A Meteor of Apparent Interstellar Origin in the CNEOS Fireball Catalog

'Oumuamua was the first interstellar object detected in the solar system by Pan-STARRS (Meech et al. 2017; Micheli et al. 2018). Several follow-up studies of 'Oumuamua were conducted to better understand its origin and composition (Bannister et al. 2017; Bolin et al. 2017; Gaidos et al. 2017; Jewitt et al. 2017; Mamajek 2017; Ye et al. 2017; Bialy & Loeb 2018; Fitzsimmons et al. 2018; Hoang et al. 2018; Trilling et al. 2018; Seligman et al. 2019; Siraj & Loeb 2019a, 2019b). Its size was estimated to be 20–200 m, based on Spitzer Space Telescope constraints on its infrared emission given its temperature (Trilling et al. 2018). The discovery of 'Oumuamua was followed by that of the second interstellar object, Borisov, in 2019 (Guzik et al. 2020). The size of Borisov's nucleus was estimated to be 0.4–1 km (Jewitt et al. 2020; Siraj & Loeb 2021).

Forbes & Loeb (2019) predicted that spectroscopy of 'Oumuamua-like objects grazing the Sun could reveal their chemical compositions. Since there should be a higher abundance of interstellar objects smaller than 'Oumuamua, we could observe small interstellar objects impacting the Earth's atmosphere, appearing as interstellar meteors (Opik 1950). Spectroscopy of the gaseous debris from such objects as they burn up in the Earth's atmosphere could reveal their composition. There is significant evidence for previous detections of dust-sized interstellar meteors (Baggaley et al. 1993; Taylor et al. 1996; Mathews et al. 1999; Baggaley 2000; Meisel et al. 2002; Weryk & Brown 2004; Afanasiev et al. 2007; Froncisz et al. 2020), as well as interstellar meteor nondetections (Hajdukova 1994; Musci et al. 2012; Engelhardt et al. 2017; Hajdukova et al. 2020) but to date no definitive evidence of any meter-scale interstellar meteors.

The CNEOS catalog includes the geocentric velocity components and geographic coordinates for bolides detected by U.S. government sensors. ¹ In this article, we identify a meteor from the CNEOS catalog that is likely of interstellar origin. Furthermore, we explore the size distribution of interstellar meteors and motivate the investigation of interstellar meteors. We present a strategy for conducting spectroscopy and obtaining physical samples of interstellar meteors.

We analyzed the bolide events in the CNEOS catalog, and found that the meteor detected at 2014 January 8 17:05:34 UTC had an unusually high heliocentric velocity at impact. ² Accounting for the motion of the Earth relative to the Sun and the motion of the meteor relative to the Earth, we found that the meteor had a heliocentric velocity of ∼ 60 km s⁻¹ at impact, which implies that the object was unbound. To uncover the kinematic history of this meteor, we integrated its motion from impact backward in time.

The Python code created for this work used the open-source N-body integrator software REBOUND ³ 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) = ( −3.4, −43.5, −10.3) km s⁻¹, located at 13 S 1476 E, at an altitude of 18.7 km, at the time of impact, t_i = 2014 January 8 17:05:34 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 2014). 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 due to air drag is estimated in Siraj & Loeb (2022).

3.1. Trajectory

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 impact speed reported by CNEOS, v_obs = 44.8 km s⁻¹, we find that the meteor was unbound with an asymptotic speed of v_∞ ∼ 42.1 km s⁻¹ outside of the solar system. In order for the object to be bound, the observed speed of v_obs = 44.8 km s⁻¹ would have to be off by more than 45%, or 20 km s⁻¹, or assuming a correct speed, a radiant off by more than 60° (Zuluaga 2019).

The typical velocity uncertainty for meter-scale impactors in the CNEOS catalog was estimated by Brown et al. (2016) and Granvik & Brown (2018) to be less than 1 km s⁻¹, but 2 of 10 events analyzed by Devillepoix et al. (2019) have discrepancies with alternate measurements of up to 28% in speed. Drolshagen et al. (2020) noted that the CNEOS catalog includes two objects that were detected in space before impact, 2008 TC₃ and 2018 LA, with respective measurement errors of 0.3 and 0.52 km s⁻¹ on the geocentric impact speed. These papers underlined the importance of uncertainties and motivated us to retrieve the measurement uncertainties for the 2014 January 8 meteor specifically.

