OSIRIS-REx is an international effort, with team members from the U.S., Canada, France, the U.K., Italy, Spain, and the Czech Republic. This diversity brings great strength to our team. I am pleased to report that last month NASA signed an international agreement with the Centre National d’Etudes Spatiales, or CNES, the French space agency. This agreement formalizes a long-standing collaboration with our French colleagues. For OSIRIS-REx, we have only developed two such agreements – one with Canada and one with France. We decided to pursue a formal agreement with CNES because of the key role that our French colleagues play in mission development.
Antonella Barucci of l’Observatoire de Paris – LESIA, Le Centre National de la Recherche Scientifique (CNRS), Universite Pierre et Marie Curie, University Paris Diderot
Antonella is a world expert on asteroid spectroscopy. She has thought a lot about telescopic characterization of asteroids, and how you turn that into a science investigation for an asteroid sample-return mission. I met Antonella in 2007, as I was finishing the first run at an OSIRIS Sample Return concept study. She is my counterpart on the Marco Polo-R Asteroid Sample Return mission, which is in concept development at the European Space Agency. This synergy makes for a great partnership. Sharing the issues and concerns that the Marco Polo team has thought about, and vice versa, makes both missions better.
Antonella studies the composition of small bodies from their spectra, or colors that make up light. However, asteroids like Bennu pose a unique challenge; it is very difficult to get spectra from such dark, carbonaceous, primitive asteroids. Only an asteroid sample-return mission can really help us to understand the composition of what we believe are these precursors of planet formation and the origin of life. One of the instruments aboard the spacecraft is the OVIRS visible and infrared spectrometer. This instrument will give Antonella and her colleagues critical information about the mineralogy of Bennu. In addition, OSIRIS-REx will give us a real sample of ancient pristine material to analyze in the lab and compare to data from telescopes and from OVIRS.
Patrick Michel, Laboratoire Lagrange – University of Nice, CNRS, Observatoire de la Côte d’Azur, Nice
Patrick Michel is a world expert on understanding what happens when asteroids collide with each other, how an asteroid family is created, and how objects like Bennu are created from catastrophic collisions in the main asteroid belt. We really want to piece together the whole history of Bennu, and Patrick focuses on a very specific and critical part, that is, when and how did Bennu form? In addition to this fundamental science, Patrick is also heavily involved in development of the Marco Polo-R Concept Study Report.
Not only is the OSIRIS-REx mission to return a sample of pristine asteroid to Earth, it is designed to discover the age, evolutionary history, dynamics, mechanical behavior and characteristics of the entire asteroid. One of the great values of getting a sample returned to Earth comes from knowing its geological history. Patrick’s knowledge of the physical properties of asteroids, and his expertise in modeling asteroid physical properties through dynamical and collisional events, help us interpret observations of Bennu’s surface and infer the asteroid properties that cannot be directly seen or measured.
For example, OSIRIS-REx scientists have a good idea of Bennu’s bulk density, but that doesn’t tell us much about the asteroid’s structure. Bennu has a bulk density of 1.2 gram per cubic centimeter, which is almost half the density of the similar meteorites we have collected. Given its bulk density, Bennu might well be a “gravitational aggregate,” or a rubble pile, a body that formed when gravity pulled together debris created when a large asteroid was smashed in a collision. That’s how we believe Bennu formed. However, assuming this scenario is correct, we don’t know whether Bennu consists of big solid rocks separated by very big voids or innumerable small voids, or whether Bennu is also porous at a very fine scale, or whether it consists of a large cloud of small debris.
Scientists also know that Bennu features a bulge and a long ridge at the equator. Whether there are craters or avalanches or other striking features is as yet unknown. OSIRIS-REx will get much more information during its 2018-2021 asteroid rendezvous. When new detailed information is available, Patrick’s research will guide the team in both choosing and interpreting the sample site as well as reveal more about the past dynamical and collisional history of the asteroid and its sample.
