OSIRIS-REx Touch-and-Go (TAG) Mission Design for Asteroid Sample Collection

The Origins Spectral Interpretation Resource Identification Security Regolith Explorer (OSIRIS-REx) mission is a NASA New Frontiers mission launching in September 2016 to rendezvous with the near-Earth asteroid Bennu in October 2018. After several months of proximity operations to characterize the asteroid, OSIRIS-REx flies a Touch-And-Go (TAG) trajectory to the asteroid's surface to collect at least 60 g of pristine regolith sample for Earth return. This paper provides mission and flight system overviews, with more details on the TAG mission design and key events that occur to safely and successfully collect the sample. An overview of the navigation performed relative to a chosen sample site, along with the maneuvers to reach the desired site is described. Safety monitoring during descent is performed with onboard sensors providing an option to abort, troubleshoot, and try again if necessary. Sample collection occurs using a collection device at the end of an articulating robotic arm during a brief five second contact period, while a constant force spring mechanism in the arm assists to rebound the spacecraft away from the surface. Finally, the sample is measured quantitatively utilizing the law of conservation of angular momentum, along with qualitative data from imagery of the sampling device. Upon sample mass verification, the arm places the sample into the Stardust-heritage Sample Return Capsule (SRC) for return to Earth in September 2023

Dynamical Evolution of Simulated Particles Ejected from Asteroid Bennu

In early 2019, the OSIRIS‐REx spacecraft discovered small particles being ejected from the surface of the near‐Earth asteroid Bennu. Although they were seen to be ejected at slow speeds, on the order of tens of cm/s, a number of particles were surprisingly seen to orbit for multiple revolutions and days, which requires a dynamical mechanism to quickly and substantially modify the orbit to prevent re‐impact upon their first periapse passage. This paper demonstrates that, based on simulations constrained by the conditions of the observed events, the combined effects of gravity, solar radiation pressure, and thermal radiation pressure from Bennu can produce many sustained orbits for ejected particles. Furthermore, the simulated populations exhibit two interesting phenomena that could play an important role in the geophysical evolution of bodies such as Bennu. First, small particles (<1 cm radius) are preferentially removed from the system, which could lead to a deficit of such particles on the surface. Second, re‐impacting particles preferentially land near or on the equatorial bulge of Bennu. Over time, this can lead to crater in‐filling and growth of the equatorial radius without requiring landslides

OSIRIS-REx Encounters Bennu: Initial Assessment from the Approach Phase

The OSIRIS-REx spacecraft launched on September 8, 2016, on a seven-year journey to return samples from asteroid (101955) Bennu. This presentation summarizes the scientific results from the Approach and Preliminary Survey phases. Bennu observations are set to begin on August 17, 2018,when the asteroid is bright enough for detection by the PolyCam. PolyCam and MapCam collect data to survey the asteroid environment for any hazards and characterize the asteroid point-source photometric properties. Resolved images acquired during final approach, starting in late October 2018, allow the creation of a shape model using stereophotoclinometry (SPC), needed by both the navigation team and science planners. The OVIRS and OTES spectrometers characterize the point- source spectral properties over a full rotation period, providing a first look at any features and thermophysical properties. TAGSAM is released from the launch container and deployed into the sampling configuration then returned to the stow position.Preliminary Survey follows the Approach Phase in early December 2018. This phase consists of a series of hyperbolic trajectories that cross over the North and South poles and the equator of Bennu at a close-approach distance of 7 km. Images from these Preliminary Survey passes provide data to complete the 75-cm resolution SPC global shape model and solve for the rotation state. Once the shape model is complete, the asteroid coordinate system is defined for co-registration of all data products. These higher-resolution images also constrain the photometric properties and allow for an initial assessment of the geology. In Preliminary Survey the team also obtains the first OLA data, providing a measure of the surface topography. OVIRS and OTES collect data as "ride-along" instruments, with the spacecraft pointing driven by imaging constraints. These data provide a first look at the spectral variation across the surface of Bennu. Radio science measurements, combined with altimetry and imagery, determine Bennu's mass, a prerequisite to placing the spacecraft into orbit in late December 2018. Together, data from the Approach and Preliminary Survey phases set the stage for the extensive mapping planned for 2019. These dates are the baseline plan. Any contingency or unexpected discovery may change this mission profile

Impact features on Europa: results of the Galileo Europa Mission (GEM)

During the Galileo Europa Mission (GEM), impact features on Europa were observed with improved resolution and coverage was compared with Voyager or the Galileo nominal mission. We surveyed all primary impact features &gt;4 km in diameter seen on Europa (through orbit E19). The transition from simple to complex crater morphology occurs at a diameter of about 5 km. We calculated the transient crater dimensions and excavation depths of all craters surveyed. The largest impact feature (Tyre) probably had a transient crater depth between 5 and 10 km and transported material to the surface from a depth of not greater than ∼4 km. Craters &lt;30 km in diameter, such as Manannàn and Pwyll, formed within targets whose immediate subcrater materials exhibited nonfluid behavior on time scales of the impact event, and that are capable, especially in the case of Pwyll, of supporting significant local topographic loads such as a central peak. These craters are nevertheless quite shallow, with very subdued floors, and we suspect that Manannàn and Pwyll's small depth-to-diameter ratios are due to the isostatic adjustment of large-scale topography, facilitated by warm, plastically deformable ice at depth. Morphological similarities between Callanish and Tyre strongly imply that conclusions reached regarding Callanish in J. Moore et al. (1998, Icarus135, 127–145) also apply to Tyre, which was that Callanish is the consequence of impact into target materials that are mechanically very weak at depth. New evidence that Callanish's circumferential rings formed before the proximal ejecta became immobile implies a low-viscosity substrate at the time of impact. We also report additional evidence that a component of the proximal ejecta of Callanish was emplaced as a fluid. Our observations of Pwyll secondaries support the conclusions stated in Alpert and Melosh (1999) that impacts on icy bodies eject smaller fragments and that fragment size decreases more gradually as velocity increases than observed for impacts on silicate bodies at equivalent ejection velocities. Examination of Pwyll's secondary craters reveals azimuthal variations, with ejecta fragment sizes being larger near the center of a ray than off the ray. Our initial analysis of the characteristic size distribution of Pwyll's secondary craters shows that they form a differential slope slightly shallower than −4. Similar steep slopes for small craters on Ganymede imply that small craters there are mostly formed by secondary impact, and the jovian system may thus be deficient in small impacts relative to the environment of the terrestrial planets