Two telescopes put the radius below 2.8 km
Before perihelion Hubble limited the radius to below 2.8 km and SPHEREx to below 2.5 km, and after perihelion Hubble measured 1.3 ± 0.2 km.
What it means
- Every direct measurement or limit falls below the 2.5-km radius the large-size answer needs.For the key question: How big is the nucleus of 3I/ATLAS?
Next steps 2
Details
Take Frincke and Seligman's compiled production rates and JPL solution 55's non-gravitational parameters, and compute the mass, and so the radius at a density of 0.5 g/cm3, separately for a CO2-driven and a water-driven push, with the dust-to-gas ratio as a stated range. A radius under 1.8 km in both cases supports the kilometre-sized answer independently of the Hubble photometry; a radius above 2.5 km would need far less push per kilogram of gas than any comet shows, and would reopen the large answer.
Details
Hui and colleagues measured the 1.3 km radius from the December 2025 and January 2026 visits of DD program 18152. The program went on to visit on 14 February and 10 April 2026, when the coma was fainter, and all its WFC3 frames are public in MAST. Fit the nucleus with coma subtraction on those later frames. A radius inside 0.35-1.8 km settles the kilometre-sized answer; one above 2.5 km revives the large one; a value between reopens it.
Sources 3
Cite it
Mysteries Department. (2026). Two telescopes put the radius below 2.8 km [Evidence EVD0DFQ48E606F6]. https://mysteriesdept.com/r/EVD0DFQ48E606F6
@misc{mysteriesdept_evd0dfq48e606f6,
title = {Two telescopes put the radius below 2.8 km},
author = {{Mysteries Department}},
year = {2026},
note = {Evidence, catalog key EVD0DFQ48E606F6},
url = {https://mysteriesdept.com/r/EVD0DFQ48E606F6}
}