The OSIRIS-REx Laser Altimeter. M. G. Daly1, O. S. Barnouin2, C. Dickinson3, J. Seabrook1, C.L. Johnson4, G. Cunningham5, T. Haltigin6, D. Gaudreau6, C. Brunet6, I. Aslam3, A. Taylor3, E. B. Bierhaus7, W. Boynton8, M. Nolan8, D. S. Lauretta8, 1The Centre for Research in Earth and Space Science, York University, 4700 Keele Street, Toronto, Ont., Canada, M3J 1P3, 2Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, MD 20723-6099, 3MacDonald Dettwiler & Associates, 9445 Airport Road, Brampton, Ont., Canada, L6S 4J3, 4The University of British Columbia, 6339 Stores Road, Vancouver, V6T 1Z4, 5Teledyne Optech, 300 Interchange Way, Vaughan, Ont., Canada, L4K 5Z8, 6 Canadian Space Agency, 6767 Route de l'Aéroport, Saint-Hubert, Que., Canada, J3Y 8Y9, 7Lockheed Martin Space Systems Company, PO Box 179, Denver, CO 80201, 8Lunar and Planetary Labor-atory, University of Arizona, Tucson, AZ, 85721. Corresponding Author’s Email: dalym@yorku.ca.

Introduction: The primary objective of the Origins, Spectral Interpretation, Resource Identification, Secu-rity-Regolith Explorer (OSIRIS-REx) is to return a sam-ple from asteroid 101955 Bennu [1]. The instruments aboard the OSIRIS-REx spacecraft will measure the properties of the asteroid to support the investigation of the geophysical and geochemical state of this B-class asteroid, a subclass within the larger group of C-com-plex asteroids, that might be organic-rich. At approxi-mately 500m in average diameter [2], Bennu is large enough to retain substantial regolith and as an Apollo asteroid with a low inclination (6◦), it is one of the most accessible primitive near-Earth asteroids.

The OSIRIS-REx spacecraft, launched in September 2016, will rendezvous with asteroid 101955 Bennu in mid 2018. As part of a suite of instruments on the OSIRIS-REx spacecraft, the OSIRIS-REx Laser Altim-eter (OLA) is the world’s first scanning laser range-finder (or lidar) to fly on a planetary mission.

The OLA instrument is very flexible in its ability to collect data because of its long and short-range laser transmitters and its scanning mirror. This flexibility is ideal for continuously improving the fidelity of topo-graphical products generated by OLA during the differ-ent phases of the OSIRIS-REx mission, as the spacecraft slowly approaches and eventually touches the asteroid. OLA has a series of scientific and mission objectives that can be divided into global and sample-site-scale in-vestigations.

At a global scale, OLA will measure the shape of Bennu to provide insights into the geological origin and evolution of the asteroid by, for example, constraining its bulk density through precise volume measurement. Combined with a carefully undertaken geodesy cam-paign OLA-based precision ranges, radio science (2-way tracking) data and stereo images will yield con-straints on any global-scale internal heterogeneity of Bennu and hence provide further clues to its origin and subsequent collisional evolution. OLA-derived global maps of slopes, geopotential elevation or altitude rela-tive to the asteroid geoid [3], and vertical roughness will provide quantitative insights [4,5] into how the surface

of Bennu evolved subsequent to the formation of the as-teroid. Establishing any connection between surface morphological features that pos- sess measurable topog-raphy and their spatial relationships to other geological features, such as craters, will provide additional con-straints on the interior structure and geophysical evolu-tion of Bennu [6].

At the sample-site scale (~25m radius), OLA will provide detailed information on the geological and geo-physical processes that influence the surface regolith at scales relevant to the samples that will be collected.

Figure 1: OLA consists of two subassemblies - the opti-cal head unit (right) and the electronics unit (left).

OLA also provides basic ranging functionality by

providing precision ranges that are used as a part of the data input to the navigation solutions. These fundamen-tal measurements provide increased confidence and faster navigation timelines, thereby improving effi-ciency, accuracy, and overall mission safety. The range measurements also allow other instrument teams to im-prove the scaling of their images or spectral sampling.

The Instrument: OLA was developed using heritage components and approaches from previous spaceflight missions. The scanning system and low-energy trans-mitter share heritage with the lidar used aboard the ex-perimental XSS-11 mission [7]. The high-energy laser is a modified version of that used in the meteorology lidar on the Phoenix Mars mission [8].

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The maximum required operational range of OLA of ~7km from the asteroid combined with the 1mJ-class la-ser pulse energy and the sensitivity of the XSS-11 de-rived constant-fraction receiver set the receiver aperture at ~75mm with a 3dB link margin. The power-aperture product of the low-energy laser provided a 750m oper-ational range, also with a 3dB link margin. To achieve global coverage of Bennu and to make efficient use of the 10kHz maximum measurement rate of OLA, a scan-ning mirror is required. The OLA scanning capability circumvents the very slow ground-track velocity of the spacecraft, which is on the order of 1 spot diameter per second, and allows the placement of thousands of meas-urements per second across the asteroid, in a very short time period.

Table 1: OLA as-built performance and key character-istics. Specifications are based on performance to a 3% Lambertian reflector.

Instrument Performance: OLA consists of two sub-assemblies ( Figure 1): a sensorhead (which contains all of the optics, lasers and circuitry to both drive the lasers and detect return signals), and the main electronics (which contains all of the system avionics such as signal processing, power, spacecraft communication and time-of-flight circuitry). The OLA as-built performance is outlined in Table 1. An example of the OLA flight in-strument performance is shown in Figure 2. The smoke-stack is at a range of ~900m and the building behind it at ~1200m.

References: [1] Lauretta, D.S. (2015) Handbook of Cosmic Hazards and Planetary Defense, ed. by Allahdadi F. and Pelton J.N., Springer, 543–567. [2] Nolan, M.C. et al. (2013) Icarus 226(1), 629–640. [3] Scheeres D.J. et al. (2016) Icarus 276, 116–140.

[4] Barnouin-Jha, O.S. et al. (2008) Icarus 198(1), 108–124. [5] Cheng, A.F. et al. (2001) Science 292(5), 488–491. [6] S. Marchi, S. et al. (2015) Cratering on Asteroids, University of Arizona Press, 725–744. [7] Nimelman, M. et al. (2005) Proc. SPIE 5798, 73–82. [8] Whiteway, J. et al. (2008) JGR: Planets 113(E3).

Figure 2: LELT raster scan in the mirror frame of ref-erence. Colour scale represents the range corrected nor-malized return intensity. Acknowledgements: There are many important con-tributors who could not be included in the author list. These contributors include staff at MDA, Teledyne Optech, CSA, York University, University of British Columbia, University of Arizona, Johns Hopkins Uni-versity Applied Physics Laboratory, Lockheed Martin and NASA Goddard Spaceflight Center. The instrument build and Canadian science support was provided by a contract with the Canadian Space Agency. The United States team contribution was supported by the National Aeronautics and Space Administration under Contract NNM10AA11C and NNG12FD66C issued through the New Frontiers Program.

1996.pdfLunar and Planetary Science XLVIII (2017)