The Lingering Death of Periodic Near-Sun Comet 323P/SOHO

Abstract

We observed near-Sun comet 323P/SOHO for the first time using ground and space telescopes. In late 2020 December, the object was recovered at Subaru showing no cometary features on its way to perihelion. However, in our postperihelion observations, it developed a long narrow tail mimicking a disintegrated comet. The ejecta, composed of at least millimeter-sized dust with power-law size distribution index 3.2 ± 0.2, was impulsively produced shortly after the perihelion passage, during which ≳0.1%–10% of the nucleus mass was shed due to excessive thermal stress and rotational disruption. Two fragments of ∼20 m in radius (assuming a geometric albedo of 0.15) were seen in Hubble Space Telescope observations from early 2021 March. The nucleus, with an effective radius of 86 ± 3 m (the same albedo assumed) and an aspect ratio of ∼0.7, has a rotation period of 0.522 hr, which is the shortest for known comets in the solar system and implies cohesive strength ≳10–100 Pa in the interior. The color of the object was freakish and changed temporally in a never-before-seen manner. Using our astrometry, we found a strong nongravitational effect following a heliocentric dependency of r H − 8.5 in the transverse motion of the object. Our N-body integration reveals that 323P has a likelihood of 99.7% to collide with the Sun in the next two millennia driven by the ν 6 secular resonance.

Perihelion Activity of (3200) Phaethon Is Not Dusty

“…What we found is that, based on the COR2 observations at small phase angles, the upper limit to the effective cross-section of the debris would be ∼ 1.7 ± 0.1 times larger than the values for ∼0.7 µm sized dust listed in Table 1, i.e., Ξ d 10 8 m 2 . In comparison, observations of 323P were able to reveal the debris trail of the object having an effective total cross-section of 10 5 m 2 within two months after the perihelion passage in 2021 (Hui et al 2022). We are unaware of any published literature that reported attempts to search for debris of Phaethon in a similar time window after perihelion.…”

“…Substituting with ād ≈ 0.7 µm and ρ d = 2.6 g cm −3 (Borovička et al 2010), we obtain the 3σ upper limit to the mass of ejected dust during a perihelion passage from the observations at large phase angles is 10 3 kg, which is at least an order of magnitude lower than the values reported by previous studies based on HI-1 observations. The COR2 observations at small phase angles also provide us with a constraint on mm-sized or larger fragments that may be shed around perihelion in a manner similar to the observed debris trail from near-Sun object 323P/SOHO (Hui et al 2022), albeit less stringent than the constraint for submicron-sized dust grains. Assuming that the debris has light scattering properties similar to the nucleus of Phaethon, their total effective cross-section can then be calculated from Equation ( 5) with the phase function of dust φ d now approximated by the one of the nucleus therein.…”

Surface Properties of Near-Sun Asteroids

“…Comet 96P/Machholz 1 had the smallest perihelion distance of any unambiguously active shortperiod comet until recent observations of 323P/SOHO, which was observed postperihelion with a long, narrow tail, likely caused by immense thermal stress or rotational instability; 323P has the shortest rotational period of the known comets in our solar system at 0.522 hr. Comet 323P/SOHO 1 was recently observed to have bluer colors than most solar system small bodies, which changed unprecedentedly over a few weeks to a color unlike any other small bodies in our solar system (Hui et al 2022). Further characterization of objects with small perihelion distances is necessary to assess whether these comets are unique or if their bluer colors are typical of objects that closely approach the Sun.…”

“…Low-q comets 322P/SOHO 1, 96P/Machholz 1, and 323P/ SOHO have been observed to have atypical blue colors (Knight et al 2016;Eisner et al 2019;Hui et al 2022). Comet 322P/ SOHO 1 has a cometary orbit and shows evidence of being active at perihelion (Lamy et al 2013), but it has an asteroid-like albedo, density, and fast rotation.…”

“…The color of comet 322P from Knight et al (2016) is shown as a magenta triangle. The first reported color of 323P from Hui et al (2022) is shown as a magenta diamond. The red circles and blue squares are low-q asteroids with T J less than and greater than 3, respectively.…”