Photometric observations of Didymos in 2003–2017, and outlook for observations in 2019 and beyond Petr Pravec and Petr Scheirich Astronomical Institute AS CR, Ondřejov Observatory and Didymos observer team Didymos Observer Workshop 2018 Prague, Czech Republic 2018 June 19

(65803) Didymos – discovery observations The asteroid was discovered by Spacewatch from Kitt Peak on 1996 April 11. Designated 1996 GT. Its binary nature was revealed by both photometric and radar observations obtained around its close approach to Earth (min. distance 0.05 AU) during 2003 November 20-24. The photometric observations were taken by P. Pravec and P. Kušnirák from Ondřejov Observatory, by D. Pray from Carbuncle Hill Observatory, and by A. Grauer and S. Larson from Steward Observatory. The radar observations were taken by L.A.M. Benner, M.C. Nolan, J.D. Giorgini, R.F. Jurgens, S.J. Ostro, J.-L. Margot and C. Magri from Goldstone and Arecibo. (Pravec et al. 2003) Mutual events (eclipses+occultations) between the binary system components observed in 2003 (a sample of the data is shown) and a model of the system: (Pravec et al. 2006) (Scheirich and Pravec 2009)

Photometric data for (65803) Didymos Date (UT) Telescope(s) # Pts V RMS Res. 2003-11-20 to 24 Ond 0.65-m, CH 0.35-m, Mt. Lemmon 1.5-m 1111 12.9 0.008 2003-11-26 to 12-04 Ond 0.65-m, CH 0.35-m, PDO 0.50-m 778 13.2 2003-12-16 to 20 Ond 0.65-m, PDO 0.50-m 458 14.9 0.012 2015-04-13 to 14 DCT 4.3-m 87 20.6 0.024 2017-02-25.1 GTC 10.4-m 75 21.0 0.017 2017-02-25.5 MMT 6.5-m 137 0.032 2017-02-23 to 03-01 VLT 8.2-m 92 20.9 0.012-0.019 2017-03-31.1 WHT 4.2-m 100 20.3 0.025c 2017-04-01 to 02 44 20.4 0.021c 2017-04-18.2 66 0.034c 2017-04-27.1 NTT 3.5-m 108 21.2 0.023 2017-05-04.3 Gemini N 8.1-m 59 21.5 0.025 c affected by clouds/sub-optimal sky conditions The 2017 data were observed and reduced by Colin Snodgrass, Ellen Howell, Simon Green, Audrey Thirouin, Javier Licandro and Joanna Thomas-Osip.

2003 data (a) Full lightcurve Mutual events (from l.c. decomposition) Pravec et al. (2006) Primary rotational lc (time axis stretched)

2003 data (b) Pravec et al. (2006)

2003 data (c) Pravec et al. (2006)

2015 data Discovery Channel Telescope 4.3-m (Observed A. Thirouin, reduced P. Kušnirák and P. Pravec)

2017 data (a) Gran Telescopio Canarias 10.4-m, Multiple Mirror Telescope 6.5-m, Very Large Telescope 8.2-m (Observed J. Licandro , E. Howell and the VLT service, reduced C. Snodgrass and E. Howell)

2017 data (b) William Herschel Telescope 4.2-m, Very Large Telescope 8.2-m (Observed S. Green and the VLT service, reduced S. Green and C. Snodgrass)

2017 data (c) Discovery Channel Telescope 4.3-m, New Technology Telescope 3.5-m, Gemini N 8.1-m (Observed and reduced A. Thirouin, C. Snodgrass, and J. Thomas-Osip)

Primary’s rotational light curve

Primary’s light curve in 2017 (a) Observed: Amplitude 0.09-0.10 mag Multiple maxima (conspicuous signal up to the 7th harmonic) Changing on monthly scale Simulated (using the Naidu & Benner 2016 preliminary shape model): Amplitude 0.07-0.09 mag One prominent maximum Only small changes over the 2-month interval. Similar misfit also for the 2015 light curve.

