Summary
A June 11, 2026 SETI Live event hosted by Dr. Franck Marchis (seti-institute) and featuring Dr. Cecilia Tubiana (INAF) reviewed compositional data from ESA’s JUICE spacecraft, which observed interstellar comet 3i-atlas across five science instruments between November 2–25, 2025 — just after perihelion on October 29. JUICE’s MAJIS instrument recorded ~2,000 kg/s of water vapor ejection on November 2 (roughly 70 Olympic swimming pools daily), while the SWI instrument revealed that most water vapor escaped from the Sun-facing hemisphere and that significant portions originated from icy coma grains rather than directly from the nucleus. Despite its extrasolar origin, JUICE’s JANUS camera found 3I/ATLAS behaving like a “typical comet from the Solar System.” An unusual chemical ratio hints at formation in an extremely cold, ancient region of space — consistent with the anomalously high D/H ratio reported via alma (src-3i-atlas-heavy-water-2026-05). The JUICE campaign generated 126 science files (11.18 gigabits); data was downlinked in February 2026. This is a new multi-instrument dataset distinct from the dual-spacecraft UVS imaging result (src-3i-atlas-dual-spacecraft-imaging-2026-05) and the FAST/ATA technosignature searches.
Key Claims
- JUICE observed 3I/ATLAS with five instruments (JANUS, MAJIS, UVS, SWI, PEP) over seven observation slots between November 2–25, 2025, at closest approach of ~0.4 AU on November 4.
- MAJIS detected ~2,000 kg/s water vapor outgassing on November 2 — “on the high side” but not exceptional for a comet near the Sun.
- SWI showed directional emission: most water vapor from the Sun-facing hemisphere; significant contributions from icy coma grains rather than the nucleus directly.
- UVS detected gas/dust tails extending over 5 million km containing oxygen, hydrogen, and carbon atoms (complementing src-3i-atlas-dual-spacecraft-imaging-2026-05).
- JANUS imaging confirmed classic comet structure: bright coma, hidden nucleus, two distinct tails.
- Early chemical-ratio findings suggest formation in “an extremely cold and ancient region of space,” consistent with the anomalous D/H ratio in src-3i-atlas-heavy-water-2026-05.
- ESA coordinated three missions (Mars Express, ExoMars TGO, JUICE) plus Hubble, JWST, XMM-Newton, and XRISM to study 3I/ATLAS simultaneously.
- The nine-month-equivalent pointing campaign was compressed into weeks — a significant operational achievement for JUICE mission control.
Notable Quotes
“3I/ATLAS is a rare and unexpected visitor, its arrival came as a complete surprise, but presented a unique opportunity to collect a once-in-a-lifetime dataset.” — Olivier Witasse, ESA JUICE Project Scientist
“The data we are already seeing from Juice’s instruments is really promising.” — Claire Vallat, ESA co-Project Scientist
“Preparations for things like payload pointing campaigns or flybys are usually in the order of nine months.” — Angela Dietz, JUICE Spacecraft Operations Manager