Arecibo Radar Observations of 15 High-Priority Near-Earth Asteroids in CY2019

Transcription

1 Arecibo Radar Observations of 15 High-Priority Near-Earth Asteroids in CY2019 Patrick A. Taylor (LPI, USRA), Anne K. Virkki, Flaviane C.F. Venditti, Sean E. Marshall, Luisa F. Zambrano-Marin (Arecibo Observatory, UCF), Edgard G. Rivera-Valentin, Sriram S. Bhiravarasu, Betzaida Aponte-Hernandez (LPI, USRA), Michael C. Nolan, Ellen S. Howell, Cassandra Lejoly, Jenna L. Crowell (U. Arizona), Jon D. Giorgini, Lance A. M. Benner, Marina Brozovic, Shantanu P. Naidu (JPL), Michael W. Busch (SETI), Jean-Luc Margot, Sanjana Prabhu Desai (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 program to observe NEAs for at least 600 hours per year. We propose radar imaging, detailed physical characterization, and orbit refinement of our 15 highest-priority NEAs during 2019 using 291 hours of telescope time. A companion proposal (Virkki et al.) with a more survey-oriented approach requests 285 hours and concentrates on basic characterization and precise astrometry for dozens more objects bringing the total proposed time request to 575 hours. Proposals to observe NEAs not included in this or the companion proposal historically account for up to 15% of time requests such that we expect to surpass 600 hours requested in In the last year, the Arecibo planetary radar program detected 50 asteroids despite significant downtime for recovery from Hurricane Maria and other equipment issues. Proposals for project R3037 have requested 428 hours of telescope time since September Of these, hours (51%) have been scheduled. The discrepancy is mainly due to recovery from Hurricane Maria, which required three months of downtime for the planetary radar system, plus reduced power output (operating in single-klystron mode), generator and equipment failures, competition for telescope time with atmospheric and radio astronomy observing campaigns, and overlap with daily maintenance activities. Pressworthy results since last September include observations of triple asteroid 3122 Florence, an asteroid with two moons and only the third such system known among the NEA population, 3200 Phaethon, an activated asteroid, likely the source of the Geminid meteors, and one of the flyby targets of the DESTINY + mission, and 2017 YE5, an equal-mass binary asteroid, one of only four known among the NEA population. The remaining four months of the calendar year, plus early January 2019, include six high-priority targets and 95.5 hours of telescope time requested. In 2019, we expect to operate with two klystrons, effectively doubling both the power output and the returned signal strength compared to observations in However, the diminished antenna gain since Hurricane Maria (by 30%) remains a significant reducing factor in our signal-strength predictions and, hence, time requests. 1

2 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 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 allow for bistatic X- or C-band experiments with resolutions of 3.75 or m. A subset of radar targets will likely be observed through a cooordinated 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) can be used to resolve the prograde/retrograde-rotation ambiguity of radar; however, none of the targets in Table 1 are known to be viable speckle targets unless their rotation periods are found to be longer than assumed. Table 2 lists specific track requests for each target. Student Participation Graduate student Luisa Zambrano-Marin (U. Granada), member of the local Arecibo team, is using radar scattering models to constrain the surface properties of asteroids and comets. Sean Marshall (Arecibo Observatory, UCF) and Jenna Crowell (U. Arizona) began working with this observing program as graduate students and continue to work with the program as postdoctoral researchers. Graduate student Sanjana Prebhu Desai (UCLA) will primarily be involved with highpriority targets, but will contribute to observations and analysis of medium-priority targets as part of her training. Graduate student Cassandra Lejoly (U. Arizona) observed Comet 45P (R3142) and is analyzing radar cross sections of Arecibo asteroid data. Undergraduate Andy Lopez-Oquendo (UPR Humacao) regularly participated in observations in 2017 and 2018 as did Research Experience for Undergraduates (REU) students Riley McGlasson (Macalester College) and Brynn Presler-Marshall (Agnes Scott College). Other undergraduate and graduate students are welcome to gain observing and research experience through this proposed work. 2

