1I/`Oumuamua and the Problem of Survival of Oort Cloud Comets Near the Sun

Abstract

A 2000-2017 set of long-period comets with high-quality orbits of perihelion distance <1 AU is used to show that the objects that perish shortly before perihelion are nearly exclusively the Oort Cloud members, especially those with perihelia within 0.6 AU of the Sun, intrinsically fainter, and dust poor. Their propensity for disintegration is much higher than predicted by Bortle's perihelion survival rule, prompting the author to propose a new synoptic index to be tested in future prognostication efforts. By their susceptibility to demise near the Sun, the nuclei of Oort Cloud comets differ dramatically from the nuclei of other long-period comets that almost always survive. In this scenario, 'Oumuamua -discovered after perihelion -is in all probability a major piece of debris of an interstellar comet that was bound to perish near perihelion if it was similar to, though much fainter than, the known Oort Cloud comets. The nondetection of 'Oumuamua by the Spitzer Space Telescope is compatible with optical data for pancake shape, but not for cigar shape, with the maximum dimension not exceeding 160 m (at an 0.1 albedo). Although the solar radiation pressure induced nongravitational acceleration requires very high porosity, 'Oumuamua's estimated mass is orders of magnitude greater than for a cloud of unbound submicron-sized dust grains of equal cross section. The acceleration could have displaced 'Oumuamua by 250 000 km in 50 days, scattering other potential debris over a large volume of space.

Are 2I/Borisov and Oort Cloud Comets Alike?

“…Yet, 'Oumuamua could be a piece of debris of a dwarf interstellar comet, which, because of its small perihelion distance, had disintegrated near perihelion (Sekanina 2019a). Intrinsically faint, dust-poor Oort Cloud comets with perihelia closer to the Sun than ∼0.5 AU are known to do just that (Sekanina 2019b). In the process of disintegration, the properties of the parent body altered to the extent that 'Oumuamua as a fragment could not be expected to provide information on the body that had arrived from interstellar space.…”

Outgassing As Trigger of 1I/`Oumuamua's Nongravitational Acceleration: Could This Hypothesis Work at All?

“…In the existing terminology, 'Oumuamua is being referred to as a pancake-shaped or cigar-shaped object, respectively. In the previous paper (Sekanina 2019b) I noted that only the pancake-like version was consistent with the nondetection by the Spitzer Telescope (Trilling et al 2018), the cigar version's dimensions exceeding the 3σ limits.…”

“…The quantities Ψ, Z 0 , v s , and Q H2O in the conditions (4) and ( 5) are heliocentric-distance dependent, while the condition (6) contains constants. In a previous paper (Sekanina 2019b) I remarked that, based on Drahus et al's (2018) data, the projected area of 'Oumuamua at peak brightness equaled 0.002/p V km 2 . With the lightcurve amplitude between 6:1 and 11:1, the mean projected area over a rotation cycle implies for the constant on the right-hand side of Equation (6) X 0 = 0.0011 km 2 .…”

The Catalogue of Cometary Orbits and their Dynamical Evolution

“…In the cases when the assumption of constant NG parameters (see Eq.1) operating within the entire data arc is not adequate (due to an unexpected variable comet activity for any reason), it was worthwhile to use other dedicated approach, in particular, in the context of studying the past or future evolution of analyzed LPCs. Therefore in such a situation when NG acceleration seems to be significantly different before and after perihelion passage, we also present separate orbit solutions basing on the observations taken before the perihelion passage and separately after the perihelion passage, or give a solution based on data taken only at large heliocentric distances; see for example different orbital In the sample of LPCs with small perihelion distances, especially for those with q ≤ 1 au, a sudden outbursts and/or split event were often observed (Sekanina 2019). Then, we derive the orbit (gravitational or NG orbit) on the basis of data restricted to period before the detected sudden activity in a given comet, see for example C/1999 S4 LINEAR, C/2010 X1 Elenin, C/2002 O4 Hönig, C/2002 O7 LINEAR, C/2012 S1 ISON, and others.…”