Interstellar Meteors Are Outliers in Material Strength

CNEOS ¹ on 2014 January 8, a meteor detected by the US Department of Defense (DOD) sensors through the light that it emitted as it burned up in the Earth's atmosphere off of the coast of Papua New Guinea in 2014, was determined to be an interstellar object in 2019 (Siraj & Loeb 2019), a conclusion that was confirmed by independent analysis conducted by the DOD in 2022 (Shaw 2022) although the numerical uncertainties were not provided to the scientific community. The object, which we refer to here as IM1, predated the interstellar object 'Oumuamua by 3.8 yr and the interstellar object Borisov by 5.6 yr. The measured peak flare apparent in the light curve of IM1 at an altitude of 18.7 km implies ambient ram pressure of ∼194 MPa when the meteor disintegrated (Siraj & Loeb 2022a). This level of material strength is ≳20 times higher than stony meteorites and ≳2 times larger than iron meteorites. IM1 was also dynamically unusual—its speed relative to the local standard of rest (LSR) is shared by less than 5% of all stars.

In this Letter, we describe an interstellar meteor candidate from the CNEOS catalog mentioned in Table 8 of Peña-Asensio et al. (2022) and in Section 5 of Siraj & Loeb (2022b), which we refer to as IM2. We then explore the statistical likelihood that interstellar meteors reflect the same distribution of material strength as noninterstellar meteors.

The Python code implemented here used the open-source N-body integrator software REBOUND ² to trace the motion of the meteor under the gravitational influence of the solar system (Rein & Liu 2012).

We initialize the simulation with the Sun, the eight planets, and the meteor, with geocentric velocity vector ( vx _obs, vy _obs, vz _obs) = (−15.3, 25.8, −20.8) km s⁻¹, located at 40.5° N 18.0° W, at an altitude of 23.0 km, at the time of impact t_i = 2017 March 9, 04:16:37 UTC, as reported in the CNEOS catalog. We then use the IAS15 adaptive time-step integrator to trace the meteor's motion back in time (Rein & Spiegel 2015). This does not account for air drag, which would lead to an even higher impact speed and therefore heliocentric speed, given the encounter geometry. The slowdown of IM1 due to air drag was estimated in earlier work (Siraj & Loeb 2022a).

There are no substantial gravitational interactions between the meteor and any planet other than Earth for any trajectory within the reported errors. Based on the geocentric impact speed reported by CNEOS, v _obs = 36.5 km s⁻¹, and the heliocentric impact speed was ∼50 km s⁻¹. We find that the meteor was unbound with an asymptotic speed of v _∞ ∼ 25.9 km s⁻¹ outside of the solar system.

We find the heliocentric orbital elements of the meteor at impact to be the following: semimajor axis a = −1.1 au; eccentricity e = 1.6; inclination i = 26°; longitude of the ascending node Ω = −12°; argument of periapsis ω = 241°; and true anomaly f = 300°. IM2 was v _LSR ∼ 40 km s⁻¹ away from the velocity of the local standard of rest (Schönrich et al. 2010).

Given the explosion energy of ∼4 × 10¹⁹ erg and the atmospheric impact speed of ∼36.5 km s⁻¹, we adopt equivalence between the preexplosion kinetic energy and the energy in the explosion, finding that the object's mass was ∼6.3 × 10⁶ g. A comparison between the properties of IM1 and IM2 is included in Table 1.

We note that the fact that both IM1 and IM2 have low orbital inclinations (i ≲ 30°) is puzzling since interstellar objects are expected to have a uniform distribution in $\cos i$. Specifically, the random likelihood of two orbital inclinations of i ≲ 30° drawn from a uniform distribution in $\cos i$ is ${(1-\cos (30^\circ ))}^{2}\sim 2 \%$. However, 2I/Borisov had an inclination of 44°, and the likelihood of drawing three inclinations of i ≲ 45° is ${(1-\cos (45^\circ ))}^{2}\sim 3 \%$, so there may be an inclination bias in the source of a certain class of interstellar objects. 1I/'Oumuamua had an orbital inclination of i = 122°, more indicative of a background population uniform distribution.

As a meteor travels through the atmosphere, it experiences friction due to air. The dynamical pressure is ρ v², and the dynamical pressure corresponding to the peak power in the meteor light curve describes the material strength of the meteor since crossing a certain level of ram pressure causes the object to deform and break apart (Trigo-Rodríguez & Llorca 2006, 2007; Popova et al. 2011).

Based on estimates for comets and carbonaceous, stony, and iron meteorites (Chyba et al. 1993; Scotti & Melosh 1993; Svetsov et al. 1995; Petrovic 2001), Collins et al. (2005) established an empirical strength–density relation for impactor density ρ_i in the range 1–8 g cm⁻³. The upper end of this range gives a yield strength of Y_i ∼ 50 MPa, corresponding to the strongest known class of meteorites, iron (Petrovic 2001). Iron meteorites are rare in the solar system, making up only ∼5% of modern falls (Zolensky et al. 2006).

We computed the ram pressure at breakup for all 273 fireballs in the CNEOS catalog. Interestingly, IM1 and IM2 display the first- and third-highest material strengths, respectively, among all of the fireballs. Figure 1 is a histogram showing all of the fireballs in the catalog and highlighting IM1 and IM2. Figure 2 shows the light curves for IM1 and IM2 with the peak ram pressures highlighted.

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The probability of randomly drawing two of the top three material strengths out of all 273 fireballs is ∼(3/273)² ∼ 10⁻⁴. Therefore, if IM2 is confirmed to be an interstellar meteor, simple random drawing dictates that there would be a ∼99.99% chance that interstellar meteors come from a population with material strength characteristically higher than meteors originating from within the solar system, a notion suggested by Peña-Asensio et al. (2022).

