OSIRIS-REx Status Report: What We've Learned about Bennu

1. OSIRIS-REx Status Report (and What We’ve Learned about Bennu) Carl Hergenrother Asteroid Astronomy Lead SBAG – 2013 July 11

2. Recent Accomplishments • TAGSAM microgravity testing @ JSC • Mission “upgraded” to Category 1 • 2012 DA14 flyby and Chelyabinsk media events • OLA authorization received for Phase B2/C • Successful Mission Preliminary Design Review • ‘Name That Asteroid’ winner announced – we are going to Bennu • NFPO directed Project to remove all “STEM education” activities and eliminate E/PO element • APMC KDP-C Confirmation Review – Confirmed! • Start of Phase C – 6/3/2013 2 OSIRIS-REx Science Team Meeting #4 – June 18-20, 2013

3. Asteroid (101955) 1999 RQ36 is now . . . • Bennu • Bennu is an Egyptian mythological bird that was born from the heart of Osiris • It is associated with the Sun, creation, and renewal • The name was selected in an international contest run by the Planetary Society Image credit: http://www.touregypt.net

4. Initial Characterization: 1999 Discovery: Sept 11, 1999 by the LINEAR survey Follow-up: ~200 astrometric observations between Sept 12 – 24, 1999 Radar: Arecibo and Goldstone observations from Sept 21 – 25, 1999 Visible Spectroscopy: McDonald Observatory for 5 nights in September 1999 LINEAR Detection limit

5. Reacquisition and Physical Characterization: 2005 Reacquisition: 71observations between Aug 8 – Sept 17, 2005 Vis-IR Spectroscopy: NASA IRTF observations on Sept 4, 2005 Photometry: Lightcurve and ECAS colors on Sept 14 – 17, 2005 Radar:observations from Sept 16 – Oct 2, 2005 Catalina Sky Survey Detection limit (Mt. Lemmon)

6. Final Campaign: 2007 - 2012 Thermal IR: Spitzer observations on May, 2007 and Aug., 2012 Radar: Further Arecibo observations in Sept, 2011 Phase function: Photometric measurements through May, 2012 Light Curve:Hubble-WFC3 observations in Sept. and Dec., 2012

7. A Quality Orbit Requires Extensive Observation Reacquisition ∆a = 4 km Second Radar ∆a = 100 m Obs. at opposition ∆a = 15 m Discovery ∆a = 376,000 km Follow-up ∆a = 20,500 km First Radar ∆a = 43 km

8. Measurements of the distribution of range and radial velocity provided two-dimensional images with spatial resolution of 7.5 m Images used to construct a geologically detailed three-dimensional model and define the rotation state Size = 492-m (±20 m, mean diameter) Shape = spheroidal “spinning top” Rotation state = 4.29 hr period, 180º obliquity Radar also probed the near-surface bulk density (1.7 g cm-3) and structural scales larger than a few centimeters Radar and Photometry Are Powerful Sources of Information about Asteroid Physical Properties

9. Finding the Right Asteroid Means Knowing What it is Made of • OSIRIS-REx seeks to return samples from a Carbonaceous Asteroid • Visible, near-infrared spectroscopy and ECAS photometry show that Bennu is a B-type asteroid • Linear, featureless spectrum with bluish to neutral slope • Near-IR thermal emission starting at 2 µm suggest an albedo of 3-5% • The hydrated CI and CM carbonaceous chondrite meteorites are the most likely analogs

10. An OSIRIS-REx First: Measuring a Planetary Mass Using Radar and Infrared Astronomy • The three precise series of radar ranging position measurements over two synodic periodsallows us to measure the Yarkovsky acceleration • The asteroid has deviated from its gravity-ruled orbit by 160 kilometersin just 12 years • This result, when combined with the thermal inertia and the shape model, constrains the mass to 6.278 (-0.942/+1.883) x 1010kg • Mass and shape constrain the bulk density to 0.980 ± 0.147 g/cm3 • Spitzer observations yield a very low albedo – 4.5 ± 1.5%

