The original asteroid is gone — scattered, ablated, and buried. But from the iron left on the ground, researchers have reconstructed it. The 425–430 km strewn field, the spread of the recovered masses, and the shallow entry angle all point back to a body far larger than anything that survives.
How big it was
More than 74 tonnes of Aletai iron have been recovered — already one of the largest iron meteorite finds anywhere. Yet that is only the remainder. Modelling the atmospheric flight backward, Li et al. (2022) estimate the original body weighed between 280 and 3,440 tonnes, with a radius of roughly 2 to 5 metres. The gap between thousands of tonnes entering and 74 surviving is a measure of how much iron was lost to ablation across the long, grazing flight.
Which way it came
The masses are not scattered at random. They lie along an array running southeast to northwest, which fixes the direction of travel: the body came in from the southeast and shed its pieces toward the northwest, where the largest masses cluster at the terminus. The modelling puts its entry speed at 11.9–14.9 km/s and its angle at a shallow 6.5–7.3° — the geometry that produced a record-length field rather than a crater.
Where it broke apart — in the air, not in space
A natural question is whether the body arrived already broken. The study argues no. Tidal forces in near-Earth space can pull apart a loose comet, but they cannot disrupt a small, solid iron body a few metres across — iron is far too strong. Instead, the break-up happened in the atmosphere, in stages: a primary fragmentation at a dynamic pressure of roughly 3–4 MPa, then a later disintegration near the northwest end at about 1–3 MPa, where several of the largest masses came down together. The body’s own internal structure — its inclusions and fractures — decided where it failed.
What it isn’t
The reconstruction also rules things out. The field is not crater ejecta: a small mass thrown from a hypothetical impact could travel only tens of kilometres, not the 425–430 km separating the farthest masses, and the nearest sizeable crater is both too distant and millions of years too old to be related. The simplest explanation that fits every piece of evidence is the one the data keeps pointing back to — a single large iron asteroid, entering shallowly, coming apart in the air. The same rare IIIE-an chemistry that makes Aletai unusual is what those surviving masses carry today.
Frequently Asked Questions
How big was the original Aletai asteroid?
Modelling its atmospheric flight, Li et al. (2022) estimate the original body weighed between 280 and 3,440 tonnes, with a radius of roughly 2 to 5 metres. Over 74 tonnes have been recovered; the rest was lost to ablation during entry.
Which direction did Aletai travel?
From southeast to northwest. The recovered masses lie along a southeast-to-northwest array, with the largest clustered at the northwestern end, marking the direction and terminus of the flight.
Did Aletai break up in space or in the atmosphere?
In the atmosphere. The study argues that tidal forces in space cannot disrupt a small, solid iron body, and that the break-up happened in stages during entry, at dynamic pressures of roughly 3–4 MPa and then 1–3 MPa.
Could the Aletai strewn field be debris from an impact crater?
No. A fragment thrown from an impact could travel only tens of kilometres, far short of the 425–430 km field, and the nearest sizeable crater is too distant and too old to be related.
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