What may be the most threatening asteroid known to humankind was discovered in 1999, tumbling through space on an unstable orbit that periodically intersects that of Earth around the sun. Astronomers eventually named the half-kilometer-wide object Bennu, after a creation god from Egyptian mythology. And indeed, if Bennu, filled with organic compounds and water-rich minerals, fell on a barren world, it might sow the seeds of life. Instead it may be destined to cause immense suffering and death. Astronomers have projected that in 2135, Bennu will pass nearer to Earth than the moon, in a flyby that could tweak the asteroid's trajectory to guarantee a strike on our planet in the late 22nd century.
No one can predict where exactly on Earth Bennu might fall, although basic arithmetic shows its impact could release as much energy as 3,000 megatons of TNT. If its 2135 flyby sets it on a collision course with Earth, global leaders would basically have two options to avert disaster: either evacuate large regions of the world or launch a mission to deflect the asteroid. To know just how big an evacuation or a deflection mission would need to be, those future planners would rely in part on data gathered more than a century earlier, by a NASA spacecraft launching this September. Called OSIRIS-REx, this spacecraft will visit Bennu with the objective of returning to Earth carrying samples of the asteroid.
The Origins of OSIRIS-REx
As leftovers from the formation of the solar system, asteroids are emissaries from the darkest depths of our history, carrying otherwise unobtainable data about events that predate Earth's geologic record by hundreds of millions of years. Samples from an asteroid might contain answers to lingering questions about the birth of the sun, the formation of planets and even the origins of life on Earth. Add to all of this the need to guard against cataclysmic asteroid strikes, and it is obvious why scientists are interested in these objects.
It is less obvious why we need to send a probe on a round-trip mission to get samples. After all, pieces of asteroids fall to Earth all the time—we call them meteorites. The problem is that few if any meteorites are pristine. All must endure a fiery, surface-melting entry into Earth's atmosphere, and most languish for years, centuries or millennia before being found, their untold stories slowly fading from extended exposure to wind and rain. Most asteroids, in contrast, have been effectively held in stasis for billions of years in the sterile environment of deep space. Visiting them is the only way to access the information they contain.
And among asteroids, Bennu is a special case. Most of the meteoritic fragments that fill Earth's museums are composed of rock and metal—materials durable enough to survive the fall to our planet's surface. Bennu, on the other hand, is a coal-black mass of delicate organic compounds. Such carbonaceous compounds could be precursors to our planet's carbon-based biochemistry. Even if Bennu were not potentially hazardous, scientists would want to study it. But potentially hazardous it is—and it is precisely because Bennu comes so perilously close to our planet that a sample-return mission is feasible.
Bennu's tale traces back at least a billion years, when it was born as a rubble pile ejected from an impact-shattered protoplanet drifting between Mars and Jupiter. The story of OSIRIS-REx only begins in February 2004, when I was a third-year assistant professor working at the Lunar and Planetary Laboratory at the University of Arizona. The aerospace company Lockheed Martin approached my boss, Michael J. Drake, to be the principal investigator on a proposed NASA asteroid-sample-return mission, and Drake approached me to be his deputy.
My early work on the mission was to define its scientific rationale. I had been studying meteorites for more than a decade and knew all the unanswered questions about them that only a sizable sample return of pristine material could provide. At the time, just one other project was comparable to ours: the Japan Aerospace Exploration Agency's Hayabusa mission, which rendezvoused with the asteroid Itokawa to gather samples in 2005. Hayabusa was only partially successful. The probe managed to gather 1,500 microscopic mineral grains, much fewer than expected. (Getting samples from an asteroid is hard!) Additionally, Itokawa was a bright, stony object, with a very different history and scientific potential from dark, carbonaceous asteroids such as Bennu. We were entering new territory.
At home one evening, I decided to draft an outline of the mission's major scientific themes and wrote down four words: origins, spectroscopy, resources and security. Pristine samples from an asteroid such as Bennu could tell us more about the origins of planets and even of life itself. Spectroscopic studies of its surface soil—the regolith—would increase the chance of successfully collecting a scientifically useful sample and could also reveal whether Bennu possesses valuable resources that could someday be mined. The more we could learn about Bennu's orbit, composition and other characteristics, the better our chances of determining whether the asteroid would pose a threat to Earth—and how we might deflect it. More broadly, the high-fidelity “ground truth” data from a spacecraft sent to Bennu would allow us to pinpoint and control for flaws that might exist in telescopic observations and theoretical models, bolstering studies of the wider variety of asteroids across the solar system.
