.

What is OSIRIS-REx and what's its mission?

OSIRIS-REx is a spacecraft. Its name stands for Origins Spectral Interpretation Resource Identification Security - Regolith Explorer. Snappy isn't it? Hence: OSIRIS-Rex. It's due to start a seven-year mission to the near-Earth asteroid Bennu, formerly known as 1999 RQ36.

The spacecraft will orbit the sun for a year before using Earth's gravitational field to help it on its way to Bennu. Using an "array of small rocket thrusters," it will match the asteroid's velocity and rendezvous.

When does OSIRIS-REx actually launch?

The launch is scheduled for 7:05 p.m. EDT (11 p.m. GMT) on Thursday, September 8 from Cape Canaveral, Florida. That is, it would be nice if it launched on Thursday, but that's just the start of a 34-day launch period. So don't hold your breath. When it launches, though, an Atlas V 411 rocket will carry OSIRIS-REx into space. The Atlas rocket is an "evolved expendable launch vehicle" operated by United Launch Alliance, a Lockheed Martin-Boeing joint venture.

When will it reach Bennu?

It should reach Bennu in 2018. It's scheduled to return to Earth in 2023 - seven years after the start.

What are the mission's science goals?

It's all in the name.

Origins: NASA hopes OSIRIS-REx will return and analyze a "pristine, carbon-rich" asteroid sample.

Spectral Interpretation: It should provide direct observations for telescopic data of the entire asteroid population.

Resource Identification: Map the chemistry and mineralogy of a primitive, carbon-rich asteroid.

Security: Measure the Yarkovsky effect. That's the effect of sunlight on a small asteroid's orbit. An asteroid such as Bennu gets a "slight push" when it absorbs sunlight and re-emits the energy as heat.

Regolith Explorer: Collect data on a loose layer of outer material known as the regolith.

And why Bennu?

Bennu is what is known as a near-Earth object (NEO). These are the closest asteroids to Earth. Close being fairly relative. NEOs orbit within 1.3 AU of the sun. That's about 120 million miles (193 million kilometers). The distance between the Earth and sun is 1 AU.

For the OSIRIS-Rex mission to be feasible, NASA had to choose an asteroid located between 1.6 AU and 0.8 AU, and it had to be one with an Earth-like orbit. When Bennu was selected, there were 7,000 known NEOs, but only 192 fit the initial criteria.

Next is Bennu's size. The asteroid had to have a diameter larger than 200 meters (about 650 feet). Any smaller than that and the asteroid spins so fast that any loose material - the regolith which OSIRIS-REx hopes to collect and investigate - can be ejected from it. Also, the asteroid had to be large enough to enable the spacecraft to make contact. This criteria slashed the 192 candidates down to 26.

Finally, it was down to the asteroid's composition. The mission aims to collect a carbon-rich sample from a primitive object. That means objects that have undergone the least changes - for instance, through collisions - since the solar system formed about four billion years ago. They contain organic molecules and other elements which we believe brought life to Earth - including water. Of the remaining 26 candidates, only 12 asteroids had a known composition, and only 5 were primitive and carbon rich.

So Bennu trumped the others because…?

Bennu - the lucky candidate - is a B-type asteroid. Its diameter is about 500 meters. It's one of the "blackest objects" in the solar system, so NASA thinks it's covered in carbon material - the "building blocks of life." And, speaking of life, Bennu comes incredibly close to Earth every six years - within about 0.002 AU. So Bennu has a "high probability of impacting Earth in the late 22nd Century." Which is as good a reason as any to find out more about it. And quick.

What's in the payload?

OSIRIS-REx has five scientific instruments to explore Bennu. They will be used for remote sensing and scanning the surface. There is a camera suite, including a MapCam to map the surface, a PolyCam - a telescope - which will image the asteroid from two million kilometers away, and a SamCam, which will capture the moment OSIRIS-REx grabs its sample from Bennu. Spectrometers will measure temperature, visible and infrared light, and the x-ray spectrum to determine the elements on Bennu and their abundance.

Any samples - if or when they make it back to Earth - will be distributed and examined by teams around the world.