NASA Is Approaching Asteroid Bennu, One of the Most 'Hazardous' Objects That Might Hit Earth

NASA’s OSIRIS-REx space probe is closing in on the 1,600-foot-wide asteroid Bennu nearly two years after launching from Cape Canaveral, Florida.

Having travelled more than 1 billion miles, OSIRIS is now "just" 1.2 million miles away from the space rock and has begun its official approach. As it gets closer, the spacecraft will begin studying the asteroid.

Understanding more about Bennu—an ancient asteroid which has remained largely unchanged since it formed nearly 4.5 billion years ago—is important for several reasons, according to NASA.

“Where did we come from? What is our destiny?" the mission website states. "Asteroids, the leftover debris from the solar system formation process, can answer these questions and teach us about the history of the sun and planets."

Rock and dust on Bennu, for example, may provide insights into the early evolution of the Solar System, while it could also contain the molecular precursors that may have led to life on Earth.

Furthermore, Bennu is considered to be one of the most dangerous of the known “potentially hazardous” asteroids due to its relatively high (although still small) probability of colliding with the Earth late in the 22nd century. OSIRIS will study the asteroid's physical and chemical properties, which will be critical if a mission is required in the future to avoid an impact.

Finally, scientists are keen to find out if Bennu contains substances such as water, organics and precious metals. It is hoped that asteroids like it could one day be mined commercially or their resources harnessed to fuel further exploration of the solar system by robotic and manned spacecraft.

OSIRIS’s official arrival at Bennu is scheduled for December 3 this year when the probe will transition to operating around the asteroid. During this time, it will conduct numerous observations, including mapping and imaging the rocky object and measuring how it moves through space.

It will even touch the surface to scoop up a sample of material in a maneuver referred to as "Touch-And-Go (TAG)." This will be no easy feat, involving a series of delicate braking exercises that will enable OSIRIS to pull up alongside Bennu, which is travelling at average speeds of around 63,000 miles per hour.

Then, a robotic arm will be extended towards to surface, and the momentum of the probe’s slow, downward trajectory will push against the asteroid’s surface for just five seconds—just enough to obtain a sample which will be safely stowed away in a special capsule.

If all goes well, OSIRIS will slowly drift away from Bennu until its main thrusters are turned on again in March 2021 and it begins its long journey back to Earth.

Once it approaches home, the capsule will be dropped into Earth's atmosphere where it will fall to the surface, landing in the Utah desert in September 2023 to be recovered, and analysis of the sample will begin.

An accurate assessment of Bennu’s probability of colliding with the Earth can only be made once scientists have more information on its movement. The current best estimates suggest there is a roughly 1 in 2,700 chance of an Earth impact in the 22nd century, although these odds are likely to change once more data comes in.

While this is obviously a very small chance, if the asteroid did strike the Earth, it would produce a blast equivalent in power to tens of thousands of atomic bombs. Aside from the massive destruction that would result from the initial impact, such a strike would alter the global climate—among other negative effects—leading to widespread crop failures that could threaten the survival of human civilization.

This is why NASA has unveiled plans to deflect or destroy the asteroid using nuclear warheads just in case Bennu's orbit around the Sun eventually puts it on a collision course with Earth.

Bennu is one of the largest of the so-called Near Earth Objects (NEOs) that could potentially threaten our planet. NEOs are any asteroid or comet whose orbits bring it into the inner solar system within about 121 million miles of the sun, and also within about 30 million miles of Earth’s orbit.

OSIRIS-REx-Spacecraft-at-Bennu OSIRIS-REx Spacecraft at Bennu. NASA Goddard Space Flight Center

If the orbit of a NEO at the time of its discovery is such that there is a (typically small) chance it will collide with Earth and cause significant damage, it is labeled "potentially hazardous," according to the Swinburne Astronomy Online Encyclopedia.

The asteroid, or other object, must have a minimum approach distance of less than 0.05 astronomical units or roughly 4.6 million miles to be classified as such. 

In total, the number of known NEOs exceeds 18,000. Researchers have detected around 90 percent of those larger than a kilometer in size, none of which are predicted to collide with Earth. There are, however, thought to be many NEOs smaller than this that are currently unaccounted for.

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