New Constraints on the Composition and Initial Speed of CNEOS 2014-01-08

Two interstellar objects have been detected so far in the solar system through their reflection of sunlight: 'Oumuamua in 2017 (Meech et al. 2017), and Borisov in 2019 (Guzik et al. 2020). CNEOS ¹ 2014-01-08, detected by U.S. 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). Recently, the light curve of CNEOS 2014-01-08 was released through the CNEOS database. ² Here, we investigate some basic implications of the light curve.

First, we convert the optical power reported in the light curve to total power. Using Equation (1) from Brown et al. (2002), combined with the total optical energy for CNEOS 2014-01-08 of 3.1 × 10¹⁷ erg (Siraj & Loeb 2019), we find that the optical efficiency is ∼6.9%. The total power as a function of time is therefore simply the optical power as a function of time, divided by 6.9%.

Next, we derive the ram pressures, ρ v², corresponding to the three major explosion flares visible in the light curve, where ρ is the ambient mass–density of air and v is the meteor speed. We adopt a straight-line trajectory for CNEOS 2014-01-08 as it moves through the atmosphere at an angle of θ = 268 relative to the ground (Zuluaga 2019). The velocity and altitude measurements reported by CNEOS, v_CNEOS = 44.8 km s⁻¹ and z_CNEOS = 18.7 km, correspond to peak brightness, ³ or Flare 3 in Figure 1. We conservatively adopt a constant speed of v_CNEOS = 44.8 km s⁻¹ between the flares; deceleration due to object breakup between Flare 1 and Flare 3 would lead to greater ram pressures. The Δt_2,3 = 0.112 s delay between Flares 2 and 3, and the Δt_1,3 = 0.213 s delay between Flares 1 and 3, imply that Flares 2 and 3 occurred at $({v}_{\mathrm{CNEOS}}{\rm{\Delta }}{t}_{\mathrm{2,3}}\sin \theta )=2.3\,\mathrm{km}$ and $({v}_{\mathrm{CNEOS}}{\rm{\Delta }}{t}_{\mathrm{1,3}}\sin \theta )=4.3\,\mathrm{km}$ above the altitude at which Flare 3 occurred, respectively. This indicates that Flare 1 and Flare 2 occurred at altitudes of z = 23.0 km and z = 21.0 km, respectively.

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We adopt the atmospheric density profile of $\rho (z)={\rho }_{0}\exp (-z/H)$, where ρ₀ = 10⁻³ g cm⁻³ is the sea-level atmospheric density and H = 8 km is the scale height of the Earth's atmosphere (Collins et al. 2005). The atmospheric densities at which Flare 1, Flare 2, and Flare 3 transpired are ρ₁ = ρ(23.0 km) = 5.64 × 10⁻⁵ g cm⁻³, ρ₂ = ρ(21.0 km) = 7.24 × 10⁻⁵ g cm⁻³, and ρ₃ = ρ(18.7 km) = 9.66 × 10⁻⁵ g cm⁻³. The resulting ram pressures for the three flares are $({\rho }_{1}{v}_{\mathrm{CNEOS}}^{2})=113\,\mathrm{MPa}$, $({\rho }_{2}{v}_{\mathrm{CNEOS}}^{2})=145\,\mathrm{MPa}$, and $({\rho }_{3}{v}_{\mathrm{CNEOS}}^{2})=194\,\mathrm{MPa}$, respectively. Figure 1 displays the flares in terms of total power as a function ram pressure.

Breakup occurs when the yield strength of the impactor Y_i is equivalent to the ram pressure: Y_i  = ρ v² (Collins et al. 2005). CNEOS 2014-01-08's active phase is bracketed by a narrow range of ram pressures, 113–194 MPa, which translates directly into the constraint on the original object's yield strength. Most conservatively, the yield strength of CNEOS 2014-01-08 was comparable to the ram pressure of Flare 1, ${Y}_{i}=({\rho }_{1}{v}_{\mathrm{CNEOS}}^{2})=113\,\mathrm{MPa}$.

Based on estimates for comets, 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). The CNEOS 2014-01-08 inferred yield strength of at least Y_i  = 113 MPa exceeds the typical yield strength of iron meteorites by a factor of ∼2. The observed yield strength is also inconsistent with stony meteorites, which exhibit a range of lower yield strengths by 1–2 orders of magnitude (Petrovic 2001; Popova et al. 2011). Finally, the natural possibilities considered for 'Oumuamua's composition are ruled out for CNEOS 2014-01-08 on the basis of insufficient strength, namely a nitrogen iceberg (Desch & Jackson 2021; Jackson & Desch 2021), an H₂ iceberg (Seligman & Laughlin 2020), or a fluffy dust cloud (Moro-Martín 2019; Luu et al. 2020).

Additionally, CNEOS 2014-01-08 experienced slowdown between its atmospheric entry and detonation. We define a slowdown factor f_s , derived from Equation (8) in Collins et al. (2005),

$$\begin{eqnarray}&&{f}_{s}(z)=\exp \left(-\displaystyle \frac{3\rho (z){C}_{D}H}{4{\rho }_{i}{L}_{0}\sin \theta }\right),\end{eqnarray} \tag{ 1 }$$

where C_D  = 2 is the drag coefficient and ${L}_{0}=2\times {(3E/2\pi {v}_{\mathrm{CNEOS}}^{2}{\rho }_{i})}^{1/3}$ is the diameter of the object, where E = (3.1 × 10¹⁷/6.9%) erg is the total explosion energy. The meteor's speed at an altitude z above the range of breakup altitudes is then v(z) ∼ [f_s (z)v_CNEOS/f_s (z_CNEOS)]. Adopting a fiducial density of ρ_i  = 8 g cm⁻³, corresponding to an iron composition and L₀ ∼ 0.5 m, this implies that the impactor's speed at the top of the atmosphere was at least v(z → ∞ ) = 66.5 km s⁻¹, which is 22 km s⁻¹ or 48% faster than v_CNEOS = 44.8 km s⁻¹, the impact speed used to evaluate the orbit and determine the interstellar origin of CNEOS 2014-01-08 (Siraj & Loeb 2019). A lower value of ρ_i would lead to greater slowdown. This increase in geocentric impact speed makes the interstellar origin of CNEOS 2014-01-08 clearer, and its motion relative to the local standard of rest even more anomalous (Siraj & Loeb 2019).

This work was supported in part by Harvard's black hole Initiative, which is funded by grants from JTF and GBMF.