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July-August 2014

Volume 102, Number 4
Page 247

DOI: 10.1511/2014.109.247

In September 2016, NASA will launch the OSIRIS-REx probe and embark on what may be the most delicate space mission ever attempted: Maneuvering next to a 500-meter-wide asteroid, brushing a robotic arm against the surface, collecting at least 60 grams of material, and flying the sample back home to a soft landing in Utah in 2023. Dante Lauretta, a planetary scientist at the University of Arizona and the mission’s principal investigator, is keenly aware of both the challenges and the potential payoff. This would be the closest-ever study of an Earth-threatening asteroid, and the first chance to study up close the type of object that may have seeded our planet with the chemicals of life 4.5 billion years ago. Corey S. Powell, interim editor of American Scientist, spoke with Lauretta about what he hopes to learn.

Image courtesy of NASA.

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Out of all the asteroids in the solar system, how did you decide where to go?

When we were making a target selection, there were around the order of half a million known asteroids. The first thing we did was say, “Well, we’re not going to get out to the main asteroid belt, get a sample, and bring it back.” That takes too much energy. We wanted to be solar powered, so we didn’t want to get too far from the Sun. And we didn’t want an asteroid that’s too small or spinning too rapidly. As we looked at the data, I learned about a fascinating trend: There’s a correlation between the rotation rate of an asteroid and its diameter, and there’s a natural break at 200 meters. Asteroids that are 200 meters and smaller are rotating really rapidly, some of them with rotation periods of less than a minute. It looks like 200 meters is the fundamental building-block size. Most asteroids are probably rubble piles, and if you took a rubble-pile asteroid a kilometer wide and spun it that fast, it would quickly fragment into those smaller pieces.

What is so scientifically interesting about this particular asteroid?

We want to understand the organic molecular evolution of the early solar system. We want to investigate whether asteroids seeded the early Earth with the fundamental molecules that led to DNA and proteins. We’re also very much interested in the origin of volatiles [compounds that vaporize at low temperatures], in particular water, and why we have so much water on Earth. If you want to understand volatile and organic evolution in the early solar system, the best place is to go is a carbonaceous primitive asteroid.

The one we selected—a 500-meter-diameter asteroid then known as 1999 RQ36, now called Bennu—also had a phenomenal data set already in place. It had made a series of close approaches to the Earth in 1999 and 2005. The Arecibo and Goldstone radio telescopes got great radar data on this object: rotation period, pole orientation, overall structure. It has one of the best known orbits of any asteroid in the solar system. And it turned out to be one of the most potentially hazardous asteroids, too. It has a 1 in about a 2,400 chance of impacting the Earth late in the 22nd century.

The most challenging part of the OSIRIS-REx mission will be nestling up to the asteroid and collecting a sample. Are you worried about whether the technology will work?

We recently did our first dress rehearsal and stepped through the whole sample acquisition sequence—it’s about five hours from when we decide we’re going for the sample to when we contact the asteroid surface. It started to dawn on me how nerve-wracking those five hours are going to be. We’re hoping to get it in one shot, but we can get three shots if needed.

This will be the first time a spacecraft has brought a sample of an asteroid back to Earth. What will a direct sample tell you that you can’t learn remotely?

OSIRIS-REx will allow us to map the distribution of water and organics in the inner solar system. That will be important for people who are thinking about going out to these asteroids as resources for use in human exploration. It will also allow us to get a much better understanding of the distribution of the kind of material that’s preserved there, and tie that back to our models for the formation of the solar system. We’re trying to compare meteorite spectra to asteroid spectra to understand why they’re different—and they really are different.

Studying the formation and delivery of organic material to the Earth is really hard with any meteorite that fell on this planet because of contamination. It’s why a sample return mission is so critical.

OSIRIS-REx will also explore the structure and dynamics of asteroid Bennu. What are your goals there?

One of the things we’re excited about is that we’re going to get up close with a rubble-pile asteroid and really get into the surface geology. If a rubble-pile asteroid comes close to the Earth it gets stretched out, almost like a cigar, and then it will snap back. That may be why Bennu has a spinning-top shape. That process may also generate particles as a result of friction, and all that material should be accumulating at the equator. I’ve challenged my science team to figure out where this asteroid originally came from. How did it get into the inner solar system? How many planetary close encounters has it had during its lifetime? Then we’re going to get a piece of it back on Earth and run all kinds of tests that are going to tell me the history of this material. How long has the surface been exposed to space? When was the last major impact on this asteroid? We’re going to test—for the first time—the geologic and dynamic theories of asteroid evolution.

How do you study the geologic history of an asteroid?

We’re going to track it very precisely for six days, and we’ll get a very nice map of the gravity field. We’re going to image the asteroid from all kinds of illumination angles and build up a three-dimensional model based on how the shadows change. From the shape and gravity field, you can tell if it’s homogeneous or if there’s a density variation inside. We’ll also look at any surface expressions of internal structure. Do we see faulting, ridges, scarps, anything that would give us some insight into deeper geophysical occurrences?

Your current thinking is that Bennu is the debris from a collision between two larger parent bodies, is that right?

Yes. We’re looking at three asteroid families in the inner main belt as the most likely source. These families are a result of major collision between two large bodies in the main asteroid belt [between Jupiter and Mars], anywhere from 200 million to 2 billion years ago. The collisions take two asteroids and shatter them to thousands and thousands of fragments.

One of the big surprises to me is that sunlight can move asteroids by heating the surface: Thermal radiation exerts a small push that can change shift its orbit (called the Yarkovsky effect). How does that affect Bennu?

What happens is the smaller you get, the more the Yarkovsky effect changes the semi-major axis [the size of the orbit]. Yarkovsky appears to be a size sorting mechanism in the main asteroid belt, where the smallest asteroid from a collision like the one that formed Bennu gets pushed really quickly and delivered into the inner solar system. This size sorting effect explains the size distribution of asteroids in the inner solar system.

What if the day comes when we find an asteroid that has a high likelihood of hitting Earth? Will OSIRIS-REx help figure out how to avoid an impact?

Absolutely. We’re building a spacecraft that’s going to launch from Earth, rendezvous with an asteroid, characterize its fundamental properties, and ultimately descend to the surface in a series of precision maneuvers to a spot of our choosing. Any kind of deflection where you want to rendezvous with the asteroids is going to require those techniques. Ultra-fine thrusting in microgravity—it’s never been done before. That’s the first critical thing.

The second thing is we’re going to measure directly the Yarkovsky effect, which is the largest uncertainty in orbit propagation into the future. If you’ve got a couple of decades before an impact is going to occur, you can actually use the Yarkovsky effect—you can direct it. You could paint some areas of the asteroid white, some areas black. You could control that Yarkovsky force, but only if the theory matches the observed acceleration. We’re going to test that. I think that is one of our most valuable contributions to an impact hazard mitigation.