A Deep Search for Emission from “Rock Comet” (3200) Phaethon at 1 au

Abstract

Abstract We present a deep imaging and spectroscopic search for emission from (3200) Phaethon, a large near-Earth asteroid that appears to be the parent of the strong Geminid meteoroid stream, using the 4.3 m Lowell Discovery Telescope. Observations were conducted on 2017 December 14–18 when Phaethon passed only 0.07 au from the Earth. We determine the 3σ upper level of dust and CN production rates to be 0.007–0.2 kg s−1 and 2.3 × 1022 molecules s−1 through narrowband imaging. A search in broadband images taken through the SDSS r′ filter shows no 100 m class fragments in Phaethon’s vicinity. A deeper but star-contaminated search also shows no sign of fragments down to 15 m. Optical spectroscopy of Phaethon and comet C/2017 O1 (ASASSN) as a comparison confirms the absence of cometary emission lines from Phaethon and yields 3σ upper levels of CN, C2, and C3 of ∼1024–1025 molecules s−1, 2 orders of magnitude higher than the CN constraint placed by narrowband imaging, due to the much narrower on-sky aperture of the spectrographic slit. We show that narrowband imaging could provide an efficient way to look for weak gas emission from near-extinct bodies near the Earth, though these observations require careful interpretation. Assuming Phaethon’s behavior is unchanged, our analysis shows that the DESTINY+ mission, currently planning to explore Phaethon in 2026, may not be able to directly detect a gas coma.