Primary’s light curve in 2017 (b) Possible reasons for the sub-optimal fit: The preliminary shape model has errors. There is a non-uniform distribution of albedo or light scattering properties over the primary’s surface. We probably cannot resolve between the two possibilities with remote observations.

Original objectives on getting Didymoon’s parameters for the 2017 campaign

Objectives for the 2017 observations We planned: Confirmation of the Orbital Pole 2 Gathering data for a future determination of orbit change by BYORP Establishing a synchronous secondary rotation and aS/bS Constraining inclination of the mutual orbit We obtained: Yes. The Orbital Pole 2 (Retrograde) was confirmed (next talk by P. Scheirich). Yes. We expect to detect or constrain a drift of the Didymoon’s orbit by BYORP in 2021 (next talk by P. Scheirich). No. A high quality data with errors ≤ 0.01 mag was not obtained. No. The data quality and the duration of the campaign were not sufficient to detect changes in the event shapes due to a possible nodal precession of the Didymoon’s orbit.

On Objective 3: Establishing a synchronous secondary rotation and aS/bS The highest quality data (rms residuals 0.008 mag) obtained during 2003-11-20 to -12-04 suggest a low secondary amplitude of ~0.02 mag outside events. But the few features might be small systematic errors in the observations. We need to confirm it, deriving PS and estimating aS/bS, with high quality observations (errors < 0.008 mag) in next apparition(s). Two full nights with a 8-10m telescope around the Didymos’ opposition in March 2019 (V ~ 19.8) or in February 2021 (V ~ 18.9) will be needed.

On Objective 4: Constraining inclination of the mutual orbit Nodal precession of the secondary’s orbit: A half of the precession cycle (180°) is completed in about 100 days. If the inclination of Didymoon’s orbit is greater than a couple degrees, we could detect effects (e.g., evolving event shapes) due to the nodal precession. Observations taken over a 3-4 month interval (with one good event coverage per month) could reveal it. A task for the 2020-2021 apparition probably.

Measurements of new Porb and Pp after the DART impact

Didymos observations after the DART impact (1) Didymos will be V = 15.1–21.7 and at solar elongations > 90° from 2022-10-15 (presumably the date by which “dust settles and fog clears” after the DART impact) to 2023-04-30. Assume that we get 0.01-mag accuracy photometry. Measurement of a new primary spin period Pravec et al. (2006) measured a synodic period Pp = 2.2593 ± 0.0001 h. Corrected to the sidereal period Pp = 2.2601 ± 0.0001 h (1-σ error) assuming the primary pole is the same as orbit pole. It was derived from just 14 days of observations from 2003-11-20 to -12-04 (rms photometric error 0.008 mag). Assuming the primary pole does not change by more than a couple degrees, we expect to obtain the same accuracy of 0.0001 h for a new Pp by the end of October 2022. We may potentially obtain an 1-σ error as low as 0.00001 h by the end of April 2023, but it will depend on how accurately we will known the primary’s pole and whether a primary lightcurve shape modeling will be possible (not sure, as there may be albedo variegation over the primary’s surface, which we may not be able to disentangle from primary’s shape).

Didymos observations after the DART impact (2) Didymos will be V = 15.1–21.7 and at solar elongations > 90° from 2022-10-15 (presumably the date by which “dust settles and fog clears” after the DART impact) to 2023-04-30. Assume that we get 0.01-mag accuracy photometry. Measurement of a new orbit period From the data taken during 2003-11-20 to -12-20 with rms errors 0.008-0.012 mag, Scheirich and Pravec (2009) obtained Porb with a 3-σ uncertainty of +0.004/-0.006 h = +14/-22 s. Assuming that the orbit pole does not change by more than a couple degrees and the eccentricity remains < 0.03, we will determine a new Porb with the same or better accuracy by mid-November 2022. A simulation of photometric observations taken after 2022-10-15, assuming we obtain 4 event observations per month with rms errors 0.010 mag, gives following (3-σ) estimated uncertainties for Porb: By 2022-11-30: ± 10 s By 2023-01-30: ± 4 s By 2023-03-30: ± 2.5 s

Thank you