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Gaia and the Asteroid Population: a Revolution on Earth, Coming from Space

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P. Tanga GREAT - EWASS Athens 2016 March 16, 2017 GAIA AND THE ASTEROID POPULATION: A REVOLUTION ON EARTH, COMING FROM SPACE P. Tanga Laboratoire Lagrange, Observatoire de la Côte d’Azur, France P. Tanga – Gaia and the Solar System 2 Gaia ID card • It is a ESA cornerstone scientific mission: • 5 years of observations – ESA: building, operation (nominal mission started in July 2014) – Scientific community: data reduction • Positioned around L2 – No instrument PI, no proprietary period • automated selection of sources , on • Astrometry: input catalog – 109 stars, V<20 • Self-monitored, onboard metrology – 25 µas at V~15 • Heritage of Hipparcos – uniform sky coverage (70-100 obs./ source) • Physical observations: • Spectro-photometry • Radial velocities, hi-res spectra (V<16.5) 3 webinar P. Tanga – Gaia and the Solar System 3 Gaia in the history of astrometry 5 webinar P. Tanga – Gaia and the Solar System 4 ESA/Gaia/DPAC/Airbus DS webinar Optics Two SiC primary mirrors 1.45 × 0.50 m2, F = 35 m Rotation axis (6 h) 106°.5 Combined focal plane (CCDs) M4: beam SiC toroidal combiner Figure courtesy EADS-Astrium structure webinar P. Tanga – Gaia and the Solar System 6 ESA/Gaia/DPAC/Airbus DS webinar Sunshield deployment test – Oct. 2011 P. Tanga – Gaia and the Solar System 8 December 19, 2013 webinar P. Tanga – Gaia and the Solar System 9 The scanning law Rotation axis movement Scan path in 4 days Scan path 4 days 4 rotations/day Spin axis Spin axis trajectory 4 days 45° Sun trajectory, 4 months Sun Spin axis trajectory, 4 months webinar P. Tanga – Gaia and the Solar System 10 Focal plane 1 pixel 60 x 180 mas 106 CCDs (4.5 x 2 kpix) = 1 Gpixel SM1-2 AF1 - 9 BP RP RVS 420 mm 0.69° WFS WFS BAM BAM sec FOV1 0s 10.6 15.5 30.1 49.5 56.3 64.1 sec FOV2 courtesy F. Mignard, OCA 0s 5.8 10.7 25.3 44.7 51.5 59.3 webinar P. Tanga – Gaia and the Solar System 11 The Astrometric Global Iterative Solution (AGIS) • Simultaneous all-sky solution of positions, parallaxes, proper motions, physical parameters (e.g. color) and instrument calibration parameters • Link to ICRS by VLBI sources directly observed by Gaia —> All aspects of the data reduction are deeply connected webinar P. Tanga – Gaia and the Solar System 12 Gaia as Micro-meteorite detector Corrective torques applied to recover spacecraft attitude after a meteoroid strike (Perseid activity peak). webinar P. Tanga – Gaia and the Solar System 13 The strengths of Gaia • Astrometry: • Accurately measure relative positions at large angles —> no zonal error • Measure time instead of positions (and control very accurately the transformation) • Average the signal over several 10.000s pixels (good for photometry too) • Spectro-photometry • provide colors and low-resolution spectra (—> classification) for all sources • Radial velocities • Add the 3rd dimension of motions in the Galaxy + automated source detection on board webinar P. Tanga – Gaia and the Solar System 14 Gaia does not produce images ~0.38 as • (Except for testing in a special mode) • Operates in TDI mode (line period 1 ms) • Small windows read from the CCD around the sources • 2D windows are binned across scan —> 1 D data sample ~1 as (typ. 6 pixels) • on board lossless compression • Fundamental measurement: crossing time of a fiducial line on the CCD by the brightness peak of a source webinar P. Tanga – Gaia and the Solar System 15 Gaia produces unusual epoch astrometry • In the along scan direction the typical accuracy is a small fraction of a pixel (~1 mas) • Across scan it is of the order of a pixel (~100 mas) ~100 mas <1 mas Dec RA • When rotated to (RA, dec) this results in highly correlated uncertainties • This is not (very) relevant for stars • It is fundamental for asteroids! Pay attention to the covariance matrix webinar P. Tanga – Gaia and the Solar System 16 The Data Processing and Analysis Consortium - since 2007 webinar P. Tanga – Gaia and the Solar System 17 Gaia data - when? GDR1: star positions + Tycho-Gaia Astrometric Solution 40 mas GDR2: star positions + parallaxes + proper motions asteroid astrometry 10 mas Final release 2 mas sept. 2016 Apr 2018 2022 webinar P. Tanga – Gaia and the Solar System 18 GDR1: brightness accuracy This plot shows the distribution of the estimated uncertainties on the weighted mean G-band photometry as a function of magnitude. The colours indicate the density of data points, from low (red) to high (blue) on a logarithmic scale. Credits: ESA/Gaia/DPAC webinar P. Tanga – Gaia and the Solar System 19 GDR1: parallax accuracy (TGAS) webinar P. Tanga – Gaia and the Solar System 20 GDR1: position errors webinar P. Tanga – Gaia and the Solar System 21 Gaia Archive https://gea.esac.esa.int/archive/ webinar P. Tanga – Gaia and the Solar System 22 The Solar System and Gaia webinar P. Tanga – Gaia and the Solar System 23 What Gaia observes • What Gaia observes = all small objects at V < 20.5 –350.000 asteroids( >700.000 known today) –comets, TNOs –small planetary satellites • Why we are interested –small bodies record the history of the Solar System –very poorly known properties: • a few 1000s spectra, 10s masses, 100k sizes, ~400 shapes webinar P. Tanga – Gaia and the Solar System 24 Where Gaia observes unobservable ~70 observations per object • Discovery space: • Low elongations (~45-60°) • Inner Earth Objects (~unknown) • Other NEOs unobservable • Some MBAs webinar P. Tanga – Gaia and the Solar System 25 What Gaia observes: how big Kuiper Belt NEA Jupiter Trojans MBO webinar P. Tanga – Gaia and the Solar System 26 Gaia single-epoch astrometric accuracyJ. Desmars et al.: Statistical and numerical study of asteroid orbital uncertainty Table 4. Statistics of di↵erent measurements provided by the MPC database for numbered and unnumbered asteroids. final attitude and calibration, single FoV (9 positions) transit, point-like source Code Type Number Percentage Timespan C CCD1 mas 88 546 921 94.12% 1986–2012 S/s Space observation 4 006 572 4.26% 1994–2011 HST 3544 0.00% 1994–2010 Spitzer 114 0.00% 2004–2004 WISE 4 002 914 4.25% 2010–2011 A Observations from B1950.0 converted to J2000.0 647 690 0.69% 1802–1999 c Corrected without republication CCD observation 462 065 0.49% 1991–2007 P Photographic 352 449 0.37% 1898–2012 T Meridian or transit circle0.1 mas 26 968 0.03% 1984–2005 X/x Discovery observation 16 741 0.02% 1845–2010 M Micrometer 12 081 0.01% 1845–1954 HHipparcos geocentric observation 5494 0.01% 1989–1993 R Radar observation 2901 0.00% 1968–2012 E Occultations derived observation 1737 0.00% 1961–2012 V Roving observer observation 388 0.00% 2000–2012 n Mini-normal place derived from averaging observations from video frames 105 0.00% 2009–2012 e Encoder 16 0.00% 1993–1995 Table 5. Accuracy of measurements for numbered asteroids from the AstDyS2 database. Type Name Number of Percentage of Accuracy measurement accepted measurement C CCD 79 569 190 99.49% 0.388 arcsec S Wise 1 526 466 99.86% 0.583 arcsec random errors S Hubble Space Telescope 867 96.54% 0.585 arcsec S Spitzer 48 33.33% 1.673 arcsec + systematic A B1950 to J2000 632 428 82.17% 1.170 arcsec cObservations Corrected CCD observationin the AstDys 423 792 service (Univ. 99.70% Pisa) 0.507 arcsec P Photographic 346 947 93.38% 1.084 arcsec T Meridian/transit circle 26 968 100.00% 0.250 arcsec M Micrometer 12 081 94.56% 1.742 arcsec X Discovery observation 9520webinar 0.81% 0.898 arcsec HHipparcos observation 5494 100.00% 0.148 arcsec E Occultations 1736 100.00% 0.085 arcsec R Ranging 612 96.41% 3.325 km 1 R Doppler 432 99.07% 6.660 km s− V Roving observer 372 49.73% 0.822 arcsec e Encoder 16 100.00% 0.558 arcsec n video frame 12 100.00% 0.319 arcsec the residual (O–C) of each measurement. All the observations asteroid detection and the mission has detected 157 000 aster- of all numbered asteroids9 have been compiled and the ac- oids, including more than 500 NEO (Mainzer et al. 2011). curacy of each measurement has been estimated by comput- ing a weighted10 root mean square of all the residuals of the measurement. 4.2. Impact of astrometric measurements on orbit uncertainty – the Apophis example Table 5 provides the number, the percentage of accepted measurement, and the estimated accuracy for each kind of ob- In order to quantify the impact of a type of measurement on the servation. After radar measurements, observations derived from orbital uncertainty of an asteroid, we specifically dealt with the occultations and Hipparcos geocentric observations appear to asteroid Apophis. This particular object belongs to the PHA fam- be the most accurate (about 0.08–0.15 arcsec). Recent mea- ily and is known to be the most threatening object of the last surements such as CCD observations or meridian circle ob- decade since it is the only asteroid that reached level 4 of the servations are accurate to between 0.2–0.4 arcsec. Older mea- Torino scale for potential impact with the Earth in April 2029 surements, such as photographic or micrometric measures, are (Sansaturio & Arratia 2008). Since new observations (optical less accurate (1.0–1.8 arcsec). Surprisingly, observations from and radar) ruled out every possibility of impact in 2029, this spacecraft (HST, Spitzer or WISE) are not any more accurate threat turned out to be a close encounter within 5.64 R of the ⊕ than ground-based data.
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