Marco Delbo, Observatoire de la Côte d’Azur, CNRS, Nice
There could be no asteroid sample-return mission without reliable estimates of asteroid surface temperatures. OSIRIS-REx depends on Marco Delbo to help determine these temperatures. The work is critical for both engineering and science.
A key task is to determine how hot Bennu will be where the spacecraft will actually touch the asteroid. In order to achieve our mission objective of collecting a pristine sample, we want to maintain sample temperatures below 75 degrees Celsius (167 ˚F). There are regions on the asteroid that are hotter than that, so our goal is to understand where the temperatures are ideal for sampling. The poles, for example, are probably too cold. The equator of the asteroid, near the local mid-day, is too hot.
Another concern is that as the spacecraft approaches the asteroid, the thermal radiation from the black asteroid surface may heat OSIRIS-REx science instruments and engineering components beyond tolerable limits. Engineers need to know temperatures in Bennu’s surrounding space environment so they can design adequate thermal control systems to keep the equipment at a safe temperature.
A very important science goal is to process data taken by the OTES thermal emission spectrometer to better characterize physical properties of the asteroid’s surface. It turns out that the time of day for peak thermal emission varies with the average grain size on the asteroid surface. Thus, Marco and his colleagues will help us find sites on the asteroid where OSIRIS-REx will be able to sample fine regolith.
Thermal emission is also key to accomplishing one of OSIRIS-REx major mission goals, which is to understand the Yarkovsky effect. Knowing the temperatures on the surface of the asteroid is fundamental, because heat that Bennu absorbs from the sun and then re-radiates back into space acts as a small thruster and changes its orbit. The Yarkovsky effect can nudge small asteroids like Bennu into Earth-impacting orbits.
Marco’s work straddles science and engineering and is a great example of the collaboration required to make a space mission successful. It is also a dream come true for Marco. His role on OSIRIS-REx reminds him of when he went for a job interview early in his career. During his interview he was asked why calculating temperatures of asteroids was important. Among several reasons, was that one-day we might land on one of these asteroids. At the time, he was not sure that would happen. And now it’s happening!
Guy Libourel, Observatoire de la Côte d’Azur, CNRS, Nice
Geologists, as well as astronomers, are needed to achieve ambitious OSIRIS-REx mission science goals. Guy Libourel is a geologist and cosmochemist who studies meteorites to answer a fundamentally important geological question: How are fine particles made from bare rock surfaces in the solar system? Layers of dust, rubble and broken rock, called “regolith,” cover surfaces of asteroids, as well as on the Earth, Moon, Mars, and other terrestrial planets and moons.
Regolith is made when rocky objects collide, creating craters and debris particles that fall back onto the surface of asteroids or planets. However, Guy is testing another way by which fine regolith might form, a process that he and co-investigators Patrick Michel and Marco Delbo call “thermal fatigue.” Because asteroids have one side facing the sun, temperatures vary from the day to the night, and this thermal variation results in mechanical stress on the rocks. Just as a spring breaks when you pull on it many times, stress can break rock into finer particles. Since Bennu rotates every 4.3 hours, there are a tremendous number of day and night cycles over her lifetime. We think this process occurs on a much more drastic manner on the “rock comet” asteroid Phaethon.
In his laboratory experiments, Guy subjects meteorites to many thermal cycles at temperature variations that have been observed on asteroid surfaces to see how quickly the meteorites fracture. He then constructs mathematical models to understand how long the process could last.
Guy’s research is important for selecting the OSIRIS-REx sample target site. It suggests that we can expect to find finer regolith where Bennu is subject to greater temperature changes. It also helps us predict what size regolith particles we can expect as a function of many thermal cycles. Guy’s laboratory experiments, which involve lasers that pulse thermal bursts on meteorite samples, further help scientists understand the thermal properties of Bennu’s surface.
OSIRIS-REx brings together the best and brightest asteroid researchers from around the world. The four French scientists bring unique expertise that is central to mission development and scientific success. It is an honor to have them as part of our team.
This article was co-authored with science writer Lori Stiles.