To Bennu and Back: A Pre-TAG Interview with Thomas Zurbuchen and Lori Glaze

After almost two years in orbit at asteroid Bennu it’s nearly go time for OSIRIS-REx—or, should I say, Touch-and-Go time! Later today, October 20, starting at 1:50 p.m. EDT (17:50 UTC) the van-sized spacecraft will begin its descent toward the surface of Bennu, culminating in the attempt to siphon up at least 60 grams of pristine material from its surface using its TAGSAM (Touch-And-Go Sample Acquisition Mechanism) instrument mounted at the end of an 11-foot-long (3.3m) articulated arm. If successful the material collected will be returned to Earth in September 2023 for study, and will be the largest amount of off-world material collected and returned by a NASA mission since Apollo.

Ultimately these precious bits of Bennu will tell us more not only about the history of the asteroid itself but also about the 4.5-billion-year history of the Solar System, how it formed, and how water and possibly even the key ingredients for life made their way to the early Earth. But maybe even more importantly OSIRIS-REx’s investigations of Bennu will help us better predict its future travels through the inner Solar System—a path that on occasion takes it close enough to Earth’s orbit to get the 490-meter-wide world classified as a potentially hazardous asteroid (PHA).

Dr. Thomas Zurbuchen and Dr. Lori Glaze

The day before OSIRIS-REx’s sample maneuver is to take place I had the opportunity to briefly talk via a Teams meeting with Dr. Thomas Zurbuchen, NASA’s Associate Administrator for the Science Mission Directorate, and Dr. Lori Glaze, NASA’s Planetary Science Mission Director, about Bennu and what OSIRIS-REx has learned about it since its arrival in December 2018. Below is a transcript of that conversation—I hope you find it as fascinating as I did!

This series of images taken by the OSIRIS-REx spacecraft shows Bennu in one full rotation from a distance of around 50 miles (80 km). The spacecraft’s PolyCam camera obtained the thirty-six 2.2-millisecond frames over a period of four hours and 18 minutes. Credit: NASA / GSFC / University of Arizona

Jason Major: What do you think is the most exciting discovery that OSIRIS-REx has made about Bennu since its arrival in December 2018?

Dr. Glaze: The composition. When the team picked Bennu as a target they were really hoping that it was an asteroid that’s carbon-rich, has organics, has water-bearing minerals, because one of the big questions that they’re trying to answer is how an asteroid could deliver organic material and water to Earth and other planets. So they were really hoping to find those things, and that’s one of the key things that they have found. They did a bang-on job, they got it exactly right, they got the right asteroid. Clearly the observations they’ve made so far is carbon-bearing, organic molecules…we’ve got great evidence that those are present on the surface. And Nightingale [the selected sample target area] in particular, there’s been recent information where they’ve been able to look at the x-ray spectrometry of that crater and it looks very fresh. They’ve dated it and it looks like a few hundreds of thousands of years…less than a million years old. So very, very fresh.

The primary “Nightingale” sample area is only the size of a few parking spaces.

Dr. Zurbuchen: So there are really two elements that I want to add. The first one is relative to the overall make, just the mechanical porosity of the rocks that are observed. It really makes clear that there’s no meteorite in our collection that’s like Bennu. So the sample that we’re going to bring back is really quite unique and historic. 

The second one I want to mention is there’s a lot of ingredients that relate to the evolution of an asteroid like that and one of them relates to how the Sun’s radiation relates to its orbit—[the] Yarkovsky Effect. Basically saying the Sun radiates to it, it thermally gets transported and then radiates into space. That is a net force on this body. Frankly there’s a lot of theory of how that should work. I would argue that’s one of the key ingredients in a paper that just came out in Science just a few weeks ago that says “hey, we have actually measured that effect,” that thermal transport at work, and from that we have a much better handle on how this effect actually works and that will help us in predicting orbits of this and many other bodies that have similar mechanical characteristics.

Dr. Glaze: Can I add one more really exciting thing that we’ve found?

JM: Please do.

Dr. G: We’ve found lots of exciting things but one of the things that was really cool that no one expected was the dynamic environment of small particles—softball-sized rocks and blocks that are being ejected from the surface of Bennu. And boy, was that a surprise when they first saw these little strings of particles coming off the surface! What an incredible discovery that was, just to see how dynamic that environment is and try to understand how that works. Some of them come off and they orbit around Bennu a couple times, eventually landing back down onto the surface…just really interesting.

“We’ve found lots of exciting things but one of the things that was really cool that no one expected was the dynamic environment of small particles—softball-sized rocks and blocks that are being ejected from the surface of Bennu.

Dr. Lori Glaze, NASA’s Planetary Science Mission Director

JM: So what’s happening there [with the rocks]? Is it leaving a trail behind it in its orbit? Or is that mostly getting scooped back up? Or we just don’t know yet?

