Arecibo Radar Observations of 19 High-Priority Near-Earth Asteroids During 2018 and January 2019

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1 Arecibo Radar Observations of 19 High-Priority Near-Earth Asteroids During 2018 and January 2019 Patrick A. Taylor, Anne K. Virkki, Sriram S. Bhiravarasu, Flaviane Venditti, Sean E. Marshall, Edgard G. Rivera-Valentin, Luisa F. Zambrano-Marin, Betzaida Aponte-Hernandez, Linda A. Rodriguez-Ford (Arecibo Observatory), Michael C. Nolan, Ellen S. Howell (U. Arizona), Jon D. Giorgini, Lance A. M. Benner, Marina Brozovic, Shantanu P. Naidu (JPL), Michael W. Busch (SETI), Jenna L. Crowell (U. Central Florida), Jean-Luc Margot (UCLA), Michael K. Shepard (Bloomsburg U.), and Christopher Magri (U. Maine at Farmington) Observing Program Radar is arguably the most powerful Earth-based technique for post-discovery physical and dynamical characterization of near-earth asteroids (NEAs). Arecibo radar observations routinely provide images with resolution as fine as 7.5 m revealing surface features, such as boulders, concavities, and ridges, that our SHAPE software (Hudson, 1994; Magri et al., 2007) inverts to obtain spin-state estimates and three-dimensional shape models limited only by echo strength and orientational coverage. Over the long term, our observations will help answer fundamental questions regarding the the diversity of asteroid morphologies and dynamical states, their internal structures and thermal properties, and the importance of non-gravitational and collisional evolution. NASA s Near-Earth Object Observations program supports the Arecibo planetary radar to observe NEAs for at least 600 hours per year. We propose radar imaging, detailed physical characterization, and orbit refinement of our 19 highest-priority NEAs for calendar year 2018 and early January 2019 using hours of telescope time. A companion proposal (Virkki et al.) with a more survey-oriented approach requests at least hours to concentrates on basic characterization and precise astrometry for many more objects bringing the total proposed time request to 570 hours. Proposals to observe NEAs not included in these companion proposals accounted for 15% of time requests in the last two years such that we expect to surpass 600 hours requested in In the last 12 months, the Arecibo planetary radar program detected 100 NEAs. The previous year s proposal (project R3037) requested hours of telescope time to observe 23 objects visible in Through August, 12 objects were visible, accounting for hours of our time request, of which 189 hours (75%) were scheduled. The discrepancy is mostly due to the S-band heat-exchanger replacement in July and August, competition for observing time with atmospheric world-day and HF campaigns, and overlap with daily maintenance activities. Noteworthy results include observations of 2014 JO25, an asteroid with a contact-binary shape very reminiscent of Comet 67P/Churyamov-Gerasimenko visited by the Rosetta spacecraft, and (190166) 2005 UP156, a rare equal-mass binary asteroid, one of only three known among the NEA population. The remaining four months of the calendar year, plus early January 2018, include 11 high-priority targets and hours of telescope time requested. We note that in this proposal, we request fewer hours, partially because there are fewer high-priority objects (19 vs. 23) and we expect to operate in one-klystron mode at half of the nominal output power. Time requests for each target are dictated by the science goals and the estimated signal-tonoise ratio (SNR). Past experience demonstrates the key factor in our ability to secure shapes and spin-state estimates is good sky and rotational coverage over several days of observations, especially when we lack prior knowledge about the target. For all targets we will measure the 1

