Sandford, Scott - AN OVERVIEW OF THE OSIRIS-REX ASTEROID SAMPLE RETURN MISSION

Abstract: 
The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security‒Regolith Explorer) sample return New Frontiers Mission will characterize near-Earth asteroid 101955 Bennu (provisional designation 1999 RQ36) - the most accessible carbonaceous asteroid and the most potentially hazardous asteroid known.  OSIRIS-REx's mission objectives are:
1. Return and analyze a sample of pristine carbonaceous asteroid regolith to study the nature, history, and distribution of its constituent minerals and organics.
2. Map the global properties, chemistry, and mineralogy of a carbonaceous asteroid to characterize its geologic and dynamic history and provide context for the returned samples.
3. Document the texture, morphology, geochemistry, and spectral properties of the sampling site regolith down to sub-centimeter scales.
4. Measure the Yarkovsky effect on a potentially hazardous asteroid and constrain asteroidal properties contributing to this effect.
5. Characterize the global properties of a primitive carbonaceous asteroid for direct comparison with ground based telescopic data of the entire asteroid population.

Asteroid Bennu is a B-type, near-Earth asteroid. B-type asteroids appear to be transitional between asteroids and comets. Bennu was discovered in 1999 by LINEAR and is an Apollo NEO with a semi-major axis of 1.126 AU. Observations support the presence of abundant regolith ideal for sampling (4-8 mm) [1-3]. Infrared spectra suggest a low albedo (0.035 ± 0.015) and that the most likely meteorite analogs for Bennu are CI or CM meteorites [4]. Bennu has a rotational period of 4.2968 ± 0.0018 hrs [1]. The rotation period and axial ratio imply a density of >0.7 g/cm3; the team estimates a 1260 ± 70 kg/m3 bulk density [5]. Bennu comes within 0.003 AU of the Earth and has the highest impact probability of any known asteroid [5].

OSIRIS-REx delivers its science using five instruments, radio science, and the Touch-And-Go Sample Acquisition Mechanism (TAGSAM). TAGSAM acquires samples using a jet of high-purity N2 gas that “fluidizes” the regolith into a collection chamber. Instruments for in situ observations include (i) the OSIRIS-REx Camera Suite (OCAMS), (ii) the OSIRIS-REx Laser Altimeter (OLA), (iii) the OSIRIS-REx Visible and Infrared Spectrometer (OVIRS), (iv) the OSIRIS-REx Thermal Emission Spectrometer (OTES), and (v) the Regolith X-ray Imaging Spectrometer (REXIS). Radio Science will determine Bennu's mass and estimate its mass distribution. With these and the shape model, the team will compute the bulk density and porosity of Bennu, thereby constraining its internal structure.

OSIRIS-REx launches in September 2016, sampling nominally occurs in 2019, and departure from Bennu occurs in March 2021. The SRC lands at the Utah Test and Training Range (UTTR) on 24 September 2023. After recovery, the SRC will be transported to NASA-JSC for sample removal. After a six-month preliminary examination period the mission will produce a sample catalog, allowing the worldwide community to request samples for detailed analysis.

References: [1] Hergenrother et al. (2013) Icarus 226, 663-670; [2] Emery J. P. (2010) Icarus 234, 17-35; [3] Nolan M. C. et al. (2013) Icarus 226, 629-640; [4] Clark B. E. et al. (2011) Icarus 216, 462; [5] Chesley S. R. et al. (2014) Icarus 235, 5-22.