Interstellar Meteors are Outliers in Material Strength

Brief

Siraj & Loeb (2022) compute peak ram-pressure yield strengths for all 273 fireballs in the NASA CNEOS catalog, using breakup altitude and atmospheric velocity as proxies for material strength. IM1 (2014-01-08) registers 194 MPa and IM2 (2017-03-09) registers 75 MPa, placing them 3.5σ and 2.6σ above a log-normal fit to the full catalog (μ = 0.65, σ = 0.47 in log₁₀ MPa); the combined Gaussian probability of both values arising by chance is ~10⁻⁶. If IM2 is confirmed interstellar, the pair's implied number densities (~1.8×10⁶ and ~2.7×10⁶ AU⁻³) would require roughly 40% of all local refractory elements to reside in meter-scale interstellar objects, a mass budget nearly two orders of magnitude beyond the minimum-mass solar nebula model and one that challenges a planetary-system origin for the population.

Metadata

Category

Phenomenon

Venue

The Astrophysical Journal Letters

Type

Peer-reviewed

Year

2022

Authors

Amir Siraj, Abraham Loeb

DOI

10.3847/2041-8213/aca8a0

arXiv

2209.09905

Access

Open access

Length

372.5 K

Programs

Galileo Project, Breakthrough Prize Foundation

Instruments

U.S. Department of Defense satellite sensors, REBOUND N-body integrator (IAS15 adaptive integrator)

Data sources

CNEOS fireball catalog (273 events), DoD satellite sensor records (IM1, IM2)

Tags

interstellar objects, meteors, material strength, fireball catalog, CNEOS, planetary formation

Key points

• IM1 and IM2 rank 1st and 3rd in peak ram-pressure material strength among all 273 fireballs in the CNEOS catalog.p.1
• IM1 peak breakup ram pressure: 194 MPa, more than 20× stony meteorites and more than 2× iron meteorites (the strongest known solar-system class, at ~50 MPa). IM2: 75 MPa.p.2
• Material strengths across the catalog follow a log-normal distribution (μ = 0.65, σ = 0.47 log₁₀ MPa); IM1 and IM2 deviate at 3.5σ and 2.6σ, with single-tailed probabilities of 2.4×10⁻⁴ and 4.5×10⁻³ respectively.p.3
• Combined Gaussian probability of both objects being drawn from the solar-system fireball population: ~10⁻⁶. Random-draw probability of landing two objects in the top 3 of 273: ~10⁻⁴.p.3
• IM2 orbital solution: e = 1.6, a = −1.1 AU, asymptotic speed v∞ ≈ 25.9 km/s, v_LSR ≈ 40 km/s, unambiguously hyperbolic.p.2
• Implied local number densities: n_IM1 ≈ 1.8×10⁶ AU⁻³, n_IM2 ≈ 2.7×10⁶ AU⁻³; corresponding ambient mass abundances ≈ 1.2 M⊕ pc⁻³ and ≈ 25 M⊕ pc⁻³.p.4
• If both detections are representative, ~40% of all local refractory elements would need to be locked in meter-scale interstellar objects, requiring ~2/3 of stellar mass budgets, roughly 100× the MMSN.p.5
• Supernovae producing iron-rich 'bullets' are identified as a candidate non-planetary origin mechanism consistent with the high material strengths observed.p.5

Verbatim

• “IM1 and IM2 are ranked 1 and 3 in terms of material strength out of all 273 fireballs in the CNEOS catalog.”

  p.1

• “The random sampling and Gaussian probabilities, respectively, of picking two objects with such high material strength from the CNEOS catalog, are ∼ 10 − 4 and ∼ 10 − 6 .”

  p.1

• “This level of material strength is & 20 times higher than stony meteorites and & 2 times larger than iron meteorites.”

  p.1

Most interesting

• IM1 predated 'Oumuamua by 3.8 years and Borisov by 5.6 years, making it the earliest known interstellar object, identified only in 2019 from archived DoD satellite data collected in 2014.
• Both IM1 and IM2 have orbital inclinations below 30°, a configuration with only a ~2% random probability for two interstellar objects drawn from an isotropic background distribution, suggesting a possible directional or structural bias in their source population.
• IM1's 194 MPa yield strength is nearly four times that of iron meteorites, the strongest material class in the known solar-system meteorite inventory, implying a composition with no terrestrial analog.
• IM2's light curve shows a distinct early flare, indicative of lower-strength surface material, followed by a dominant central flare consistent with a metallic interior, a layered structural signature atypical of solar-system fireballs.
• The observed paucity of refractory elements in the gas phase of the interstellar medium may itself be explained if those elements are sequestered inside meter-scale interstellar objects of the type IM1 and IM2 represent.
• Iron meteorites account for only ~5% of modern falls in the solar system, yet both interstellar candidates exhibit strengths at or above the iron threshold, a composition that is rare locally but may be common in the interstellar population.