11. Surface Properties are Consistent with Abundant Loose Regolith Available for Sampling • Radar polarization shows transition to a “rough” surface at a scale smaller than the shortest (3.5-cm) wavelength • The thermal inertia is substantially below the bedrock value – regolith grains are significantly smaller than the scale of the skin depth (~1 cm) • The asteroid’s shape, dynamic state, and geomorphology provide additional evidence for the presence of loose particulate regolith • There is one ~10–m boulder apparent on the surface

12. Backup Charts

13. Thermal IR Observations Provide Critical Knowledge for Spacecraft Design • Spitzer observations yield a very low albedo – 4.5 ± 1.5% • Combining the asteroid shape, rotation state, ephemeris, and albedo yields a global temperature model • Thermal IR observations provide ground truth for this model • Direct input into the mission Environmental Requirements Document

14. Mission Design Constraints 2: Size and Rotation • OSIRIS-REx must be able match the rotational rate of the target, achieving a spacecraft attitude where we “hover” over the sampling site • This constraint translates into a limit on the rotation period of the asteroid • The majority of asteroids <200 m are rapid rotators, with rotation periods as short as one minute • Rapid rotation greatly increase the risk during proximity operations • Centrifugal forces have also likely ejected most regolith particles from the surface • Lightcurve reveals a rotation period of 4.297  0.002 hours

15. OSIRIS‐REx is Developing Critical Technologies for Exploring Near‐Earth Asteroids • Astronomical characterization in support of mission design • Measurement of asteroid global characteristics • Detailed characterization of an asteroid surface at sub-cm scales • Mission-critical data processing and analysis on a tactical timeline • Accurate navigation in microgravity • Delivery to a specific location on the asteroid surface • Successful contact and acquisition of material from an asteroid surface • Safe return of the sample to Earth Without guidance With guidance

16. Mission Design Constraints 1: Orbit • Use of solarpower: aphelion < 1.6 AU • Thermal constraints: perihelion > 0.8 AU • These two requirements constrain both the semi-major axis and the orbital eccentricity of the target • Mission propellant and sample return capsule (SRC) performance requirements: inclinations <10˚ • Objects on low-inclination orbits require a minimum amount of delta-V for rendezvous and provide low re-entry velocities for the SRC • Quality of orbital knowledge: sufficiently precise to allow us to design a trajectory ensuring the spacecraft could rendezvous with the target • The orbit of Bennu meets all of our mission-target criteria Note: Bennu inclination = 6.03˚

17. Knowledge of Asteroid Mass Substantially Enhances Mission Planning • Mass and shape constrain the bulk density to 0.980 ± 0.147 g/cm3 • They are combined to produce a global gravity-field model that facilitates orbital stability analysis • Combining the gravity-field model and rotation state yields global surface-slope distributions and accelerations • All this information is critical to evaluating our ability to safely deliver the spacecraft to the asteroid surface and maintain nominal attitude during sampling 17

18. The Great Value of Asteroid Samples are in the Detailed Knowledge of Sample Context • Dynamical studies characterize the asteroid history and provide sample context • Combined dynamical and spectral information to identify the most likely main-belt origin • Discovered the “Eulalia family” – formed between 900–1500 Myr ago from the breakup of a 100–160 km parent body • Found compelling evidence for an older and more widespread primitive family in the same region • Either one of these families could be the source of 1999 RQ36 – need sample return to discriminate between the two

19. Study of this Potentially Hazardous Asteroid is Strategically Important • 1999 RQ36 is classified as a potentially hazardous object • Diameter larger than 150 meters • MOID of 0.0027 AU with the Earth • The Yarkovsky effect is the most significant non-gravitational acceleration acting to alter the asteroid’s orbit • We can confidently predict eleven approaches to Earth closer than 0.05 AU over a span of 481 years • ~10-3 probability of a 3000 MT impact late in 22nd century

20. The Rotation State is Well Constrained from Lightcurve Measurements • Achieved a frequency of observations which resulted in a lightcurve covering afull rotation cycle each night for 4nights of observing • Lightcurve reveals a rotation period of 4.297  0.002 hours • The low amplitude is consistent with the rotation of a nearly spherical body

