OSIRIS-REx Mission to Asteroid Bennu – NASA (2018)

How did our Sun, the Earth, and the planets form and evolve? Asteroids and comets are the early building blocks of the solar system. They even may hold clues to how life has started on our and perhaps in other planets. The asteroid Bennu contains information going back to four and a half billion years. Visitation and sample collection from asteroid Bennu will be very informative. Here you can watch a concatenation of four videos produced by the NASA Goddard Space Flight Center that focuses on OSIRIS-REx Mission. The acronym stands for: O – Origins, SI – Spectral Interpretation, RI – Resource Identification, S – Security, REx – Regolith Explorer.

The first animated short feature is a cheerful summary of how asteroid Bennu was chosen for the OSIRIS-REx Mission. From a rich collection of over 500,000 known asteroids, scientists narrowed the choices down to 5 candidates. Asteroid’s proximity to Earth, its orbit and size, and its chemical composition were key for the finalists.

Bennu is a B-type asteroid with a diameter of 500 meters. It completes an orbit around the Sun every 436.604 days (1.2 years) and every 6 years it comes very close to Earth, within 0.002 AU. Bennu’s size, primitive composition, and potentially hazardous orbit make it one of the most fascinating and accessible near-Earth objects, and the ideal target for the OSIRIS-REx Mission.

The second video highlights phases of the mission. OSIRIS-REx was launched in September 2016, and arrived at asteroid Bennu on December 3, 2018. The spacecraft will be spending more than a year surveying and mapping Bennu before collecting a sample. The spacecraft will also try to understand the Yarkovsky effect by measuring the orbit deviation caused by non-gravitational forces.

The third video interviews members of the mission. Many people worked on this mission including the ARECIBO Observatory team who created the first three dimensional model of the asteroid.

The final fourth video provides a cosmological scenario for how Bennu came to be in the near-Earth orbit that we observe today. Although this asteroid was formed at the outer solar system as a part of a proto-planet it later traveled towards the inner solar system. The video also introduces chondrule as a geological term. Accordingly, a vast cloud of hydrogen, helium, and dust called a “stellar nursery” provided the raw ingredients of Bennu, and our solar system. This is much earlier than the formation of our own Sun. Supernova explosions from hot stars destabilized the nursery, causing it to collapse.

Very rapidly, within a hundred thousand years, gravity and angular momentum flattened the cloud into a swirling disc like a frisbee. In the center, a proto-star began to form by building incredible pressures and temperatures. Deep within the disc, clumps of dust not much larger than a grain of wheat are flash heated into droplets of molten rock, called chondrules. The source of this heat is still unknown.

Chondrules aggregated and grew into the first asteroids and later into planets. The asteroids are rubble piles of rock, metal, ice and organics. At some point the proto-star in the center underwent fusion and ignited to become our Sun. But the solar system continued to evolve. The gas giant Jupiter which formed near its outer edge began to move inwards just 500 million years after the Sun ignited. Its massive gravity disrupted countless asteroids and comets, catapulting them toward the Sun. The period known as the Late Heavy Bombardment is a result of that, hammering and re-melting large portions of crust on inner planets including the Earth. Did these comets and asteroids deliver organics and water, key ingredients for life?

Asteroid Bennu has survived this period of heavy bombardment. It continued to orbit in the outer solar system without being deflected by Jupiter as a part of the proto-planet. Things calmed down. The solar system and Jupiter assumed the orbits that we see today. Then according to one hypothesis, a billion years ago, a collision scattered the proto-planet. Some of the debris loosely reunited into a new, smaller body which we now call Bennu. Still Bennu would not stay in place. Its surface is dull and non-reflective. Solar heating turned its warm side into a low-intensity thruster.

Through millions of years, Bennu’s orbit gradually tightens. It slowly migrates toward the Sun. Eventually it also meets the fate of many other outer solar system asteroids: Like a pinball Saturn’s gravity catapults it into the inner solar system. Earth and Venus exert gravitational pulls which may have repeatedly stretched and reformed Bennu. Turning it inside out and pulling off loose material. We are all holding our breath for what Osiris-Rex will reveal about Bennu. Watch this space…