3 References Busch, M.W., et al. Determining asteroid spin states using radar speckles. Icarus 209, , Chesley, S.R., et al. Direct detections of the Yarkovsky effect: Status and outlook, Proc. IAU 318, Greenberg, A.H., et al. Yarkovsky drift detections for 159 near-earth asteroids, 2017, 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, , Mainzer, A.K., et al., NEOWISE diameters and albedos V1.0. EAR-A-COMPIL-5-NEOWISEDIAM- V1.0. NASA Planetary Data System, Masiero, J.R. et al., NEOWISE reactivation mission year three: Asteroid diameters and albedos, AJ 154, Nugent, C.R., et al., NEOWISE reactivation mission year two: Asteroid diameters and albedos, AJ 152, Warner, B.D., Harris, A.W., Pravec, P., The asteroid lightcurve database, Icarus 202, , Updated February

4 Object H Diam P spin Prev Next Start-End RTT SNR Notes mag [km] [h] Obs? App Dates [s] /day 433 Eros Y 2056 Jan 18-Feb G Y (2016 JP) Apr P G (2002 QF15) Y 2044 May P G (1999 KW4) Y 2036 May 29-Jun B P G (2010 NY65) Y 2020 Jun 17-Jul P G Y (2003 YE45) Y 2056 Jun 27-Jul P (2013 BZ45) Y 2065 Jul 31-Aug P N (2002 HK12) Y 2036 Aug P G (2001 WN5) Y 2023 Aug P G 2100 Ra-Shalom Y 2022 Sep Y 1998 FF Sep P G (1998 HL1) Oct P G 2015 JD Oct 31-Nov P G A (2006 SF6) Nov P N G 2011 YS Y 2074 Nov G Table 1: We propose to observe our 15 highest-priority NEAs in a combined 291 hours (including transmitter warm-up time; see Table 2 for detailed time requests). Absolute magnitudes H are taken from the JPL Small- Body Database. Diameters are taken from previous radar observations, spacecraft observations (Eros), or infrared observations by NEOWISE (Mainzer et al., 2016; Nugent et al., 2016; Masiero et al., 2017) when available; otherwise italicized diameters are estimates based on H assuming a brighter-than-average optical albedo of 0.2. 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. Assumptions of more rapid spins and brighter albedos (smaller sizes) lead to more conservative estimates for the signal-to-noise ratio (SNR). 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), VLBA speckle-tracking targets (S), possible IRTF near- and thermal-infrared targets (I), Yarkovsky-drift detections or candidates (Y) from Chesley et al. (2016) and Greenberg et al. (2017), 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 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 (1999 VF22), (2010 NY65), (2001 WN5), and 2100 Ra-Shalom will help us prepare for their close approaches in the next few years and possibly allow for confirmations of Yarkovsky drift (and inference of the asteroid mass and density).

5 Observing Requests Table 2. We request 82 tracks and 291 hours to observe 15 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 unless estimated from previous radar observations. When unknown, the sizes and spin rates used tend to give conservative estimates of the SNR. Nominal system parameters are assumed: transmitter power = 700 kw (dual-klystron mode), sensitivity 7 K/Jy (post-maria, also a function of declination), and system temperature = 24 K. 5

6 Request: 10 tracks, hours Notes: 7 consecutive tracks densely sample complete rotation; 3 tracks over 7 days coarsely sample complete rotation 433 Eros [s] [h] [deg] /run /day 2019-Jan :49-01: Jan :37-01: Jan :28-02: Jan :21-02: Jan :14-02: Jan :08-02: Jan :03-02: Jan :59-02: Jan :54-02: Jan :50-02: Jan :47-02: Jan :44-02: Jan :41-02: Jan :38-02: Jan :36-02: Feb :33-02: Feb :31-02: Feb :29-02: Feb :28-02: Feb :26-02: Feb :25-02: Feb :24-02: Feb :22-02: Feb :21-02: Feb :21-02: Feb :20-02: Feb :19-02: Feb :19-02: Feb :18-01: Feb :18-01: Feb :17-01: Feb :17-01: Feb :17-01: Feb :17-01: Feb :17-01: Feb :17-01: Feb :17-01: Feb :17-01: Feb :17-01: Feb :17-01:34 6

