In September 2016 NASA will launch a very special mission. The target: near-Earth asteroid 101955 Bennu. The primary objective of the Origins, Spectral Interpretation, Resource Identification, and Security– Regolith Explorer (OSIRIS-REx) mission is to return the first pristine samples of carbon-rich material from the surface of a primitive asteroid.
“Primitive” in regard to asteroids means original material formed at the beginning of the solar system that’s been little altered by heat or pressure. Look around you and you won’t find these sorts of rocks on Earth. Most have been reworked and recycled through water and wind erosion and the great engine of plate tectonics.
While there are plenty of primitive asteroids out there, Bennu stands out as the most easily accessible one by space probe.
Third grader Michael Puzio from North Carolina named it after an Egyptian mythological bird associated with the sun, creation and rebirth. A fitting name given Bennu’s origin at the time the clock first starting ticking in our planetary system.
With a girth of about 1,600 feet (0.5 km) Bennu’s a larger than average Earth-approaching asteroid. It orbits around the sun every 436 days (1.2 years) and passes close to the Earth every six years.
Why should you care about this hunk of rock? During those close approaches, Bennu comes within 278,867 miles (448,794 km) of our blue orb. Calculation of its future orbits indicate that Bennu has one of the highest impact probabilities in the next few centuries of any known asteroid.
Currently there’s a 1 in 1800 chance of impact with Earth on September 24, 2182. Another study by mathematician Andrea Milani reveals eight potential hit opportunities between 2169 and 2199. Little Bennu could turn into an angry bird.
Not only do scientists need to study the asteroid up close to gain a deeper understanding of the origin and evolution of asteroids and planets, but also to precisely determine its orbit and composition. In particular, we need to know how solar heating affects Bennu’s orbit. Something as simple as baking in the afternoon sun can have dramatic consequences over an asteroid’s lifetime.
Heat absorbed by Bennu during the day radiates back into space, pushing back against the asteroid like a rocket thruster. All those tiny pushes add up over the centuries, causing Bennu to slow down and its orbit to shrink. Whether a shrinking orbit will ultimately reduce or increase the probability of an impact, no one knows. That’s why getting up close will help nail down Bennu’s future amblings.
OSIRIS-REx is set to launch in September 2016 and reach the asteroid in October 2018. Once in orbit, the probe will map and study the surface from a distance of between 3 and 0.4 miles (5 and 0.7 km) for 505 days.
During this time the probe will extend the SAM or Sample Acquisition Mechanism down to the surface and collect at least 2.1 ounces (60 grams) of pristine material. The sample will be returned to Earth in a capsule that will float down by parachute over the Utah desert in September 2023.
Based on studies from Earth, Bennu most resembles the carbon and organic chemistry-rich carbonaceous chondrite meteorites. The Sutter’s Mill meteorite that fell in California last April is similar to what we expect for Bennu – dark, crumbly and rich in ancient clays and organics.
That chip-off-an-asteroid we could handle – it broke up harmlessly in the atmosphere. If Bennu came to Earth and remained intact, well, that would be another thing altogether.