21. TAGSAM – The OSIRIS-REx Sampling Strategy is Designed to Collect Abundant Pristine Regolith

22. Mission Timeline • Selection: May 25, 2011 • Preliminary Design Review (PDR): March, 2013 • Critical Design Review (CDR): April, 2014 • System Integration Review (ATLO): February, 2015 • Launch: September, 2016 • Earth Gravity Assist (EGA): September, 2017 • Asteroid Arrival (AA): August, 2018 • Asteroid Departure (Dep): March, 2021 • Sample Return: September, 2023 • End of Mission (Sample Analysis – SA): September, 2025

23. Our Sample is Collected During a Five-Second Touch-and-Go Maneuver • Approach surface within vertical and horizontal speed constraints • Surface contact is made with sampler head • Compression of spring in the Touch-and-Go Sample Acquisition Mechanism (TAGSAM) arm • Rebound from surface using stored energy in spring • Fire thrusters to accelerate away from RQ36

24. Our Payload Performs Extensive Characterization at Global and Sample-site-specific Scales OCAMS (UA) SamCamimages the sample site, documents sample acquisition, and images TAGSAM to evaluate sampling success MapCamprovides landmark-tracking OpNav, performs filter photometry, maps the surface, and images the sample site PolyCamacquires 1999 RQ36 from >500K-km range, performs star-field OpNav, and performs high-resolution imaging of the surface OLA (CSA)provides ranging data out to 7 km and maps the asteroid shape and surface topography

25. Spacecraft-based Remote Sensing Provides Ground Truth for our Astronomical Data OVIRS (GSFC)maps the reflectance albedo and spectral properties from 0.4 – 4.3 µm OTES (ASU)maps the thermal flux and spectral properties from 4 – 50 µm RadioScience (CU)reveals the mass, gravity field, internal structure, and surface acceleration distribution REXIS (MIT)maps the elemental abundances of the asteroid surface

26. Our Design Reference Mission Provides Substantial Operational Margin

27. Phase Function for 1999 RQ36

28. Finding a Boulder in Space Catalina Sky Survey Detection limit (Mt. Lemmon)

29. Carbonaceous Boulder Statistics • A carbonaceous asteroid with a diameter < 10 meters and albedo < 0.07 would have an absolute magnitude > 28.5. • As of April 2013, 78 NEAs with H > 28.5 have been discovered. • 72 of the 78 were discovered by surveys operated by the University of Arizona (Catalina Sky Survey, Mount Lemmon Survey, Spacewatch)

30. Asteroid Boulder Orbits • Of the 78 only 8 have been characterized in any way • 6 Rotation Periods (1991 VG, 2006 RH120, 2008 JL24, 2008 TC3, 2010 TD54, 2012 KT42) • 3 Taxonomy (2008 TC3, 2010 TD54, 2012 KT42) • 3 Radar Observations (2006 RH120, 2012 XB112, 2013 EC20) • Quality of Orbits • The following plots were obtained from MPC orbit data for the 78 NEAs fainter than H of 28.5 • Nearly 2/3rd were only observed for 2 days or less, nearly 1/3rd were followed for less than 1 day • Only 27 of the 78 have positional uncertainties less than ~1 million km. • Only 15 of the 78 have positional uncertainties less than ~1/4 of a million km • Only 3 have positional uncertainties less than ~14,000 km

31. Boulder Search Strategy • The plot below compares the length of observations (in days) with the minimum delta-V for each of the H > 28.5 objects • Note that the objects with the longest arcs of observations also have some of the lowest delta-Vs • This is due to two reasons: • One these objects were specifically observed because they have low delta-Vs • Low delta-V objects spend a longer time in the vicinity of Earth due to their lower relative velocity and, as a result, are observable for longer • The take away … • Instead of looking for every small asteroid flying near cis-lunar space, effort should be focused on the low delta-V objects • Search for temporary captures • Search for objects leading or trailing Earth by a few degrees

32. 1999 RQ36 Semi-major Axis Drift Uncertainty

33. 1999 RQ36 Semi-major Axis Uncertainty