Initial Visible and Mid-IR Characterization of P/2019 LD₂ (ATLAS), an Active Transitioning Centaur Among the Trojans, with Hubble, Spitzer, ZTF, Keck, APO and GROWTH Imaging and Spectroscopy

We present visible and mid-infrared imagery and photometry of Jovian co-orbital comet P/2019 LD₂ (ATLAS) taken with Hubble Space Telescope/WFC3 on 2020 April 1, Spitzer Space Telescope/IRAC on 2020 January 25, Zwicky Transient Facility between 2019 April 9 and 2019 Nov 8 and the GROWTH telescope network from 2020 May to July, as well as visible spectroscopy from Keck/LRIS on 2020 August 19. Our observations indicate that LD₂ has a nucleus with radius 0.2-1.8 km assuming a 0.08 albedo and that the coma is dominated by ∼100 μ m-scale dust ejected at ∼1 m/s speeds with a ∼1" jet pointing in the SW direction. LD₂ experienced a total dust mass loss of ∼10⁸ kg and dust mass loss rate of ∼6 kg/s with Afρ/cross-section varying between ∼85 cm/125 km² and ∼200 cm/310 km² between 2019 April 9 and 2019 Nov 8. If the Afρ/cross-section increase remained constant, it implies that LD₂ has remained active since ∼2018 November when it came within 4.8 au of the Sun, a typical distance for comets to begin sublimation of H₂O. From our 4.5 μm Spitzer observations, we set a limit on CO/CO₂ gas production of ∼10²⁷/∼10²⁶ mol/s. Multiple bandpass photometry of LD₂ taken by the GROWTH network measured in a 10,000 km aperture provide color measurements of g-r = 0.59±0.03, r-i = 0.18±0.05, and i-z = 0.01±0.07, colors typical of comets. We set a spectroscopic upper limit to the production of H₂O gas of ∼80 kg/s. Improving the orbital solution for LD₂ with our observations, we determine that the long-term orbit of LD₂ is that of a typical Jupiter Family Comet having close encounters with Jupiter coming within ∼0.5 Hill radius in the last ∼3 y to within 0.8 Hill radius in ∼9 y and has a 95% chance of being ejected from the Solar System in < 10 Myr.