Comparing Likely Observation of 3I/Atlas with Juno Versus James Webb

by

Juno would offer ~20-24x better angular resolution due to proximity (distance ratio ~24:1), enabling finer details of extended features like jets or tail asymmetry, if the coma is active. Juno excels in intensity and resolution for extended structures, offering complementary data if diverted. Juno’s closer vantage enables brighter, potentially resolved views of active features—valuable for a rare, fast-moving target.

This as a low-risk repurposing of Juno, which is nearing mission end (extended operations conclude ~September 2025). With Juno’s mission winding down, this is a worthwhile high-reward gamble.

The diversion would likely allow Juno to collect unique, closer-range data not possible from JWST or other Earth-based facilities. We could potentially answering key questions about the composition and origin of such ancient, rare interstellar objects. Given the rarity of such visitors and the potential insights into the broader galactic environment, there is strong scientific justification for making an extra effort.

James Webb telescope

The 3I/Atlas object is about 3-4 AU (450-600 million km) from Earth, yielding an angular size for the 15 km nucleus of ~0.005-0.01 milliarcseconds (mas)—far below JWST’s diffraction limit (60 mas at 2 μm). Any coma (gas/dust envelope, potentially 100,000-500,000 km across if active) would span ~10-50 arcseconds, resolvable by JWST’s NIRCam (pixel scale ~0.03 arcsec/pixel).

At 25 million km, the nucleus angular size is 0.12 mas—still unresolved for Juno. A coma could span ~0.2-1 degree, potentially filling multiple pixels in JunoCam (resolution ~0.06 deg/pixel) or JIRAM (0.01 deg/pixel), allowing coarser but closer-view structure mapping.

Brightness and Signal-to-Noise Ratio (S/N):Flux scales inversely with distance squared (1/d²) for reflected/emitted light. Juno’s view would make 3I/ATLAS ~500-1,000x brighter than from JWST (magnitude boost of ~6-8), dramatically improving S/N for short exposures. The object is currently magnitude ~12-14 (observable but faint); from Juno, it could appear as bright as magnitude 4-6, easing detection amid background noise.

Advantage: Juno, for time-limited observations during a flyby-like pass.

JunoCam (visible RGB imaging) for color photometry of coma/tail. JIRAM (2-5 μm IR imager/spectrometer, resolving power ~100-300) for thermal mapping and basic gas detection (e.g., auroral-like emissions if magnetic). UVS for ultraviolet spectra. Particle detectors (e.g., JEDI) could sense dust impacts if close enough. Multi-instrument synergy for real-time data.
Limitations: Not a dedicated telescope—lower spectral resolution, smaller apertures (JIRAM ~9 cm), no deep-space sensitivity. Pointing constrained by Jupiter orbit; fuel use for repositioning.

JWST for detailed composition/chemistry and Juno for higher-flux, multi-wavelength snapshots (visible/IR/UV) and potential dust environment probing.

Scientific Yields

JWST: Ideal for global properties—spectral analysis of interstellar ices/minerals, linking to thick disk chemistry (e.g., ancient carbon-rich material). Could confirm age proxies via isotope ratios or volatile depletion.

Juno: Better for dynamic features (e.g., outgassing variability) and higher-S/N on faint emissions. Unique geometry: observing from “inside” the Solar System could reveal phase-angle effects (forward-scattering dust).