Born from the rubble of a violent collision, hurled through space for millions of years and dismembered by the gravity of planets, asteroid Bennu had a tough life in a rough neighborhood: the early solar system. "Bennu's Journey," a new animation created at NASA's Goddard Space Flight Center in Greenbelt, Maryland, shows what's known and what remains mysterious about the life of Bennu and the origin of the solar system.
"We are going to Bennu because we want to know what it has witnessed over the course of its evolution," said Edward Beshore of the University of Arizona, Deputy Principal Investigator for NASA's asteroid-sample-return mission OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security – Regolith Explorer). The mission will be launched toward Bennu in late 2016, arrive at the asteroid in 2018, and return a sample of Bennu’s surface to Earth in 2023. "Bennu's experiences will tell us more about where our solar system came from and how it evolved. Like the detectives in a crime show episode, we'll examine bits of evidence from Bennu to understand more completely the story of the solar system, which is ultimately the story of our origin."
The video opens with an establishing shot of the galaxy and moves in to a nebula – a vast cloud of gas and dust ejected from the explosions of dying stars. From observations of other star-forming regions in our galaxy, scientists have a good idea of the basic outlines of how our solar system came to be, according to Beshore. As shown in the animation, a nearby exploding star disrupts material in the nebula, causing part of it to collapse under its own gravity and form a disk of material surrounding the infant Sun.
Within this disk, bits of dust are flash heated to molten rock and solidify to become chondrules — some of the building blocks of the solar system. Chondrules are shown in the animation as they clump together via electrostatic and gravitational forces to become asteroids and planets.
Chondrules may make up a large part of the material in Bennu. "On planets like Earth, the original materials have been profoundly altered by geologic activity and chemical reactions with our atmosphere and water. We think Bennu may be relatively unchanged, so this asteroid is like a time capsule for us to examine," said Beshore. By analyzing the sample collected from Bennu, the OSIRIS-REx team will be able to examine some of the most pristine material to be found anywhere in the solar system.
Bennu may also harbor organic material from the young solar system. Organic matter is made of molecules containing primarily carbon and hydrogen atoms and is fundamental to terrestrial life. The analysis of any organic material found on Bennu will give scientists an inventory of the materials present at the beginning of the solar system that may have had a role in the origin of life. "By bringing this material back to Earth, we can do a far more thorough analysis than we can with instruments on a spacecraft, because of practical limits on the size, mass, and energy consumption of what can be flown," said Beshore. "We will also set aside returned materials for future generations to study with instruments and capabilities we can't even imagine now."
The mission also will contribute to NASA’s Asteroid Redirect Mission (ARM), which will identify, capture and redirect a near-Earth asteroid to a stable orbit around the moon, where astronauts will explore it in the 2020s, returning with samples. ARM is part of NASA’s plan to advance new capabilities needed for future human missions to Mars. OSIRIS-REx also will support the agency's efforts to understand the population of potentially hazardous near-Earth objects and characterize those suitable for future asteroid exploration missions.
The early solar system was quite chaotic. Giant impact craters throughout the inner solar system indicate there may have been a "late heavy bombardment" by asteroids approximately 4.1 billion to 3.8 billion years ago, right around the origin of life on Earth. The video illustrates one theory for this. The massive "gas giant" planet Jupiter began to migrate inward closer to the Sun due to gravitational interactions with the outer gas giant planets. Jupiter's gravity disrupted the asteroid belt, tossing many asteroids closer to the Sun, where some collided with the terrestrial planets, including Earth. This asteroid bombardment may have been a significant source of organic matter and water for the early Earth.
After this bombardment, things calmed down a little, but massive collisions still happened occasionally, like the one the video shows happening between an asteroid and a planetesimal about one billion years ago. Scientists think a collision like this may have resulted in the birth of Bennu, and the video illustrates the asteroid forming as some of the rubble from the collision slowly coalesces under its own weak gravity.
Measurements reveal that Bennu's density is less than that of rock, so scientists think the asteroid may have voids in its interior, according to Beshore. An asteroid like this is called a "rubble pile" — a loosely bound collection of boulders, rock, and dust.
Bennu is also quite dark. Like an asphalt road on a hot day, it absorbs most of the sunlight that hits it and later radiates this energy away as heat. This radiation gives Bennu a tiny push, called the Yarkovsky effect, which gradually changes its orbit over time. The animation shows how the Yarkovsky effect causes Bennu to migrate until it encounters a so-called gravitational resonance with the planet Saturn. Regular tugs by this resonance eventually push Bennu into the inner solar system, where it has repeated close encounters with Venus and Earth. These encounters pull apart the rubble pile that is Bennu, turning it inside out and reshaping the asteroid.
Because Bennu comes close to Earth, there is a tiny chance – about 1 in 2,500 – that it could hit Earth late in the 22nd century, according to Beshore. "We'll get accurate measurements of the Yarkovsky effect on Bennu by precisely tracking OSIRIS-REx as it orbits the asteroid," said Beshore.
This story has been brought to you courtesy of NASA. Learn more about OSIRIS-REx, and its epic journey to Bennu, here.
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