Recently, the U.S. Department of Defense, which houses the classified data pertaining to the uncertainties involved in the CNEOS 2014-01-08 detection, released a public statement dated 2022 March 1 and addressed to the NASA Science Mission Directorate, referencing this discovery preprint and mentioning our analysis that the meteor originated from an unbound hyperbolic orbit with high confidence (Shaw 2022). The letter then states "Dr. Joel Mozer, the Chief Scientist of Space Operations Command, reviewed analysis of additional data available to the Department of Defense related to this finding. Dr. Mozer confirmed that the velocity estimate reported to NASA is sufficiently accurate to indicate an interstellar trajectory" (Shaw 2022). We rely on the official letter released by the U.S. Department of Defense for the confirmation of the interstellar origin of the 2014 January 8 meteor. Given the confidence level expressed in the official letter from the U.S. Department of Defense, we calculate and quote here the corresponding Gaussian symmetric errors on each parameter related to the object's orbital trajectory.

We find that the heliocentric orbital elements of the meteor at time of impact are as follows: semimajor axis, a = −0.47 ±0.15 au, eccentricity, e = 2.4 ± 0.3, inclination i = 10 ± 2°, longitude of the ascending node, Ω = 108 ± 1°, argument of periapsis, ω =58 ± 2°, and true anomaly, f = −58 ± 2°. The trajectory is shown in Figure 1. The origin is toward R.A. 49.4 ± 4.1° and decl. 11.2 ± 1.8°. The heliocentric incoming velocity at infinity of the meteor in right-handed Galactic coordinates is v_∞(U, V, W) = (32.7 ± 5.8, −4.5 ± 1.5, 26.1 ±2.0) km s⁻¹, which is 58 ± 6 km s⁻¹ away from the velocity of the local standard of rest (LSR), (U, V, W)_LSR = (−11.1, −12.2, −7.3) km s⁻¹ (Schonrich et al. 2010).

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3.2. Size Distribution

Given the impact speed of the meteor, ∼44.8 km s⁻¹, and the total impact energy, 4.6 × 10¹⁸ ergs, the meteor mass was approximately 4.6 × 10⁵ g. Adopting bulk density values of 1.7 and 0.9 g cm⁻³ for Type II and Type IIIa objects respectively, yields a radius of 0.4–0.5 m for a spherical geometry (Ceplecha 1988; Palotai et al. 2019). Adopting a density similar to an iron meteorite, ∼8 g cm⁻³, as suggested by Siraj & Loeb (2022) based on the light curve for CNEOS 2014-01-08, yields a diameter of ∼0.5 m.

The CNEOS catalog includes bolide events at a relatively high frequency for the past decade, so we approximate the yearly detection rate of interstellar meteors to be at least∼ 0.1 yr⁻¹. This estimate agrees with that of Hajdukova et al. (2019) within the error budget for the size of CNEOS 2014-01-08. We estimate the number density of similarly sized interstellar objects by dividing the yearly detection rate by the product of the impact speed of the meteor and the cross sectional area of the Earth, finding the approximate number density of interstellar objects with a size of order R ∼ 0.45 m and a speed v ∼ 60 km s⁻¹ km s⁻¹ relative to the LSR to be

$$\begin{eqnarray}&&n\sim \displaystyle \frac{0.1\,{\mathrm{yr}}^{-1}}{(13\ \mathrm{AU}/{\mathrm{yr}}^{})(5.7\times {10}^{-9}{\mathrm{AU}}^{2})}\sim {10}^{6}\ {\mathrm{AU}}^{-3}.\end{eqnarray} \tag{ 1 }$$

Given 95% Poisson uncertainties (Gehrels 1986), the inferred ⁴ local number density for interstellar objects of this size is $n={10}^{{6}_{-1.5}^{+0.75}}\,{\mathrm{au}}^{-3}$. This figure necessitates $6\times {10}^{{22}_{-1.5}^{+0.75}}$ similarly sized objects, or 0.2–20 Earth masses of material, to be ejected per local star. This is at tension with the fact that a minimum-mass solar nebula is expected to have about an Earth mass of total planetesimal material interior to the radius where the orbital speed is ∼60 km s⁻¹ (Desch 2007), with similar values for other planetary systems (Kuchner 2004). Our inferred abundance for interstellar meteors should be viewed as a lower limit since the CNEOS data might have a bias against detection of faster meteors (Brown et al. 2016).

CNEOS 2014-01-08, 'Oumuamua, and Borisov together serve as important calibration points for the size distribution of interstellar objects. Figure 2 illustrates the three classes of interstellar objects considered here in size-abundance parameter space. We use a Monte Carlo simulation to characterize the slope of this size distribution. For each run of the simulation, we select a size and a population abundance for the three interstellar objects. The sizes for the CNEOS 2014-01-08, 'Oumuamua, and Borisov are drawn from the triangular distributions spanning 0.3–0.7 m, ⁵ 20–200 m (Trilling et al. 2018), and 0.4–1 km (Jewitt et al. 2020; Siraj & Loeb 2021), respectively, with peaks at 0.45 m, 100 m, and 0.7 km. The abundances are drawn from the Poisson distributions for sample size of one with central values of 10⁶, 10⁻¹ (Do et al. 2018; Levine & Laughlin 2021; Siraj & Loeb 2021), and 9 × 10⁻³ au, respectively. Given the three size-abundance tuples, each run of the simulation fits a linear least square to the log-log transformed values, resulting in a power-law size distribution fit with normalization k and slope q, corresponding to a size distribution with the form N(>R) ∼ kR^1−q . We repeat this routine 10⁵ times, thereby constructing a distribution of q corresponding to the uncertainties in CNEOS 2014-01-08–like, 'Oumuamua-like, and Borisov-like populations. Our model applies if 'Oumuamua was not primarily composed of hydrogen or helium.

