First measurement of decimeter-sized rocky material in the Oort cloud
Bibliographic record
Abstract
Abstract The Oort cloud is thought to be a reservoir of icy planetesimals and a source of long-period comets (LPCs) implanted from the outer Solar System during the time of giant planet formation. The presence of rocky ice-free bodies is much harder to explain. The rocky fraction in the Oort cloud is a key diagnostic of Solar System formation models as this ratio can distinguish between "massive" and "depleted" proto-asteroid belt scenarios and thus disentangle competing planet formation models. Objects of asteroidal appearance have been telescopically observed on LPC orbits, but from reflectance spectra alone it is uncertain whether they are asteroids or extinct comets. Here we report a first direct observation of a decimeter-sized rocky meteoroid on a retrograde LPC orbit (e ≈ 1.0, i = 121°). The ~2 kg object entered the atmosphere at 62 km/s. The associated fireball terminated at 46.5 km, 40 km deeper than cometary objects of similar mass and speed. During its flight, it experienced dynamic pressures of several MPa, comparable to meteorite-dropping fireballs. In contrast, cometary material measured by Rosetta have compressive strengths of ~1 kPa. The earliest fragmentation of this fireball occurred at >100 kPa, indicating it had a minimum global strength well in excess of cometary. A numerical ablation model produces bulk density and ablation properties consistent with asteroidal meteoroids. We estimate the flux of rocky objects impacting Earth from the Oort cloud to be ~0.7 × 106 km2 per year to a mass limit of 10 g. This is ~6% of the total flux of fireballs on LPC-orbits to these masses. Our results suggests there is a high fraction of asteroidal material in the Oort cloud at small sizes and gives support to migration-based dynamical models of the formation of the Solar System which predict that significant rocky material is implanted in the Oort cloud, a result not explained by traditional Solar System formation models.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".