7 Request: 8 tracks, hours Note: 8 consecutive tracks sample complete rotation (2016 JP) [s] [h] [deg] /run /day 2019-Apr :27-14: Apr :53-13: Apr :18-13: Apr :43-13: Apr :08-12: Apr :33-12: Apr :59-11: Apr :27-11: Apr :55-10: Apr :26-09: Apr :58-09: Apr :33-09:01 Request: 6 tracks, hours (2002 QF15) [s] [h] [deg] /run /day 2019-May :03-22: May :08-23: May :18-23: May :31-23: May :45-00: May :59-00: May :13-00: May :26-01: May :38-01: May :49-01: May :58-01: May :07-01: May :14-02:00 7

8 Request: 7 tracks, 24.0 hours Note: Known binary, all dates with SNR/day > 100 requested (1999 KW4) [s] [h] [deg] /run /day May :36-00: May :29-00: May :28-00: Jun :29-01: Jun :30-01: Jun :31-01: Jun :31-01: Jun :31-01: Jun :30-01: Jun :29-01:14 Request: 6 tracks, 19.5 hours (2010 NY65) [s] [h] [deg] /run /day 2019-Jun :30-18: Jun :30-18: Jun :35-18: Jun :47-18: Jun :21-18:22... Target is north of the Arecibo declination window Jun :31-02: Jun :11-03: Jun :09-03: Jul :10-03: Jul :11-03:57 8

9 Request: 5 tracks, 18.0 hours Note: 5 tracks over 9 days nominally sample complete rotation (2003 YE45) [s] [h] [deg] /run /day 2019-Jun :55-13: Jun :33-13: Jun :14-13: Jun :57-13: Jun :42-13: Jul :28-13: Jul :15-12: Jul :03-12: Jul :52-12: Jul :42-12: Jul :33-12: Jul :24-12: Jul :17-11: Jul :10-11: Jul :04-11:23 Request: 4 tracks, 14.0 hours Note: 4 consecutive tracks sample complete rotation (2013 BZ45) [s] [h] [deg] /run /day 2019-Jul :43-20: Aug :40-20: Aug :38-20: Aug :38-20: Aug :41-20: Aug :49-19: Aug :11-19:18 9

10 Request: 7 tracks, 26.0 hours Note: 7 consecutive tracks sample complete rotation (2002 HK12) [s] [h] [deg] /run /day 2019-Aug :06-19: Aug :49-19: Aug :32-19: Aug :15-18: Aug :57-18: Aug :39-18: Aug :21-18: Aug :02-17: Aug :44-17: Aug :26-16: Aug :09-16: Aug :52-15: Aug :35-15: Aug :20-14:49 Request: 4 tracks, hours (2001 WN5) [s] [h] [deg] /run /day 2019-Aug :11-18: Aug :59-18: Aug :46-18: Aug :34-18: Aug :21-17: Aug :09-17: Aug :56-17: Aug :44-17:04 10

11 Request: 5 tracks, 17.0 hours Note: 5 consecutive tracks sample complete rotation 2100 Ra-Shalom (1978 RA) [s] [h] [deg] /run /day 2019-Sep :29-08: Sep :24-08: Sep :19-07: Sep :14-07: Sep :10-07: Sep :06-07: Sep :02-07: Sep :59-07: Sep :57-06: Sep :56-06: Sep :56-06:18 Request: 2 tracks, 7.0 hours (1998 FF14) [s] [h] [deg] /run /day Sep :49-17: Sep :16-19:04 Request: 4 tracks, hours (1998 HL1) [s] [h] [deg] /run /day Oct :59-05: Oct :46-05: Oct :52-05: Oct :10-05: Oct :46-05:01 Request: 4 tracks, hours Note: Requires optical astrometry prior to radar observations ( 10 arcmin) (2015 JD1) [s] [h] [deg] /run /day 2019-Oct :14-23: Nov :25-00: Nov :50-01: Nov :27-02: Nov :19-02:09 11

12 Request: 5 tracks, 17.5 hours (2006 SF6) [s] [h] [deg] /run /day 2019-Nov :26-06: Nov :16-06: Nov :07-05: Nov :58-05: Nov :49-05: Nov :40-05: Nov :34-04: Nov :31-04: Nov :38-03:52 Request: 5 tracks, 18.5 hours (2011 YS62) [s] [h] [deg] /run /day 2019-Nov :15-09: Nov :06-09: Nov :02-09: Nov :01-09: Nov :05-09: Nov :11-09: Nov :22-09:49 12