Covertly Active Comet (139359) 2001 ME1

TL;DR

This paper reports the covert activity of the near-Earth minor planet (139359) 2001 ME$_1$, revealed by a dramatic forward-scattering dust brightening as it passed near perihelion in 2018 with SOHO/LASCO and STEREO-A/HI1. By combining forward-scattering modeling via the Fournier--Forand phase function with nucleus-subtracted photometry across multiple viewpoints, the authors derive a dust-size distribution with $μ_d^*\approx3.1$ and find dust contributes about $d_{90}=(20\pm11)\%$ of the brightness near peak geometry, while gas emission dominates the rest; the inferred water production rate is $ ext{Q}_{ ext{H}_2 ext{O}} oughly3 imes10^{26}$ molecules s$^{-1}$, corresponding to an active fraction of only $ ext{0.02--0.03}\%$ for a nucleus of a few kilometers. Dynamical simulations with 100 orbital clones indicate a long-lived near-Earth orbit for $ exists ext{(} ext{10 kyr)}$, implying sustained solar heating and substantial volatile depletion over time. Complementary meteoroid modeling reproduces a 2006 CMOR radar outburst and forecasts past and near-future meteor activity tied to dust ejected in the last several centuries, consistent with observed meteor signatures. Overall, the object appears to be an aging Jupiter-family comet whose near-surface ice is largely exhausted except for residual activity near perihelion, highlighting a population of dormant or weakly active NEOs that still exhibit episodic gas/dust emission and measurable meteor debris.

Abstract

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.

Figures (8)

• Figure 1: Trajectory of (139359) 2001 ME$_1$ during its 2018 apparition across the STEREO-A HI1 (HI1A; left) and SOHO LASCO C3/C2 (center/right) fields of view. The size and position of the Sun behind the LASCO occulters are indicated by the dotted circles ($\odot$). The Sun is off the left edge of the HI1 frame.
• Figure 2: Stacked frames of (139359) 2001 ME$_1$ showing its tail during its 2018 apparition from STEREO-A HI1, SOHO LASCO C3 (clear-filtered) and C2 (orange-filtered), and Palomar ZTF ($r'$-filtered). Prior to stacking, the constituent HI1 and ZTF frames were rotated to place the sunward direction ($\odot$) upward, while those of the LASCO stacks were rotated to place the direction of the comet's heliocentric velocity ($+v$) upward. In the ZTF images, the comet appears slightly diffuse with a hint of an antisunward tail on December 5 but becomes indistinguishable from a point source by December 9 as the comet moved away from the Sun ($r=0.63$ au to 0.70 au) and the nucleus brightened relative to the dust with the decreasing phase angle ($\alpha=98^\circ$ to $89^\circ$) over this period.
• Figure 3: Photometry of (139359) 2001 ME$_1$ measured from SOHO LASCO C2/C3 and STEREO-A HI1 imagery in 2014 (left) and 2018 (right) with $3'$ radii apertures, normalized to $\varDelta=1$ au. The times of perihelion are indicated by $T_p$. During its 2018 apparition when the comet was simultaneously observed by both spacecraft, the comet appeared much brighter from SOHO than from STEREO due to the much higher $\alpha$ of the comet from SOHO, demonstrating the high efficiency of forward scattering by cometary dust.
• Figure 4: Forward scattering brightness surge of 2P/Encke seen by SOHO LASCO C2/C3 with simultaneous baseline photometry from STEREO-A HI1 during the comet's 2017 apparition, with $T_p$ indicating the time of perihelion.
• Figure 5: Comparison of comets (139359) 2001 ME$_1$ (top) with 2P/Encke (bottom) as seen by SOHO LASCO C2 under similar forward scattering geometry near $\alpha=175^\circ$. Syndyne curves (solid) corresponding to dust of $\beta=1$, 0.3, 0.1, and 0.01, and synchrone curves (dotted) corresponding to ages $\Delta t=1$, 2, 4, and 8 days are overlaid. Note the larger coma of 2P/Encke, which reflects the higher dust ejection velocity supported by its stronger outgassing.