What are the physical characteristics of 3I/Atlas sightings (size, speed, altitude)?

1. Why observers disagree on size — Hubble’s upper limits vs. continuing uncertainty

Hubble Space Telescope imaging and follow-up analyses published in October 2025 set an upper limit on 3I/ATLAS’s nucleus diameter of about 3.5 miles, while alternative reductions and estimates indicate it could be as small as roughly 1,444 feet across, a spread that reflects signal faintness, coma contamination, and rapid motion that blur nucleus measurements ^[1]. Ground-based photometry and morphology studies show a brightening coma and active outgassing that makes isolating the nucleus difficult; when a substantial coma is present the nucleus can be hidden beneath diffuse light and any derived diameter becomes an upper limit rather than a precise measurement. Observers explicitly treat size estimates as provisional: imaging constraints deliver upper bounds driven by instrument resolution and subtraction of coma light, and different teams use distinct deconvolution methods and assumptions about albedo and activity, producing the range seen across reports ^{[1] [5]}.

2. Speed and velocity: consistent high-velocity consensus with numeric spread

Multiple teams report that 3I/ATLAS is traversing the inner solar system at velocities on the order of 50–70 km/s (roughly 130,000–152,000 mph), a speed typical for interstellar interlopers and consistent with a hyperbolic escape trajectory ^{[6] [5] [3]}. Differences in quoted numbers reflect whether speed is stated relative to the Sun or Earth and the epoch of measurement as the object accelerates near perihelion; published values of ~58 km/s and up to ~152,000 mph are mathematically compatible when converted between units and reference frames. Observers emphasize the high tangential motion across the sky, which complicates long-exposure imaging and forces short integrations on large telescopes; the rapid apparent motion is a key reason photometry and morphology remain uncertain and why specialized facilities were needed for post-perihelion recovery ^{[6] [3]}.

3. Altitude and distance: multiple AU, close approach still far from Earth

Reports consistently place the comet many astronomical units from Earth in late 2025: contemporaneous ephemerides list distances of roughly 2.2–2.9 AU at various observation epochs, with the predicted closest approach to Earth near 1.797 AU on December 19, 2025 (about 170 million miles), meaning the object never comes close in human terms and remains beyond Mars’ orbit at perihelion ^{[2] [3]}. Published perihelion distances cluster near ~1.36 AU, and positional data show the comet currently crossing constellations visible with telescopes once solar elongation allows longer observations. The practical consequence is that although 3I/ATLAS is scientifically compelling as an interstellar sample, its visual faintness and distance limit high-SNR spectroscopy and high-resolution imaging, reinforcing the reliance on large aperture facilities and space telescopes for detail ^{[2] [4]}.

4. Orbit shape and dynamics: unequivocally interstellar but some orbital elements vary

All analyses agree the orbit is strongly hyperbolic — eccentricities reported around 6.3 confirm the object is not gravitationally bound to the Sun and will not return; this flag of interstellar origin is robust across datasets ^{[3] [1]}. However, secondary orbital parameters such as reported inclination show inconsistencies between sources — one dataset lists an inclination near 175° while another lists ~85° — differences that arise from distinct element conventions (e.g., reference plane, retrograde vs. prograde sign conventions) and interim orbit refinements as more observations arrive. Teams also report measurable non-gravitational accelerations caused by outgassing, complicating inverse orbit fitting and yielding small course changes and mass-loss-driven acceleration that are factored into updated orbital solutions ^{[1] [4]}.

5. Activity and mass loss: cometary behavior, unexpected chemistry, and measurement implications

Spectroscopy and imaging show classic cometary activity — coma, water outflow (reported values such as ~40 kg/s at ~2.9 AU), and episodes of brightening — yet chemical and polarimetric signatures differ from typical solar-system comets, prompting headlines about unusual composition and polarization properties ^{[7] [4]}. Observers report a ~13% mass loss inferred from monitoring and that outgassing produced non-gravitational acceleration, which affects both size inference and trajectory fits; when mass and activity change, so do brightness and apparent morphology, feeding back into nucleus-size uncertainty. These activity-driven effects are central to reconciling divergent size and brightness claims and underscore why teams emphasize ongoing observations from Hubble, Webb, ground large telescopes, and survey facilities to refine physical parameters ^{[1] [4] [7]}.

6. What remains unresolved and why further observation matters

The core uncertainties are tightly coupled: nucleus size, precise non-gravitational acceleration, and detailed composition/polarization remain only partially constrained because the object is faint, fast-moving, and distant, and because activity masks the nucleus. Different groups publish slightly different numeric values depending on epoch, instrument, and reduction choices; no single measurement yet displaces the need for continued multi-wavelength monitoring and improved orbit solutions that incorporate measured outgassing forces ^{[1] [3] [8]}. The scientific payoff for resolving these specifics is high — better nucleus size plus