2 circular polarization ratio and radar cross section, which are gauges of near-surface roughness and near-surface density, provide precise astrometry, and constrain the size, shape, and spin state, which when combined with photometric and/or spectroscopic measurements constrain the optical albedo and composition. Table 1 describes our targets and lists synergistic observations. The objects requested at the Goldstone radar (more maneuverable, but less sensitive than Arecibo) will have greater coverage from longer daily tracks and observations outside the Arecibo declination window, which may lead to tighter constraints on physical parameters. Overlapping tracks with Goldstone allows for bistatic X- or C-band experiments with resolutions of 3.75 or m. Eleven asteroids are possible targets for a coordinated program with the NASA InfraRed Telescope Facility (IRTF) for spectral characterization and application of radar-derived shape information to thermophysical modeling. Speckle tracking (Busch et al., 2010) with the Very Long Baseline Array (VLBA) will be used to resolve the prograde/retrograde-rotation ambiguity of at least two targets. Table 2 lists specific track requests for each target. Student Participation Graduate student Luisa Zambrano-Marin (Granada), member of the local Arecibo team, is using radar scattering models to constrain the surface properties of asteroids and comets. Graduate student Sean Marshall (Cornell) works on shape and thermal modeling of asteroids observed with radar and the IRTF (Marshall et al., 2017) and will join the local Arecibo team in late Graduate student Jenna Crowell (Central Florida) used radar in the shape and thermal modeling of asteroid 1627 Ivar (R2831; Crowell et al., 2017) and will lead observations of Ivar in Graduate student Adam Greenberg (UCLA) led observations of asteroids (1566) Icarus (R2960; Greenberg et al., 2017a) and (441987) 2010 NY65 (R3037) and is publishing Yarkovsky-drift measurements based partly on radar astrometry (Greenberg et al., 2017b). Graduate student Cassandra Lejoly (Arizona) observed Comet 45P (R3142) and is analyzing radar cross sections of Arecibo asteroid data. Benjamin Sharkey (now a graduate student at Arizona) was part of the Research Experience for Undergraduates (REU) program in 2015 and a summer research assistant at Arecibo in 2016 shape modeling asteroid (52760) 1998 ML14 (R1172 and R2831) and participated in radar observations. Undergraduate Andy Lopez-Oquendo (UPR Humacao) regularly participated in observations in Other undergraduate and graduate students are welcome to gain observing and research experience through this proposed work. References Busch, M.W., et al. Determining asteroid spin states using radar speckles. Icarus 209, , Crowell, J.L., et al., Radar and lightcurve shape model of near-earth asteroid (1627) Ivar, Icarus 291, , Greenberg, A.H., et al., Asteroid 1566 Icarus s size, shape, orbit, and Yarkovsky drift from radar observations, AJ 153, 108, 2017a. Greenberg, A.H., et al. Yarkovsky drift detections for 159 near-earth asteroids, 2017b, submitted. Hudson, S., Three-dimensional reconstruction of asteroids from radar observations. Remote Sens. Rev. 8, , Magri, C., et al. Radar observations and a physical model of asteroid 1580 Betulia. Icarus 186, , Marshall, S.E., et al., Thermal properties and an improved shape model for near-earth asteroid (162421) 2000 ET70, Icarus 292, 22-35, Warner, B.D., Harris, A.W., Pravec, P., The asteroid lightcurve database, Icarus 202, , Updated February

3 Object H Diam P spin Prev Start-End RTT SNR Notes Next mag [m] [h] Obs? Dates [s] /run App 2015 BN Y Feb P G I SR Feb P G I Camillo Y Feb G I Midas Mar P G I (2008 TZ3) Apr 26-May P G S I (2005 SE71) Apr P G I US Apr 28-May P N G (2001 KB67) May P G I (1999 KW4) Y May B P G I DP Jun P N G A (2010 NY65) Y Jun P G Y Ivar Y Jul I Agni Y Jul 23-Aug P G I FP Aug 24-Sep P G A (2004 DV24) Sep P G I VE Nov P G I (2003 SD220) Y Dec P N G X S AZ Jan P N G A XP Jan P G 2047 Table 1: We propose to observe our 19 highest-priority NEAs in a combined hours (including transmitter warm-up time; see Table 2 for detailed time requests). Absolute magnitudes H are taken from the JPL Small- Body Database. Rotation periods P spin are taken from the asteroid Lightcurve Database [Warner et al., 2009] when available. Previously observed objects ( Prev Obs? column) have radar-estimated spin periods consistent with P spin. Italicized periods are assumed very rapid at 2.1 h for H < 22, which gives a more conservative signalto-noise ratio (SNR), and 0.5 h for H > 22. Diameters are taken from previous radar observations if available; otherwise italicized diameters are estimates based on H assuming an optical albedo of 0.2. Start-End dates bracket the requested tracks. The closest approach is given by the round-trip time, RTT, for light to reach the target and return. Notes include binary asteroids (B), potentially hazardous asteroids (P), NHATS objects (N), Goldstone radar targets (G), possible bistatic X-band targets (X), VLBA speckle-tracking targets (S), possible IRTF near- and thermal-infrared targets (I), Yarkovsky-drift candidates (Y) from Greenberg et al. (2017b), and objects requiring optical astrometry prior to radar observations (A). Next App indicates the next comparable close approach to Earth of less than 1.2 times the RTT of the 2018/2019 apparition. Many are not re-observable at the same proximity for several decades meaning this is our best chance to characterize them, while observing 2008 TZ3, 1999 KW4, and 2010 NY65 will help us prepare for their close approaches in the next few years and possibly allow for future confirmations of Yarkovsky drift.

4 Observing Requests Table 2. We request 98 tracks and hours to observe 19 asteroids. Requested tracks are marked with a +; unmarked tracks are acceptable alternatives. The rise/set times do NOT include one hour of transmitter warm-up time prior to the source rising. Several days of observations spread over the observing window allow for complete rotational coverage (assuming typical rotation periods) and better constraints on the spin state. Calculations assume the physical parameters from Table 1 and a radar albedo of 0.1. When unknown, the sizes and spin rates used tend to give conservative estimates of the SNR. Nominal system parameters are assumed: transmitter power = 450 kw (single-klystron mode), sensitivity = 10 K/Jy, and system temperature = 24 K. Request: 5 tracks, hours (2015 BN509) [s] [h] [deg] /run /day 2018-Feb :16-06: Feb :12-06: Feb :11-06: Feb :25-05:31... Target is north of the Arecibo declination window Feb :26-19: Feb :55-19: Feb :39-19: Feb :28-19: Feb :21-19:03 Request: 3 tracks, hours (2014 SR339) [s] [h] [deg] /run /day 2018-Feb :28-01: Feb :47-02: Feb :44-02: Feb :00-02: Feb :39-01:59 4