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That outline came to define the mission and provided its awesomely convoluted acronym: OSIRIS-REx stands for Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer. NASA selected OSIRIS-REx for flight on May 25, 2011, and the team gathered to celebrate our success. Unfortunately, Drake passed away shortly thereafter, in September 2011. I was promoted from deputy to replace him as principal investigator. The OSIRIS-REx team works every day in Mike's honor, knowing that he would be proud of us as we prepare to open a new scientific frontier.
From Life's Origins to Off-World Economies
OSIRIS-REx's journey will begin when the spacecraft launches from Cape Canaveral, Fla., on an Atlas V rocket in September. It will travel across the solar system for nearly two years before arriving at Bennu in August 2018. It will then orbit Bennu for more than three years, thoroughly mapping the asteroid and ultimately collecting a sample weighing at least 60 grams.
The samples returned by OSIRIS-REx will record a vast expanse of time, from before the solar system existed to the present day. Bennu's oldest minerals will be microscopic “presolar” grains that formed in the stellar winds emanating from dying stars. Eventually these grains were incorporated into the sun and its planets. Bennu's youngest constituents will be minerals and compounds altered by micrometeorite impacts, cosmic rays and solar flares. OSIRIS-REx's studies of these “space-weathering” processes on a carbonaceous asteroid will be unprecedented.
Like other carbonaceous asteroids, however, the bulk of Bennu's material will be organic molecules and water-rich clay minerals—the same stuff thought to have served as the raw feedstock for DNA, RNA, proteins and other building blocks of life on Earth. Some of Bennu's water was once liquid, kept warm in the asteroid's heart by the radioactive decay of short-lived isotopes such as aluminum 26 and iron 60. Immense numbers of carbonaceous asteroids must have fallen on the early, prebiotic Earth. But whether those asteroids actually brewed life's recipe cannot be easily answered on our planet—there are no unaltered rocks here old enough to tell the tale.
OSIRIS-REx is not only a mission to discover our deep origins; it will also gather information that will be important for our future. Several companies and nations are seriously investigating asteroid mining as a solution to resource limitations both on Earth and outside of it, examining ways to extract precious metals for use back on our planet or to use water ice for in-space production of rocket fuel. With its ability to precisely map and maneuver around an asteroid, OSIRIS-REx will serve as a pathfinder for future asteroid-mining missions.
Although it is not the mission's sole purpose, the value of OSIRIS-REx for improving asteroid-impact forecasts and prevention methods cannot be overstated. Determining whether an asteroid will hit Earth requires measuring the object's orbit to extreme precision. To appreciate the difficulty of that task, consider the distances and forces involved. Bennu circles the sun every 1.2 years, with an orbital velocity of more than 28 kilometers per second, and approaches Earth every six years. In a single orbit, this asteroid travels more than a billion kilometers; at its most distant, the asteroid is more than 340 million kilometers away from Earth.
Because it regularly comes relatively near to Earth, astronomers have been able to study Bennu's orbit closely enough to make it the most precisely recorded in asteroid catalogs. The uncertainty in its semimajor axis (the radius of its orbit at its two most separated points) is just six meters out of a total length of 168,505,699.049 kilometers. That is equivalent to measuring the distance from New York City to Los Angeles with an accuracy of about a third of a millimeter. But orbital accuracy alone is not enough, because many external forces can cause an asteroid's orbit to change.
To plot a course for Bennu, the OSIRIS-REx team uses high-fidelity models to compute the influence of all forces on the asteroid's orbit. These models must account for the gravitational effects of the sun, the moon, and the eight planets, as well as other large asteroids and the dwarf world Pluto. Even Earth's oblateness plays a role because it induces significant variations in a closely passing asteroid's trajectory. The models predict that Bennu will pass within 300,000 kilometers of Earth in 2135. What happens after that is harder to predict. This much is certain, however: if Bennu passes through one of several “keyhole” regions around Earth during its 2135 flyby, the accumulated gravitational effects will place it on track to strike the planet near the end of the 22nd century.
We simply do not know enough about Bennu to predict whether it will indeed pass through one of those keyholes. We currently calculate an approximately one in 10,000 chance of impact in 2196; tabulating all potential Earth impacts yields an estimated impact chance of roughly one in 2,700 sometime between 2175 and 2196. Yet Bennu appears just as likely to be ejected from the inner solar system entirely as it is to strike Earth. If it avoids these outcomes, it has an almost even chance of eventually falling into the sun and slightly less favorable odds of hitting Venus. Alternatively—although this scenario is much less likely—it could strike Mercury, Mars or Jupiter. Better models of Bennu's interior, surface and orbital interactions—models that OSIRIS-REx can provide—will allow us to increase the accuracy of our forecasts.
