Small Bodies, Big Plans
With the successful July 30 launch of our Mars 2020 mission, with the aptly named Perseverance rover and Ingenuity helicopter, our priorities in the Planetary Science Division (PSD) shift to the next major mission events on our calendar. First up, on October 20, will be the OSIRIS-REx touch-and-go (TAG) event. During TAG the spacecraft’s Sample Acquisition Mechanism will be used to collect a sample of Bennu, which will eventually be returned to Earth. Since its arrival at the B-type carbonaceous asteroid in December 2018, OSIRIS-REx has been carefully and systematically surveying the asteroid’s surface and characteristics. And over the course of this past spring and summer, the OSIRIS-REx team conducted two TAG rehearsals — the “Checkpoint” and “Matchpoint” rehearsals over the primary sampling site, Nightingale — when the spacecraft was flown down to approximately 75 m and 40 m above Bennu’s surface, respectively.
Although the sample collection, and its ultimate return to Earth, will be the pinnacle of the OSIRIS-REx mission, the science results that continue to be produced by the mission team illustrate the fantastic insights we can gain from visiting the small bodies in our solar system and observing them from orbit or flybys. For example, we have recently seen a series of research articles published in JGR Planets and Nature Astronomy based on OSIRIS-REx results. In a planetary instance of giving and taking away, the team identified several pieces of another asteroid — likely Vesta — on Bennu’s surface and witnessed regular particle ejection events, during which material was ejected from the asteroid’s surface. Even more recently, in six papers published in Science and Science Advances, OSIRIS-REx scientists presented new findings on Bennu’s surface material, geological characteristics, and dynamic history. So even before the precious pieces of Bennu are brought back to Earth for study in state-of-the-art laboratories, OSIRIS-REx is repeatedly revealing the dynamism of asteroids.
Of course, Bennu isn’t the only small body we’ll be visiting in the next few years. The next scheduled PSD launches will be the Double Asteroid Redirection Test (DART) and Lucy in July and October 2021, respectively, with Psyche following closely in 2022. Plus, New Horizons continues to explore the Kuiper belt after its fruitful flybys of Pluto and Arrokoth.
DART — the exciting first mission from the Planetary Defense Coordination Office — will use a kinetic impactor to demonstrate the asteroid deflection technique. The Didymos binary asteroid system (consisting of the larger Didymos and its smaller moonlet Dimorphos, with diameters of ~780 and 160 m, respectively) will be DART’s target, with the spacecraft hitting Dimorphos with a nearly head-on impact in September 2022. The Light Italian Cubesat for Imaging of Asteroid (LICIACube), from the Italian Space Agency, will ride along with DART and will support the mission goals by capturing images of the impact effects on the surface and the expected ejecta plume it will generate. The relatively low energy of the impact will cause a change in the speed and path of Dimorphos rather than disrupt the integrity of the asteroid itself (there is no concern that the path of the system will intersect with that of Earth at any point). Earth-based telescopes, as part of an international observing campaign, will be used to determine the change in the orbit of Dimorphos after the impact.
Although the focus of DART will be the defense technology test (i.e., to help prevent future impacts of potentially hazardous asteroids to Earth), the mission will also play a larger role in an exciting international scientific collaboration known as the Asteroid Impact and Deflection Assessment (AIDA). As the second part of this collaboration, the European Space Agency’s Hera mission will rendezvous with the Didymos system in 2026 (i.e., about four years after the DART impact) and will conduct a detailed survey of both bodies in the system. By measuring numerous properties of Dimorphos, including its mass, surface characteristics, porosity, and internal structure, as well as gaining a detailed look at the DART impact crater, the AIDA collaboration will provide unique science results. The combination of DART and Hera will revolutionize our current paradigms for understanding fundamental processes, including collision physics and cratering behaviors, as well as solar system formation in general.
Likewise, Lucy will provide a fantastic opportunity to gain new insights into planet formation processes — particularly for the outer solar system. The mission will make a tour of seven Trojan asteroids — asteroids trapped in Jupiter’s orbit that are thought to be remnants of material that originally accreted to form the outer planets. The Lucy spacecraft will be equipped with four payload instruments to characterize the surface geology, surface color and composition, interior and bulk properties, as well as satellites and rings of each asteroid visited. Although searching for satellites is one of the mission goals, earlier this year the Lucy science team discovered — with the use of the Hubble Space Telescope (HST) — a satellite of one of the original asteroids slated for Lucy’s tour. The satellite of Eurybates thus increased Lucy’s total asteroid itinerary from seven to eight (including a main-belt asteroid as well), and the discovery of more satellites is definitely possible before Lucy arrives at Jupiter and begins its prime mission. The team is continuing to use HST to pin down the precise orbit of this new “moon.”
Adding to the collection and variety of asteroid-types our upcoming missions will visit is Psyche — the mission and the asteroid. This Discovery mission will be the first time we send a spacecraft to explore a world that appears to be made of significantly more metal than other asteroids that are primarily made of rock, ice, or gas. Thought to be representative of the metallic — and inaccessible — cores of terrestrial planets, the asteroid Psyche provides us with the ability to study a key component of rocky planets and gain new understanding of planetary formation. The mission’s payload of four instruments — a multispectral imager, a gamma-ray and neutron spectrometer, a magnetometer, and an X-band gravity science investigation — will be used to meet a set of science objectives, which will include determining if Psyche is indeed a protoplanet’s core or if it is never-melted material, determining the relative ages of different regions on the asteroid’s surface, and determining if the asteroid contains similar light elements than are thought to be present in the cores of terrestrial planets.
As the COVID pandemic continues, and as we round out the year of 2020, I gaze back at what our community was able to accomplish in the midst of a long string of unforeseen challenges. We truly persevered in every sense of the word. Thinking about these upcoming missions and bearing witness as they progress through their various development stages, however, is continuing to enthuse and engage me — and I hope you all as well. I know I speak for the whole planetary science community when I express my thanks to all the engineers, managers, and scientists for their hard work during these difficult times in non-ideal conditions. I am immensely grateful for their efforts in keeping these missions on track and on schedule. I hope you agree that these ongoing and upcoming missions to some of the smaller (and least known) bodies in our solar system are providing hope, excitement, and inspiration during these trying times. I’m thrilled to see the diversity of our planetary science portfolio, and I can’t wait to see what solar system discoveries we uncover next.
— Lori S. Glaze, Director, NASA’s Planetary Science Division, October 2020