Determination of the orbit of Didymoon from past
Determination of the orbit of Didymoon from past and future photometric observations Petr Scheirich and Petr Pravec Astronomical Institute AS CR, Ondřejov, Czech Republic Didymos Observer Workshop Prague, 2018 June 19
Long period component of the lightcurve 2003 2015 2003 2017 2003 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Past apparitions
Didymos orbital model We modeled the observed mutual events of Didymos using the method of Scheirich and Pravec (2009). Following function is minimized: Shapes of the components are approximated with ellipsoids if only lightcurve data are available, or a shape model (e. g. , from radar observations) is used. Main assumptions: • Same albedo of both components • Uniform surface light scattering properties • i = 0 (i. e. , there is not present a nodal precession of the mutual orbit)
Didymos orbital model Main parameters Diameter ratio DS/DP = 0. 21 ± 0. 01 (Scheirich and Pravec 2009) Eccentricity e ≤ 0. 03 (Scheirich and Pravec 2009) Orbital pole Lorb, Borb = 270°, -87° (Scheirich and Pravec 2009, updated using 2015 & 2017 data ) Orbital period (Porb = 11. 92164 ± 0. 00003 h) Assuming zero BYORP! Allowed (conservative 3 -σ uncertainty) area plotted. Black outline: 2003+2015 data Red outline: 2003+2015+2017 data
BYORP effect Ćuk & Burns (2005), Mc. Mahon & Scheeres (2010 a, b) Binary YORP – shrinking or expanding of the orbit of a synchronous satellite due to the unisotropic reradiation of thermal energy. The change of semimajor axis change in mean motion quadratic rate of change of mean anomaly.
BYORP solutions from the 2003 -2017 data DMd (°/yr 2) allowed solutions 0. 1 (consistent with 0) -2. 3 3. 2 0. 9 Prediction (scaled from the 1999 KW 4 secondary): 2. 8°/yr 2 May be any value between -6 and +6°/yr 2 perhaps.
BYORP solutions from the 2003 -2017 data Porb Five allowed solutions in Porb vs DMd plot. DMd Each period represents different number of cycles between 2003 and 2015. 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Past apparitions
BYORP solutions from the 2003 -2017 data 2003 2015 2003 2017 2003 Model lightcurves for the five BYORP solutions. 2003 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Past apparitions
Questions for the next two oppositions (2019, 2021) • Will we be able to determine or constrain the orbital drift by BYORP? • What time distribution and quality of the data will be needed to predict the position of Didymoon with an uncertainty in mean anomaly < 20° at the time of the DART impact in October 2022? • How could additional observations with the HST in Aug-Sep 2020 improve the orbit solution? 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Past and future apparitions
Simulated future apparitions Properties of simulated data Cadence: 2003 Events covered: Mimics the 2017 apparitions 1 – 2 per lunation RMS residuals: 0. 01 or 0. 02 mag Time span: 2003 1 or 3 lunations in each apparition Assumed DMd: The five solutions derived from the 20032017 data. 2003 2015 2017 2019 2021 Example of simulated data with RMS residuals of 0. 02 mag. 2021 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Past and future apparitions
Simulated future apparitions Properties of simulated data Cadence: 2003 Events covered: Mimics the 2017 apparitions 1 – 2 per lunation RMS residuals: 0. 01 or 0. 02 mag Time span: 2003 1 or 3 lunations in each apparition Assumed DMd: The five solutions derived from the 20032017 data. 2003 2015 2017 2019 2021 Example of simulated data with RMS residuals of 0. 01 mag. 2021 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Past and future apparitions
RMS 0. 02 mag. RMS 0. 01 mag. 2019 2021 BYORP resolved Oct 2022 HST (RMS 0. 01 from 19&21 M 3σ unc. mag) will help 3 lunations YES +/-12° --- 3 lunations YES(!) +/-12° 3 lunations --- NO 1 lunation YES(!$) --- 1 lunation NO 1 lunation 3 lunations YES(!) +/-12° 3 lunations YES +/-9° --- 3 lunations YES(!) +/-9° 3 lunations --- NO 1 lunation YES(!) --- 1 lunation NO 1 lunation 3 lunations YES +/-12° +/-9° NO NO NO YES(#) YES(#) YES(#) NO YES(#) YES(#) +/-9° (!) Other solutions excluded based on 2015 data (so, not a robust solution) ($) Second BYORP solution excluded based on reliable resolving of events in 2019 (#) Depending on actual BYORP value 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Past and future apparitions
RMS 0. 02 mag. RMS 0. 01 mag. 2019 2021 BYORP resolved Oct 2022 HST (RMS 0. 01 from 19&21 M 3σ unc. mag) will help 3 lunations YES +/-12° --- 3 lunations YES(!) +/-12° 3 lunations --- NO 1 lunation YES(!$) --- 1 lunation NO 1 lunation 3 lunations YES(!) +/-12° 3 lunations YES +/-9° --- 3 lunations YES(!) +/-9° 3 lunations --- NO 1 lunation YES(!) --- 1 lunation NO 1 lunation 3 lunations YES +/-12° +/-9° NO NO NO YES(#) YES(#) YES(#) NO YES(#) YES(#) +/-9° (!) Other solutions excluded based on 2015 data (so, not a robust solution) ($) Second BYORP solution excluded based on reliable resolving of events in 2019 (#) Depending on actual BYORP value 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 Past and future apparitions
RMS 0. 02 RMS 0. 01 2019 2021 BYORP resolved Oct 2022 HST (RMS 0. 01 from 19&21 M 3σ unc. mag) will help 3 lunations YES 3 lunations --- NO 1 lunation 3 lunations YES +/-12° NO YES +/-9° Conclusions • Will we be able to determine or constrain the orbital drift by BYORP? Yes. • What time distribution and quality of the data will be needed to predict the position of Didymoon with an uncertainty in mean anomaly < 20° at the time of the DART impact in October 2022? We will need to observe at least 1 event in 1 lunation in 2019 and at least 1 event in each of the 3 lunations in 2021, with photometric rms errors 0. 01 mag. • How could additional observations with the HST in Aug-Sep 2020 improve the orbit solution? Not significantly (assuming the campaign in 2021 is successful).
Acknowledgements • Access to computing and storage facilities owned by parties and projects contributing to the National Grid Infrastructure Meta. Centrum provided under the programme "Projects of Large Research, Development, and Innovations Infrastructures" (CESNET LM 2015042), is greatly appreciated.
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