But OSIRIS-REx's greatest contribution to asteroid forecasting will be its investigations of a recently discovered nongravitational phenomenon called the Yarkovsky effect [see box above]. The Yarkovsky effect describes the force that acts on a small asteroid when it absorbs sunlight and radiates the energy back into space as heat. When not evenly distributed across the entire asteroid, this thermal radiation acts like a minuscule thruster, causing the asteroid to drift and change its orbit over time. Asteroids with prograde rotations (spinning from west to east, like Earth does) drift away from the sun under this thrust. Asteroids with retrograde rotations, like Bennu, drift inward instead.
Already we have used ground- and space-based telescopes to measure the Yarkovsky effect on Bennu, revealing that its position has shifted more than 160 kilometers since its discovery in 1999. These measurements reveal that Bennu probably originated farther out in the asteroid belt, somewhere between Mars and Jupiter, before migrating inward to its present position. Uneven solar illumination and thermal reradiation can also influence an asteroid's spin, which handily explains Bennu's spinning-top shape. The shape comes from sunlight falling asymmetrically on Bennu's surface, boosting its rotation rate over long periods and steadily driving surface material from its poles to its equator. The resulting wide-scale resurfacing may have brought fresh, unweathered material to Bennu's surface—ideal for obtaining a pristine sample.
OSIRIS-REx will perform a detailed study of the Yarkovsky effect by measuring Bennu's spin, surface area and thermal emission. We will also directly measure the Yarkovsky acceleration over the course of our encounter. This study will improve our theory of the Yarkovsky effect and allow us to incorporate it into impact-hazard assessments for all near-Earth asteroids. Additionally, better understanding the Yarkovsky effect could prove vital for future asteroid-deflection missions, which might exploit the effect to help nudge a hazardous space rock onto a different, less threatening trajectory.
The Grand Finale
From beginning to end—from its origins in the mid-2000s to its conclusion in the 2020s and onward into its multigenerational legacy—OSIRIS-REx will represent decades of work and hundreds of millions of dollars of investment. All of that effort and expense will culminate in an act that lasts a mere five seconds: the touch-and-go maneuver, which the spacecraft must perform to gather a sample from the asteroid's surface.
OSIRIS-REx will gather its sample using an instrument called the Touch-and-Go Sample Acquisition Mechanism, or TAGSAM. TAGSAM consists of two major components, a sampler head and an articulated positioning arm. The head acquires the bulk sample by releasing a jet of nitrogen gas that “fluidizes” the regolith and propels it into a collection chamber. The articulated arm positions the head for collection, brings it back for visual documentation and places it in a capsule for the return to Earth. As a backup, 24 separate surface contact pads on the TAGSAM base plate will acquire fine-grained material on touching the asteroid surface.
Most of OSIRIS-REx's three-year stay at Bennu will be spent preparing for this final maneuver. With its cameras, lasers, radio antennas and spectrometers, the probe will create multiple high-resolution global surveys of the asteroid. From these surveys, we will construct a “treasure map,” which will identify a primary and a backup sample-collection site based on safety, the estimated ease of obtaining samples and the expected scientific value of any sampled material. The safest regions to visit will likely be near the equator, where the spacecraft can more easily match the velocity of the spinning asteroid to touch down on the surface. The most scientifically valuable sites should contain a diversity of organic compounds, water-rich minerals and other materials that could help us learn whether asteroids contributed to the origin of life on Earth.
Once the OSIRIS-REx team has chosen the primary sample-collection site and performed extensive dress rehearsals, the actual touch-and-go maneuver will begin. At this time, Bennu will probably be at the far end of its orbit, more than 18 light-minutes away from Earth. After we send the command to initiate the maneuver, we can only sit back and wait as the automated process unfolds. In a series of three propulsive burns over a period of hours, OSIRIS-REx will deorbit, align with its sampling site and then slowly descend toward the asteroid's surface. It will touch down at a maximum velocity of 10 centimeters per second. TAGSAM will have five seconds to collect samples before the craft blasts off from Bennu and rises to an altitude of roughly 10 kilometers above the asteroid. There it will perform a series of tests to ensure the sampling was a success. TAGSAM contains enough nitrogen for three sampling attempts—three strikes, and we will be out.
If all goes well, in 2021 the spacecraft will fire its main engines to return its precious sample to Earth. In late 2023, just after jettisoning the sample-return capsule into Earth's atmosphere, OSIRIS-REx will fire its engines again to enter a safe, stable graveyard orbit around the sun. The sample-return capsule will hit the top of the atmosphere at a speed in excess of 45,000 kilometers per hour, protected by a heat shield that will bleed off more than 99 percent of its reentry energy. At an altitude of three kilometers, the capsule will deploy a parachute, slowing to a soft landing in Utah's West Desert, seven years after its journey began. A team of specialists will recover the sample and transport it to the NASA Johnson Space Center for long-term storage and distribution so that the global scientific community can study it for generations to come.