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We find that, over the size range from CNEOS 2014-01-08 (meter-scale) to Borisov (kilometer-scale), the best-fit slope is q ≈ 3.6 ± 0.5, where the quoted uncertainty corresponds to 2σ. The result is in good agreement with q = 3.5 the analytical collisional model by Dohnanyi (1969), but is not inconsistent with the q = 4 scale-free power-law model (Siraj & Loeb 2021), which contains equal mass per logarithmic bin. The power-law extrapolation may not hold for all bolide radii down to dust particles.

The cores of meteoroids with radii larger than ∼5 cm can reach the ground in the form of meteorites (Kruger & Grun 2014). Additionally, meteoroids on smaller size scales could be accelerated from the Poynting–Robertson effect (Wyatt & Whipple 1950) and could have potential origins in the interstellar medium. Hence, interstellar meteors with size ≳5 cm are optimal for a systematic study of physical extrasolar material (in addition to the spectroscopy of the hot gases as the meteor burns up). We consider their potential detection here even though they represent a size scale from CNEOS 2014-01-08. Since we expect interstellar meteors with size ≳5 cm to strike the Earth at least a few times per year based on extrapolating the size distributions considered here (q = 3.6 ± 0.5) to objects ∼5 times smaller in size than CNEOS 2014-01-08, a network of all-sky camera systems monitoring the sky above all land on Earth could detect an interstellar meteor with size ≳5 cm every few years, given that the land fraction of Earth's surface is ∼30%. Such detections can be made with science-grade video cameras, such as those used in AMOS, CAMO, and CAMS ⁶ (Jenniskens et al. 2011; Weryk et al. 2013; Tóth et al. 2015; Jenniskens et al. 2016). These systems could employ the new trajectory estimation software (Vida et al. 2020a, 2020b) that seems to be promising.

A conservative estimate for the total area of ∼70 km altitude atmosphere visible from a system of two all-sky camera systems separated by 100 km is 5 × 10⁵ km², so to cover all land on Earth would require ∼300 systems, or ∼600 total all-sky camera systems, similar to CAMO but with an all-sky field of view, like AMOS and CAMS.

We therefore advocate for a network of all-sky camera systems to conduct real-time remote spectroscopy of the hot gases as r ≥ 5 × 10⁻² m interstellar meteors burn up, and to precisely determine their trajectories for the immediate retrieval of interstellar meteorite samples.

By extrapolating the trajectory of each meteor backward in time and analyzing the relative abundances of each meteor's chemical isotopes, one can match meteors to their parent stars and reveal insights into planetary system formation. R-processed elements, such as Eu, can be detected in the atmospheres of stars (Frebel et al. 2016), so their abundances in meteor spectra can serve as important links to parent stars.

If interstellar objects typically have ejection speeds near zero from their parent stars (because they are tidally removed by passing stars or the Milky Way from the outskirts of their planetary system), we would expect interstellar objects to reflect the dynamics of local stars. As a result, most interstellar objects should originate from within the local stellar velocity dispersion away from the LSR (Siraj & Loeb 2020). The large deviation for this object from the LSR of ∼58 ± 6 km s⁻¹ suggests that it perhaps originated in the thick disk, which has velocity dispersion components of (σ_U , σ_V , σ_W ) = (50, 50, 50) km s⁻¹ relative to the LSR (Bland-Hawthorn & Gerhard 2016). However, the ratio of local thick disk stars to thin disk stars is 0.04, making this a minority population. Alternatively, for a parent planetary system with a more typical velocity relative to the LSR, the object could have originated in the deep interior, where the orbital speeds of objects are of the necessary magnitude. Either way, the meteor seems to have an unusual origin. There is also a possibility that the parameters are erroneously determined.

Siraj & Loeb (2022) analyzed the recently released light curve for CNEOS 2014-01-08, ⁷ finding that the measured velocity and three observed flares down to an altitude of 18.7 km imply ambient ram pressure in the range of 113–194 MPa when the meteor disintegrated. The required yield strength is ≳20 times higher than stony meteorites and ≳2 times larger than iron meteorites. Additionally, the implied slowdown in the atmosphere suggests a significantly higher initial speed, strengthening the case for an interstellar origin of this meteor and making it an outlier relative to the velocity dispersion of local stars. Using the same data, Peña-Asensio et al. (2022) corroborated our finding of a high material strength, quoting an even higher value of 222.8 ± 67.6 MPa. However, the reality of the combination of the fireball's reported altitude and speed remains questionable.