The material strength data follow a log-normal distribution, shown in Figure 1 with mean μ = 0.65 and σ = 0.47 MPa. As a result, IM1 and IM2 represent 3.5σ and 2.6σ deviations from the mean, respectively. These deviations correspond to 2.4 × 10⁻⁴ and 4.5 × 10⁻³ single-tailed probabilities, respectively. Combining these independent events, we find a ∼10⁻⁶ probability of getting the material strengths of IM1 and IM2 by random chance. This Gaussian perspective implies a ∼99.9999% chance that interstellar meteors are characteristically stronger than meteors from within the solar system.

We note that the ram pressure could be overestimated by the fact that DOD satellites only detected the brightest sections of their luminous paths. For instance, the US government sensor data on the Chelyabinsk event gives a dynamic strength 2 or 3 times as large as that measured in recovered meteorites (Peña-Asensio et al. 2022).

For a background population on random trajectories drawn from an isotropic distribution in the LSR, the number density implied by the detection of an interstellar meteor is

$$\begin{eqnarray}&&n\simeq \displaystyle \frac{{\rm{\Gamma }}}{{v}_{\infty }\pi {R}_{\oplus }^{2}\left[1+{\left({v}_{\mathrm{esc}}/{v}_{\infty }\right)}^{2}\right]},\end{eqnarray} \tag{ 1 }$$

where Γ is the implied rate, v_∞ is the speed outside of the solar system, ³ R_⊕ is the radius of the Earth, and ${v}_{\mathrm{esc}}=\sqrt{2{{GM}}_{\odot }/{d}_{\oplus }}$ is the escape speed from the Earth's position, where M_⊙ is the mass of the Sun and d_⊕ the distance between the Earth and the Sun. Gravitational focusing is accounted for by the term in the denominator $[1+{({v}_{\mathrm{esc}}/{v}_{\infty })}^{2}]$. We adopt Γ ∼ 0.1 yr⁻¹ for both IM1 and IM2 (Siraj & Loeb 2019) and speeds outside of the solar system of v_∞ ∼ 42 km s⁻¹ and v_∞ ∼ 26 km s⁻¹, respectively. We find that the number density implied in the detections of IM1 and IM2 are ${n}_{\mathrm{IM}1}\sim 1.8\times {10}^{6}\,{\mathrm{au}}^{-3}$ and ${n}_{\mathrm{IM}2}\sim 2.7\times {10}^{6}\,{\mathrm{au}}^{-3}$, as shown in Figure 3.

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Given the respective masses of ∼4.6 × 10⁵ g and ∼6.3 × 10⁶ g, we find that the detections of IM1 and IM2 imply, respectively, ambient local abundances of ∼1.2 M_⊕ pc⁻³ and ∼25 M_⊕ pc⁻³ of similar objects.

The local stellar mass density is ∼0.04 M_⊙ pc⁻³ (Bovy 2017). The local density of the interstellar medium is 1.2 cm⁻³ (McKee et al. 2015), implying ∼0.03 M_⊙ pc⁻³. All refractory elements (metals and silicates) sum to a total mass fraction of ∼0.3% at solar metallicity, implying that the local budget of metals and silicates in stars and dust is ∼70 M_⊕ pc⁻³. We conservatively assume that IM1 and IM2 are composed of refractory elements even though their material strengths imply that they were primarily metallic in composition. If IM2 is indeed an interstellar object, the detections of IM1 and IM2 together imply that ∼40% of all refractory elements locked from stars and the ISM are locked in meter-scale interstellar objects. Refractory elements are also locked in objects bound to the Sun, with the largest theorized reservoir being the Oort cloud (Stern & Shull 1990).

If interstellar meteors are formed in planetary systems, the natural limit to the scale of mass ejected is the total budget of the minimum mass solar nebula model (MMSN), which is of order ∼1% of stellar mass (Weidenschilling 1977; Hayashi 1981; Desch 2007; Crida 2009). The result reached here indicates that if IM2 is confirmed an interstellar object, the detections of IM1 and IM2 combined imply that ∼2/3 of the mass budget in stars is necessary to provide the refractory elements to produce a population of interstellar meteors that would make the detections of IM1 and IM2 likely. This result thereby provides a new constraint on planetary system formation since it requires nearly 2 orders of magnitude more mass than the MMSN (Weidenschilling 1977; Hayashi 1981; Desch 2007; Crida 2009). Note that the mass budget discussed exceeds that of objects "'Oumuamua-sized and larger," which itself is an unsolved puzzle (Loeb 2022; Siraj & Loeb 2021). The extraordinary mass budget required to produce interstellar meteors seemingly defies planetary system origins and suggests some other highly efficient route for creating meter-scale objects made of refractory elements. Interestingly, there is a paucity of refractory elements observed in the gas phase in the interstellar medium (Savage & Sembach 1996; Maas et al. 2005; Delgado Inglada et al. 2009), an observation that could potentially reflect refractory elements being locked in interstellar objects. Supernovae have been observed to produce iron-rich "bullets," which could be a possible origin of IM1 and IM2 (Loeb et al. 1994; Strom et al. 1995; Stone et al. 1995; Wang & Chevalier 2002; Tsunemi & Katsuda 2006; Perret & Timmes 2009; Miceli et al. 2013; Tsebrenko & Soker 2015; Sandoval et al. 2021).

This work was supported in part by a grant from the Breakthrough Prize Foundation and by research funds from the Galileo Project at Harvard University.