5 Request: 5 tracks, hours 3752 Camillo (1985 PA) [s] [h] [deg] /run /day 2018-Feb :55-00: Feb :17-01: Feb :01-01: Feb :55-01: Feb :56-01: Feb :03-01: Feb :19-01:00 Request: 5 tracks, hours 1981 Midas (1973 EA) [s] [h] [deg] /run /day 2018-Mar :28-00: Mar :08-00: Mar :20-23: Mar :47-23: Mar :27-22: Mar :18-22: Mar :22-21:40 Request: 6 tracks, hours (2008 TZ3) [s] [h] [deg] /run /day 2018-Apr :07-05: Apr :03-05: Apr :00-05: Apr :57-05: Apr :55-05: Apr :53-05: Apr :52-05: Apr :51-05: Apr :52-04: May :57-04: May :07-04:28 5

6 Request: 4 tracks, hours (2005 SE71) [s] [h] [deg] /run /day 2018-Apr :38-05: Apr :58-05: Apr :31-05: Apr :14-04: Apr :07-04: Apr :12-03:47 Request: 4 tracks, hours (2013 US3) [s] [h] [deg] /run /day Apr :54-03: Apr :36-03: Apr :39-03: May :10-02:32 Request: 8 tracks, hours (2001 KB67) [s] [h] [deg] /run /day 2018-May :27-14: May :41-14: May :53-14: May :59-13: May :00-12: May :58-11: May :57-09: May :54-08: Jun :54-07: Jun :06-06: Jun :29-06: Jun :01-06: Jun :39-06: Jun :21-05:50 6

7 Request: 8 tracks, hours (1999 KW4) [s] [h] [deg] /run /day 2018-May :04-09: May :17-09: May :36-09: May :59-08: May :24-08: May :53-07: May :25-06: May :00-06: Jun :38-05: Jun :18-05: Jun :00-04: Jun :43-03:58 Request: 7 tracks, hours (2015 DP155) [s] [h] [deg] /run /day 2018-Jun :36-05: Jun :17-06: Jun :14-06: Jun :18-07: Jun :28-07: Jun :43-07: Jun :04-07: Jun :34-06:59 Request: 5 tracks, hours (2010 NY65) [s] [h] [deg] /run /day 2018-Jun :22-18: Jun :19-18: Jun :19-18: Jun :23-18: Jun :38-18: Jun :31-17:41... Target is north of the Arecibo declination window Jun :45-03: Jun :27-03: Jun :23-04: Jul :21-04: Jul :21-04:07 7

8 Request: 5 tracks, hours 1627 Ivar (1929 SH) [s] [h] [deg] /run /day 2018-Jun :41-02: Jun :38-02: Jun :35-02: Jun :32-02: Jun :30-01: Jun :27-01: Jun :25-01: Jun :22-01: Jul :20-01: Jul :18-01: Jul :16-01: Jul :14-01: Jul :13-01: Jul :11-01: Jul :10-01: Jul :09-01: Jul :08-01: Jul :08-01: Jul :08-00: Jul :08-00: Jul :08-00: Jul :09-00: Jul :10-00: Jul :11-00:29 8

9 Request: 4 tracks, hours Agni (2010 LE15) [s] [h] [deg] /run /day 2018-Jul :57-06: Jul :46-06: Jul :35-06: Jul :24-06: Jul :13-05: Jul :02-05: Jul :50-05: Jul :38-05: Jul :26-05: Jul :14-04: Jul :03-04: Jul :52-04: Jul :42-04: Jul :34-03: Aug :27-03: Aug :24-03: Aug :31-02:25 Request: 6 tracks, hours (2015 FP118) [s] [h] [deg] /run /day 2018-Aug :49-21: Aug :47-21: Aug :47-21: Aug :48-21: Aug :52-21: Aug :02-21: Aug :28-20:37... Target is north of the Arecibo declination window 2018-Sep :58-06: Sep :23-07: Sep :08-07: Sep :58-07: Sep :50-07:22 9

10 Request: 3 tracks, 9.50 hours (2004 DV24) [s] [h] [deg] /run /day Sep :18-23: Sep :32-00: Sep :41-23:52 Request: 6 tracks, hours (2002 VE68) [s] [h] [deg] /run /day Nov :55-02: Nov :30-02: Nov :17-02: Nov :11-01: Nov :09-01: Nov :10-01: Nov :15-01: Nov :23-00: Nov :42-00:19 Request: 5 tracks, hours (2003 SD220) [s] [h] [deg] /run /day Dec :27-15: Dec :24-15: Dec :37-16: Dec :04-16: Dec :53-16:31 Request: 4 tracks, hours (2016 AZ8) [s] [h] [deg] /run /day 2019-Jan :33-17: Jan :56-18: Jan :28-18: Jan :07-17: Jan :54-17: Jan :09-16:16 10

11 Request: 5 tracks, hours (2004 XP14) [s] [h] [deg] /run /day Jan :40-02: Jan :06-02: Jan :51-02: Jan :49-02: Jan :00-01:52 11