The discovery of additional interstellar meteors will serve as an important calibration for population-wide parameters of interstellar objects, including their abundance and origin.

We estimate the impact rate of similarly sized objects with the Earth, given 95% Poisson distribution confidence intervals, to be at least ${0.1}_{-0.097}^{+0.457}$ yr⁻¹. Future meteor surveys could flag incoming objects with excess heliocentric velocities for follow-up preimpact observations. Spectroscopy of gaseous debris from these objects as they burn up in the Earth's atmosphere would reveal their composition. Given that some isotope ratios are expected to be markedly different for objects of interstellar origin compared to the solar system, one could validate an interstellar origin (Lingam & Loeb 2018; Forbes & Loeb 2019). Precision tracking with the upcoming Vera Rubin Observatory's Legacy Survey of Space and Time (LSST ⁸ ) could determine the trajectory of meteors of interstellar origin to their parent systems in the Gaia catalog. ⁹ Our discovery also implies that at least $4.5\times {10}^{{8}_{-1.5}^{+0.75}}$ similarly sized interstellar bolide events have occurred over Earth's lifetime. Potentially, interstellar meteors could deliver life from another planetary system and mediate panspermia (Ginsburg et al. 2018). Interestingly, the high speed for the meteor discussed here implies a likely origin in the habitable zone of the abundant population of dwarf stars, indicating that similar objects could carry life from their parent planetary systems. Note that this discussion of panspermia and the habitable zone in connection with CNEOS 2014-01-08 is only speculation.

We identified one other interstellar meteor candidate from the entire catalog, a bolide that struck (40.5N, 18.0W) at 04:16:37 UTC on 2017 March 9, and appeared to have a heliocentric speed of ∼50 km s⁻¹. We advocate for the declassification of the uncertainties associated with the USG measurements for CNEOS 2017-03-09 so its interstellar nature can be confirmed. The apparent lack of a set of velocities that disperse continuously beyond the parabolic limit is consistent with the notion that the CNEOS catalog is of high quality for the determination of interstellar meteors. Peña-Asensio et al. (2022) display the dynamical properties of all of the fireballs reported in the CNEOS catalog in Figure 2, and enumerate the velocities for the apparent hyperbolic orbits in Table 8. Three of the five objects with apparent hyperbolic orbits have heliocentric speeds within ≲5% of the hyperbolic threshold, and the remaining are CNEOS 2014-01-08 and the other bolide mentioned here, CNEOS 2017-03-09, which have heliocentric speeds ∼45% and ∼19%, respectively, above the hyperbolic threshold.

We presented and analyzed impact data from the meteor detected at 2014 January 8 17:05:34 UTC. The U.S. Department of Defense verified in a letter referencing this work that "the velocity estimate reported to NASA is sufficiently accurate to indicate an interstellar trajectory" (Shaw 2022). Based on the data provided by CNEOS the meteor had an asymptotic speed of v_∞ ∼ 42.1 ± 5.5 km s⁻¹ outside of the solar system. Assuming these parameters correspond to the reality, its size, trajectory, and excess speed exclude the possibility that it was gravitationally scattered within the solar system prior to impact (Wiegert 2014).

We also analyzed the updated size distribution of interstellar meteors, deriving a range of possible slopes of q ∼ 3.6 ± 0.5, consistent with the analytical model by Dohnanyi (1969). Finally, we presented a strategy for studying interstellar meteors: using a network of ∼600 all-sky camera systems to determine the orbits and chemical compositions of r ≥ 5 cm meteors. This method also allows for the possibility of retrieving interstellar meteorite samples. This new field of astronomical research is significant as it would save the trip and allow us to study samples of materials from other planetary systems.

We thank Matt Daniels for his exceptional help in procuring official confirmation from the U.S. government that the measurement uncertainties of the 2014 January 8 meteor were small enough to confirm its interstellar nature. We thank Pete Worden for introducing the authors to Matt Daniels. We thank Lt. Gen. John Shaw, Joel Mozer, and an anonymous analyst, for their detailed examination of the relevant Department of Defense data that led to the confirmation of the meteor's interstellar origin. We thank Alan Hurd and Matthew Heavner for initiating the search for clarification regarding the quality of the data for the 2014 January 8 meteor. Without the help of all aforementioned individuals, the interstellar origin of this meteor would have been enshrouded in uncertainty. We also thank Nathan Golovich, Jeff Kuhn, Manasvi Lingam, Matthew Payne, Steinn Sigurdsson, and Peter Veres for helpful comments on the manuscript. This work was supported in part by a grant from the Breakthrough Prize Foundation.