Dr. G: A lot of them are coming back and resting back down. Some of them may reach some type of escape velocity and they go off into space and be left behind. But a lot of them like I said are just coming right back down. You can see their little trails, they go off and come right back down.

This view of asteroid Bennu ejecting particles from its surface on January 6, 2019 was created by combining two images taken by the NavCam 1 imager onboard NASA’s OSIRIS-REx spacecraft: a short exposure image (1.4 ms), which shows the asteroid clearly, and a long exposure image (5 sec), which shows the particles clearly. Credit: NASA/Goddard/University of Arizona/Lockheed Martin

Dr. Z: If I could just add just one more point about the Yarkovsky Effect. To actually measure the delta-v, that takes longer time to do that right. But because of the emission characteristics measured all over in a global sense and because of the thermal characteristics all the ingredients are there to fully measure it.

So if you stand back from it you basically say any rock that’s in space, what is the likelihood for it to hit the Earth within a given time? Well the first one is: what’s the initial orbit? But then there are two things that could bump it around. The first one is the [Yarkovsky] effect we were just talking about. That creates uncertainty, there are error bars to it. The other one is the mass distribution of the asteroid belt. The rocks that are out there [in the belt] are all pulling on it at different intervals. So if you said what’s the biggest limitation for us to make predictions it’s those two effects. So if the error bar on the Yarkovsky effect is down—if we can decrease that—it actually has the impact of better predictions of these bodies. So it has a really important consequence I believe—for Bennu of course, but also for multiple bodies like it. So relative to planetary defense this is a really important result.

“If you stand back from it you basically say any rock that’s in space, what is the likelihood for it to hit the Earth within a given time? Well the first one is: what’s the initial orbit? But then there are two things that could bump it around…the Yarkovsky Effect and the mass distribution of the asteroid belt.”

Dr. Thomas Zurbuchen, NASA’s Associate Administrator for the Science Mission Directorate

JM: So Bennu is not slated to impact Earth in the foreseeable future, but we have to take all of these margins of error and effects on its delta-v and other stuff into consideration and narrow it down and figure it out. But it is on the short list of potentially hazardous asteroids, correct?

Dr. G: It is on the list, and there is a one-in-2700 chance that it could intersect with Earth in about 150 years at the end of the 2100s…between 2175 and 2199. But the kinds of things that Thomas is talking about and trying to understand the Yarkovsky Effect are not just going to help us better predict Bennu, but better understanding the effect will help us predict any of the near-Earth asteroids. We start to really hone in and reduce that uncertainty. As we do, we put our models together and predict future trajectories of any of the near-Earth asteroids.

JM: So would Bennu be typical of other asteroids like it? Because it seems a little different from other main belt asteroids that we’ve imaged. As we learn more about asteroids, will we find that there are more differences than similarities?

Dr. Z: The way you’ve got to think about these bodies is in families. There are families that look like the Bennus, these kind of dark objects. And it turns out that those are really the most important providers of meteorites on Earth…the chondritic meteorites, they’re pretty abundant. So over time they were really important in delivering rocks to Earth. There are other families with different characteristics, some of them more iron-based…they have a different conduction of heat.

Dr. G: We now know there’s over a million, maybe up to two million asteroids in the Solar System. And we find them in different parts of the Solar System: there’s the main belt asteroids, the near-Earth asteroids, the Trojan asteroids, the Centaurs, you’ve got Kuiper Belt Objects…each one of those we expect to be slightly different and even within each we see some diversity. So it’s kind of a population question and a statistics question, trying to get statistically-significant samples of each one of those families to help us understand “what is a normal asteroid, what is an average main-belt asteroid, what’s an average near-Earth asteroid?” Until we get additional data, it’s hard to answer that question. 

JM: OK. Well we’ll learn a lot more with the Touch-and-Go sample mission! Good luck, and go OSIRIS-REx and TAGSAM and thanks so much for taking the time to talk to me. 

Dr. Zurbuchen (L) and Dr. Glaze (R) were safely masked up during the 20-minute Teams meeting we held on October 19, 2020. (I was alone in my office at the time.) Listen to scientists!

Dr. Z: Thanks to you, and thanks for all you do to help us communicate the amazing miracles of science on your blog and social media presence. I really appreciate it, thank you.

JM: Thanks so much—I appreciate that. 

You can learn more about the OSIRIS-REx mission here, and follow along with the news of the sample maneuver on October 20 covered live from Lockheed Martin’s space facility in Denver, CO starting at 5:00 p.m. EDT (21:00 UTC) on NASA’s live feed here, right here on Lights in the Dark, or in the window below:

Main image:  A rehearsal on August 11, 2020 that brought the spacecraft through the first three maneuvers of the sampling sequence to a point approximately 131 feet (40 meters) above the surface, after which the spacecraft performed a back-away burn. Credit: NASA / Goddard / University of Arizona

Thanks to Emily Furfaro for reaching out to me and helping arrange this interview.

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