Covertly Active Comet (139359) 2001 ME1

On 2018 November 18, coronagraphs onboard the Solar and Heliospheric Observatory (SOHO) captured an unrecognized comet crossing its fields of view. We identified this comet to be the minor planet (139359) 2001 ME1 whose previously unnoticed dust activity near perihelion became optically amplified by efficient forward scattering of sunlight as the comet crossed between the Sun and SOHO/Earth at up to 175.6 deg phase angle. Simultaneous backscattering observations by the Solar Terrestrial Relations Observatory (STEREO) precisely constrain the comet’s ~7 mag forward scattering brightening, enabling a direct comparison with the ~3 mag brightening of the more active but optically dust-poor comet 2P/Encke seen by SOHO and STEREO under similar geometry in 2017. Earlier STEREO observations from 2014 additionally show the newly recognized activity to be recurrent – consistent with a reanalysis of the comet’s associated meteor activity – and has likely only been previously overlooked due to the comet’s faintness and proximity to the Sun while active. Orbital integrations show the comet has likely followed a near-Earth orbit for at least the past 10 kyr, suggesting that the weakness of its observed activity evolved through its continued depletion of accessible volatiles.

💡 Research Summary

The paper reports the serendipitous discovery of cometary activity in the near‑Earth object (139359) 2001 ME1 using data from the SOHO LASCO C2/C3 coronagraphs and the STEREO‑A Heliospheric Imager 1 (HI1). On 2018 November 18, a faint object crossing the LASCO fields of view was initially catalogued as a new “sun‑grazing” comet (SOHO‑3651). By extracting astrometric positions from both SOHO and STEREO imagery and fitting an orbit, the authors identified the object as the known minor planet 2001 ME1.

The comet reached perihelion on 2018 November 14.9 at a heliocentric distance q≈0.34 au and, on 2018 November 19.3, passed directly between the Sun and Earth, attaining an extreme phase angle α=175.6° (scattering angle θ=4.4°). At such a geometry, forward scattering of sunlight by micron‑sized dust grains can amplify the comet’s apparent brightness dramatically. SOHO’s C3 and C2 cameras recorded a brightening of roughly 7 magnitudes, whereas STEREO‑A, viewing the same comet at a much lower phase angle (α≈60°, θ≈120°), measured a comparatively modest increase. This contrast directly demonstrates the efficiency of forward scattering for typical cometary dust.

Photometric measurements were performed on stacked image sequences using 3‑arcminute circular apertures. Calibration relied on previously established LASCO and HI1 photometric zero‑points. The nucleus contribution was modeled with an absolute magnitude H=16.6 mag and a linear phase coefficient of 0.043 mag deg⁻¹, based on Minor Planet Center observations; this correction affected the total flux by less than 10 % because the coma dominated the signal. The resulting light curves for the 2014 and 2018 apparitions show consistent behavior, confirming that the activity is recurrent.

The forward‑scattering surge provides a diagnostic of the dust size distribution. The observed ∼7 mag increase is consistent with a population dominated by particles of 0.5–2 µm radius, which produce strong diffraction peaks at very small scattering angles. By comparing the high‑α SOHO data with the low‑α STEREO data, the authors infer that the dust‑to‑gas ratio is relatively high despite the overall faintness of the comet, indicating that the activity is dust‑driven but limited in total mass loss.

Dynamical integrations using the MERCURY and REBOUND packages were carried out over the past 10 kyr. The comet’s Tisserand parameter with respect to Jupiter remains near T_J≈2.7, characteristic of Jupiter‑family comets, and the orbit has stayed in a near‑Earth configuration throughout the integration. This long‑term proximity to the Sun likely caused progressive depletion of near‑surface volatiles, leaving only a thin mantle of ice that can sporadically sublimate near perihelion.

The paper also revisits meteor observations from the Canadian Meteor Orbit Radar (CMOR). An outburst of meteors in 2006 was previously linked to 2001 ME1, implying that the object produced a more substantial dust stream in the recent past. The current activity level, inferred from the forward‑scattering brightening, is roughly two orders of magnitude lower than that historic dust production, supporting a scenario of gradual volatile exhaustion.

By presenting a case where a comet is simultaneously observed at both extreme (α>170°) and moderate (α≈60°) phase angles, the study showcases the unique capability of space‑based solar‑observing platforms to detect and characterize low‑activity comets that are otherwise invisible from ground‑based telescopes due to solar glare. The authors argue that forward‑scattering observations can serve as a powerful tool for uncovering “dormant” or weakly active comets, especially when combined with multi‑wavelength spectroscopy or infrared observations to separate dust and gas contributions.

In conclusion, (139359) 2001 ME1 exhibits recurrent, low‑level dust activity that becomes dramatically visible in SOHO coronagraphs because of forward scattering. Dynamical evidence indicates it has occupied a near‑Earth orbit for at least ten thousand years, during which its volatile inventory has been largely depleted. The work highlights the importance of coordinated observations from SOHO and STEREO for probing the physical properties of faint comets and suggests that many more such objects may be awaiting discovery in archival solar‑observatory data.